Geochemical Behavior of Levoglucosan in Tibetan Plateau Glacier Snow and Ice [1st ed.] 9789811579721, 9789811579738

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Springer Theses Recognizing Outstanding Ph.D. Research

Chao You

Geochemical Behavior of Levoglucosan in Tibetan Plateau Glacier Snow and Ice

Springer Theses Recognizing Outstanding Ph.D. Research

Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected for its scientific excellence and the high impact of its contents for the pertinent field of research. For greater accessibility to non-specialists, the published versions include an extended introduction, as well as a foreword by the student’s supervisor explaining the special relevance of the work for the field. As a whole, the series will provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today’s younger generation of scientists.

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Chao You

Geochemical Behavior of Levoglucosan in Tibetan Plateau Glacier Snow and Ice Doctoral Thesis accepted by the Chinese Academy of Sciences, Beijing, China Excellent Doctoral Dissertation of Chinese Academy of Sciences, 2017

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Author Dr. Chao You Institute of Tibetan Plateau Research Chinese Academy of Sciences Beijing, China

Supervisor Prof. Tandong Yao Institute of Tibetan Plateau Research Chinese Academy of Sciences Beijing, China

Chongqing University Chongqing, China

ISSN 2190-5053 ISSN 2190-5061 (electronic) Springer Theses ISBN 978-981-15-7972-1 ISBN 978-981-15-7973-8 (eBook) https://doi.org/10.1007/978-981-15-7973-8 © Springer Nature Singapore Pte Ltd. 2021 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Supervisor’s Foreword

As the supervisor of Dr. You Chao’s Ph.D. thesis, I am pleased to introduce the influential and significant work by Dr. You. Fire exerts a crucial influence on global ecosystems and human society. This thesis focuses on geochemical behavior and ancient records of the specific biomarker, levoglucosan, in Tibetan glacier ice. You’s work first developed a convincing method for determination of trace concentration of levoglucosan in glacier ice. Then, You’s work discussed the snow–ice interface processes of levoglucosan on glacier surfaces, which are necessary in understanding the environmental significance of levoglucosan records. Finally, You’s work provided annually resolved levoglucosan records and fire changes since 1990 in a Tibetan ice core and discussed the interaction among fire, climate change, and human activities. This is the first effort to reconstruct annual resolution fire records in Tibetan ice, which provides crucial information for improving the knowledge of fire changes over the subtropical Asia. I believe that the research results of this thesis can substantially improve the analytical methods and extend the content of Tibetan ice core records. It will contribute significantly to the advance of understanding the past fire changes. Beijing, China September 2020

Prof. Tandong Yao

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Abstract

Levoglucosan is a widely used biomarker for tracing fire changes in environmental studies. This thesis summarized our understandings about geochemical behavior of levoglucosan in Tibetan Plateau glaciers. The main findings of this thesis are as follows: Develop a novel method for determination of levoglucosan at trace concentration levels in complex matrices of Tibetan glacier samples, by using ultraperformance liquid chromatography system combined with triple tandem quadrupole mass spectrometry. The required sample volume is 0.50 mL. The Limit of Detection is only about 0.11 ng mL−1, and with recovery higher than 90% at a concentration of 1 ng mL−1. This method ensures to obtain continuous levoglucosan records in Tibetan Plateau ice cores to reveal detailed characteristics of fire changes. Evaluate the spatial–temporal variations of levoglucosan in Tibetan Plateau glacier snow and ice. Results supported that Tibetan glaciers mainly acted as receptors of fire emissions from surrounding regions. Opposite distribution patterns of levoglucosan were observed on glaciers under different climate backgrounds over northern and southern parts of the Tibetan Plateau. The highest levoglucosan concentration was detected near the equilibrium line altitude on glacier surfaces. Temporal variability in levoglucosan distributions in snow and ice layers may be affected by chemical leaching on the glacier surface after deposition. Approve levoglucosan record in ice core that can reflect both long-term fire change trends and strong fire event insights. Levoglucosan record in Tibetan ice core indicated a rapid increase in fire activities since 1990 over adjacent regions. Although human activities have substantially modulated fire changes, climate change acted as a controlling factor for recent increasing fire activities. Increasing strong wildfires can potentially enhance black carbon deposits on Himalayan glaciers, which would impact glacial melting during the pre-monsoon wildfire seasons. The climate–fire relationships revealed in this work hint at high wildfire risks under the current warming and drying climate situation over the regions adjacent to the Tibetan Plateau.

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Abstract

Based on above-mentioned points, we suggested that levoglucosan records in Tibetan ice cores can be used as a creditable tool for calibrating and reconstructing past fire changes. Keywords Levoglucosan Glacier geochemistry


Fire changes
Tibetan Plateau
Ice core records


Main publications for supporting this thesis 1. You C*, Yao T (2019) Fire records in glacier ice. National Science Review 6(3): 384–386. (Reproduced with Permission. Copyright (2019) Oxford University Press and Science China Press). 2. You C*, Yao T, Xu C (2019) Environmental significance of levoglucosan records in a central Tibetan ice core. Science Bulletin 64(2):122–127. (Reproduced with Permission. Copyright (2019) Elsevier and Science China Press) 3. You C*, Yao TD, Xu C (2018) Recent increases in wildfires in the Himalayas and surrounding regions detected in central Tibetan ice core records. Journal of Geophysical Research: Atmospheres 123(6):3285–3291. (Reproduced with Permission. Copyright (2018) American Geophysical Union) 4. You C*, Xu C (2018) Review of levoglucosan in glacier snow and ice studies: Recent progress and future perspectives. Science of the Total Environment 616-617:1533–1539. (Reproduced with Permission. Copyright (2018) Elsevier) 5. You C*, Yao TD, Xu C, et al. (2017) Levoglucosan on Tibetan glaciers under different atmospheric circulations. Atmospheric Environment 152:1–5. (Reproduced with Permission. Copyright (2017) Elsevier) 6. You C*, Yao TD, Xu BQ, et al. (2016) Effects of sources, transport, and postdepositional processes on levoglucosan records in southeastern Tibetan glaciers. Journal of Geophysical Research: Atmospheres 121(14):8701–8711. (Reproduced with Permission. Copyright (2016) American Geophysical Union) 7. You C*, Xu C, Xu B, et al. (2016) Levoglucosan evidence for biomass burning records over Tibetan glaciers. Environmental Pollution 216:173–181. (Reproduced with Permission. Copyright (2016) Elsevier) 8. You C*, Song L, Xu B, et al. (2016) Method for determination of levoglucosan in snow and ice at trace concentration levels using ultra-performance liquid chromatography coupled with triple quadrupole mass spectrometry. Talanta 148:534–538. (Reproduced with Permission. Copyright (2016) Elsevier)

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Acknowledgements

This thesis is a summary of works done since my Ph.D. days. At this very moment, I would like to express my highest respect to my supervisor Prof. Yao Tandong. Prof. Yao lives up to the Chinese traditional standard of “the teacher, proselytizes instructs dispels doubt.” I am deeply influenced by his great personality and scientific insights, which will benefit me for life. I would also like to express my sincere gratitude toward Prof. Wu Guangjian, Prof. Xu Baiqing, Prof. Liu Yongqin, and Prof. Tian Lide, for their constructive suggestions in solving problems in my research. I also want to thank my colleagues in Prof. Yao’s team: Prof. Yu Wusheng, Prof. Gao Jing, Dr. Zhao Huabiao, Dr. Yang Wei, Dr. Yang Xiaoxin, Dr. Gao Yang, Dr. Li Shenghai, Dr. Zhu Meilin, Dr. Dai Yufeng, Dr. Deji, Xia Cuihui, Danzengzhuoga, Wen Xu, and Wang Ping, I learned a lot from their help. I also appreciate all the help I get from others during fieldworks. Finally, I sincerely thank my beloved family. With their support, I have no fear when I confront with difficulties. The research in support the thesis was funded by the Second Tibetan Plateau Scientific Expedition and Research Program (2019QZKK0201), the Youth Innovation Promotion Association CAS (2020071), the Chinese Academy of Sciences (YIPA2020071, and QYZDY-SSW-DQC003), and the National Natural Science Foundation of China (41725001, 41190081 and 41701078).

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Contents

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2 Introduction of the Zangsgangri Ice Core . . . . . . . . . 2.1 Basic Information of the Zangsegangri Glacier . . . 2.2 Basic Information for the Zangsegangri Ice Core . . 2.3 Additional Glacier Surface Snow and Ice Samples . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 Determination of Levoglucosan in Tibetan Glacier Snow and Ice Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Background Information . . . . . . . . . . . . . . . . . . . . . . . 3.2 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Sample Preparation . . . . . . . . . . . . . . . . . . . . . 3.2.3 Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . 3.2.4 Method Validation . . . . . . . . . . . . . . . . . . . . . 3.3 Optimization of the Instrumental Performance . . . . . . . 3.4 Extraction of Levoglucosan in Samples . . . . . . . . . . . . 3.5 Method Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4 Spatio–Temporal Variations of Levoglucosan on Tibetan Glaciers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Background Information . . . . . . . . . . . . . . . . . . . . . . . 4.2 Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Levoglucosan in Tibetan Plateau Glaciers . . . . . . . . . .

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1 Research Background . . . . . . . . 1.1 Fire in Earth System . . . . . 1.2 Fire Records in Glacier Ice . 1.3 Challenges . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . .

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4.4 Impact Factors for Spatial Distribution of Levoglucosan . 4.5 Levoglucosan at Different Elevations . . . . . . . . . . . . . . . 4.6 Temporal Variations in Levoglucosan and Black Carbon 4.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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5 Levoglucosan Records in the Zangsegangri Ice Core . . 5.1 Background Information . . . . . . . . . . . . . . . . . . . . . 5.2 Identify the Possible Fire Sources . . . . . . . . . . . . . . 5.3 Levoglucosan Records in the Zangsegangri Ice Core 5.4 The Annually Resolved Levoglucosan Records . . . . 5.5 Possible Influences of Strong Fire Events . . . . . . . . 5.6 Increasing Fire Activities Since 1990 . . . . . . . . . . . 5.7 Possible Linkages to Recent Climate Changes . . . . . 5.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 1

Research Background

1.1 Fire in Earth System Fire is a key factor affecting many aspects of the earth system, including climate change, ecosystems, land surface processes, the carbon cycle, atmospheric chemistry, and human society [3, 5, 7, 11, 12, 15, 28]. Understanding fire dynamics can provide insights into the interactions between fire and these components. Satellite data can provide high spatial–temporal resolution of fire changes for the most recent decades, while some historical information—such as the burned area in a certain region—can be traced back to the beginning of the twentieth century [1, 2, 25, 26]. Fire changes over longer time scales (historical to ancient) can be derived from some specific proxy records in the lithosphere, biosphere, cryosphere, hydrosphere, and anthroposphere [5, 15, 23, 30]. A combination of several evidence sources can yield a good estimate of the true fire variability [5, 11, 15, 23, 30]. Charcoal fossils in sediments can provide fire change information since the Silurian (about 420 million years ago, Ref. [5]), potentially yielding detailed evidence of fire distribution, intensity, and frequency [5, 15–17, 21]. The resolution of such records is decadal at best, and some have been disturbed by human activities over the past few decades to several centuries in densely populated regions such as the subtropical Asia [15–17]. Fire changes, and their influences on the environment and human society, are an important issue because the recent warming and drying trend has been reported to favor increased fire frequency [17, 19, 35]. High-resolution reconstructions of ancient fires in glacier ice are therefore crucial, and will constrain modeling predictions and inform policymaking [5, 15, 35, 37].

© Springer Nature Singapore Pte Ltd. 2021 C. You, Geochemical Behavior of Levoglucosan in Tibetan Plateau Glacier Snow and Ice, Springer Theses, https://doi.org/10.1007/978-981-15-7973-8_1

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1 Research Background

1.2 Fire Records in Glacier Ice The normal focus of fire studies in glacier ice is the chemical components, including characteristic ions (e.g., ammonium), black carbon, trapped air (e.g., methane and carbon monoxide), low-molecular-weight organic acids (e.g., formate and oxalate), persistent organic pollutants, resinic acids, and monosaccharide anhydrides (e.g., levoglucosan) [11, 23, 30]. Each of these components provides a different line of evidence [32]. These chemical species often have multiple potential sources in addition to fire emissions [11, 23, 30], and consequently, more specific tracers—related only to fire emissions—are preferred. Levoglucosan can only be generated from the degradation of cellulose and hemicellulose when the combustion temperature is higher than 300 °C [22, 30]. The lifetime of levoglucosan varies from several hours to more than ten days under different atmospheric conditions, and thus ensures its long-range transport and global distribution [8, 9, 22, 30, 37]. Levoglucosan has been widely detected in the Antarctic and Greenland ice sheets, and in mid-low-latitude mountain glaciers [8, 9, 11, 13, 14, 18, 20, 30, 31, 34–37]. Furthermore, levoglucosan displays almost no apparent degradation in the freezing and anaerobic conditions in glacier ice layers [9, 30, 32], thus possibly enabling its use as a specific biomarker for ancient fires in glacier ice records [4, 9, 20, 32, 35, 37]. Due to the long-distance transport, fire signals detected in glacier ice are usually at extremely low levels when compared to those of the source regions [9, 30, 33]. Although post-depositional photochemical or leaching processes can modify the records of organic compounds on glacier surfaces [34, 36], it should be noted that they can be used as proxies for fire changes at least on seasonal to annual scales over the accumulation zone, in either polar ice sheets or mountain glaciers [9, 30, 33]. Furthermore, some tracers can even capture event-based signals [9, 33, 34]. Nevertheless, there is not a one-to-one relationship between fire events in the source region and fire records detected in glacier ice [30, 33]. Polar ice sheets can provide more detailed information on fire changes after the Last Glacial Period [4, 10, 11, 37], and in particular can provide seasonal to annual reconstructions covering the past millennium [37] to past few decades [35]. For instance, higher levoglucosan concentrations were reported in inter-glacial ice than in glacial ice in the Dome C ice core from Antarctica [30]. Levoglucosan records reconstructed from both Greenland and Antarctic ice cores have revealed significant increases since the last glacial, reaching a maximum around 2500 years before present, then decreasing until the present; these changes may have been caused by anthropogenic activities [4, 37]. Some periods of high fire activities were detected by peaks in levoglucosan, vanillic acid, and p-hydroxybenzoic acid concentrations over the past millennium [38]. Fire changes over the past few decades remain as a controversial issue across high-latitude regions [11], and further, high-resolution evidence from alpine glacier ice is needed to resolve this dispute [11, 18, 33, 35]. Alpine glacier ice can provide some essential information about fire changes on regional scales [8, 33, 35]. The fire regime over southern Siberia since AD 1250 was reconstructed using nitrate, potassium, and charcoal records in the Belukha ice

1.2 Fire Records in Glacier Ice

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core from the Siberian Altai [6]. Fires increased at around 1980, followed by a decrease in the 1990s, as deduced from levoglucosan and black carbon records in ice cores from the Muztagh Ata and the Caucasus—both of which are affected by the middle latitude westerlies [27]. Intensified fire emissions due to anthropogenic activities were detected during the 1940s to 1950s [11, 27, 30]. However, to better understand ancient fire changes reconstructed in glacier ice, several associated issues need further attention in future works.

1.3 Challenges The most notable issue regards the geochemical significance of fire tracers. A large concentration of photochemically active groups is present in the snowpack [11, 30], which could potentially degrade biomarkers after deposition [30]. Much stronger evidence is needed to validate the stability of these tracers in glacier ice. Postdepositional processes, such as leaching and wind scour, on glacier surfaces can also reshape the initial records [9, 11, 33, 34]. Such processes might affect the reliability of any reconstructed fire records, yet their impacts remain poorly understood [11, 30]. Therefore, combining biomarkers with traditional proxies (e.g., black carbon) are necessary to achieve more confidence in fire reconstructions [30, 32]. Challenges in tracing fire sources are also an urgent issue. Those based on back trajectories can only provide rough results [4, 8, 9, 35, 37], and more precise results should be obtained using the isotopic or isomeric methods [18]. With the continued development of analytical methods, we can potentially detect levoglucosan and its isomers at trace (ng g−1 ) or ultratrace (pg g−1 ) concentrations or even lower [30], which is helpful for identifying fire source regions. Furthermore, developing greater sensitivity in new proxies is important, because a large quantity of unknown chemical components (mostly organics) exist in glacier ice [11, 30]. Precise dating of ice cores is critical for capturing the detailed characteristics of fire variations. Obtaining continuous results can reveal the frequency of fire events as well as longer-term temporal changes [35], especially from the multi-decadal to millennium scales. Besides, continuous results can potentially improve our understanding of the causes of differences between records obtained from various proxies [11]. Some years with extreme fire events could be used as absolute layers for calibrating ice core dating results, notably for those ice cores from alpine regions which usually lack absolute layers for cross-validation [24]. Indeed, glacier ice in low- and middle-latitude alpine regions can yield valuable data for better understanding fire variations [30], particularly in densely populated regions (e.g., subtropical Asia). The recent, rapid retreat of high elevation alpine glaciers makes collecting such records an urgent objective in low- and mid-latitude regions [29].

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1 Research Background

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18. Natalie K, Jeramy Roland J, Melissa ED, Davis GF, Erich CO, Joshua K, Jeremy H, Larry BB, Sarah KF (2020) Boreal blazes: biomass burning and vegetation types archived in the Juneau Icefield. Environ Res Lett 19. Pechony O, Shindell DT (2010) Driving forces of global wildfires over the past millennium and the forthcoming century. Proc Natl Acad Sci USA 107:19167–19170 20. Pokhrel A, Kawamura K, Kunwar B, Ono K, Tsushima A, Seki O, Matoba S, Shiraiwa T (2020) Ice core records of levoglucosan and dehydroabietic and vanillic acids from Aurora Peak in Alaska since the 1660s: a proxy signal of biomass-burning activities in the North Pacific Rim. Atmos Chem Phys 20:597–612 21. Power MJ, Marlon J, Ortiz N, Bartlein PJ, Harrison SP, Mayle FE, Ballouche A, Bradshaw RHW, Carcaillet C, Cordova C, Mooney S, Moreno PI, Prentice IC, Thonicke K, Tinner W, Whitlock C, Zhang Y, Zhao Y, Ali AA, Anderson RS, Beer R, Behling H, Briles C, Brown KJ, Brunelle A, Bush M, Camill P, Chu GQ, Clark J, Colombaroli D, Connor S, Daniau AL, Daniels M, Dodson J, Doughty E, Edwards ME, Finsinger W, Foster D, Frechette J, Gaillard MJ, Gavin DG, Gobet E, Haberle S, Hallett DJ, Higuera P, Hope G, Horn S, Inoue J, Kaltenrieder P, Kennedy L, Kong ZC, Larsen C, Long CJ, Lynch J, Lynch EA, McGlone M, Meeks S, Mensing S, Meyer G, Minckley T, Mohr J, Nelson DM, New J, Newnham R, Noti R, Oswald W, Pierce J, Richard PJH, Rowe C, Goni MFS, Shuman BN, Takahara H, Toney J, Turney C, Urrego-Sanchez DH, Umbanhowar C, Vandergoes M, Vanniere B, Vescovi E, Walsh M, Wang X, Williams N, Wilmshurst J, Zhang JH (2008) Changes in fire regimes since the Last Glacial Maximum: an assessment based on a global synthesis and analysis of charcoal data. Clim Dynam 30:887–907 22. Simoneit BRT, Schauer JJ, Nolte CG, Oros DR, Elias VO, Fraser MP, Rogge WF, Cass GR (1999) Levoglucosan, a tracer for cellulose in biomass burning and atmospheric particles. Atmos Environ 33:173–182 23. Suciu LG, Masiello CA, Griffin RJ (2019). Anhydrosugars as tracers in the Earth system. Biogeochemistry 24. Thompson LG, Yao T, Davis ME, Mosley-Thompson E, Wu G, Porter SE, Xu B, Lin P-N, Wang N, Beaudon E, Duan K, Sierra-Hernández MR, Kenny DV (2018) Ice core records of climate variability on the Third Pole with emphasis on the Guliya ice cap, western Kunlun Mountains. Quaternary Sci Rev 188:1–14 25. van der Werf GR, Randerson JT, Giglio L, Collatz GJ, Kasibhatla PS, Arellano AF (2006) Interannual variability in global biomass burning emissions from 1997 to 2004. Atmos Chem Phys 6:3423–3441 26. van der Werf GR, Randerson JT, Giglio L, van Leeuwen TT, Chen Y, Rogers BM, Mu M, van Marle MJE, Morton DC, Collatz GJ, Yokelson RJ, Kasibhatla PS (2017) Global fire emissions estimates during 1997–2016. Earth Syst Sci Data 9:697–720 27. Wang M, Xu B, Kaspari SD, Gleixner G, Schwab VF, Zhao H, Wang H, Yao P (2015) Centurylong record of black carbon in an ice core from the Eastern Pamirs: estimated contributions from biomass burning. Atmos Environ 115:79–88 28. Ward DS, Kloster S, Mahowald NM, Rogers BM, Randerson JT, Hess PG (2012) The changing radiative forcing of fires: global model estimates for past, present and future. Atmos Chem Phys 12:10857–10886 29. Yao T, Xue Y, Chen D, Chen F, Thompson L, Cui P, Koike T, Lau WKM, Lettenmaier D, Mosbrugger V, Zhang R, Xu B, Dozier J, Gillespie T, Gu Y, Kang S, Piao S, Sugimoto S, Ueno K, Wang L, Wang W, Zhang F, Sheng Y, Guo W, Ailikun X, Yang Y, Ma SSP, Shen Z, Su F, Chen S, Liang Y, Liu VP, Singh K, Yang D, Yang X, Zhao Y, Qian Y Zhang, Li Q (2019) Recent Third Pole’s rapid warming accompanies cryospheric melt and water cycle intensification and interactions between monsoon and environment: multidisciplinary approach with observations, modeling, and analysis. B Ame Meteorol Soc 100:423–444 30. You C, Xu C (2018) Review of levoglucosan in glacier snow and ice studies: recent progress and future perspectives. Sci Total Environ 616–617:1533–1539 31. You C, Xu C, Xu B, Zhao H, Song L (2016) Levoglucosan evidence for biomass burning records over Tibetan glaciers. Environ Pollut 216:173–181

6

1 Research Background

32. You C, Yao T (2019) Fire records in glacier ice. Natl Sci Rev 6:384–386 33. You C, Yao T, Xu C (2019) Environmental significance of levoglucosan records in a central Tibetan ice core. Sci Bull 64:122–127 34. You C, Yao TD, Xu BQ, Xu C, Zhao HB, Song LL (2016) Effects of sources, transport, and postdepositional processes on levoglucosan records in southeastern Tibetan glaciers. J Geophys Res-Atmos 121:8701–8711 35. You C, Yao TD, Xu C (2018) Recent increases in wildfires in the Himalayas and surrounding regions detected in Central Tibetan Ice Core Records. J Geophys Res-Atmos 123:3285–3291 36. You C, Yao TD, Xu C, Song LL (2017) Levoglucosan on Tibetan glaciers under different atmospheric circulations. Atmos Environ 152:1–5 37. Zennaro P, Kehrwald N, Marlon J, Ruddiman WF, Brucher T, Agostinelli C, Dahl-Jensen D, Zangrando R, Gambaro A, Barbante C (2015) Europe on fire three thousand years ago: Arson or climate? Geophys Res Lett 42:5023–5033 38. Zennaro P, Kehrwald N, McConnell JR, Schupbach S, Maselli OJ, Marlon J, Vallelonga P, Leuenberger D, Zangrando R, Spolaor A, Borrotti M, Barbaro E, Gambaro A, Barbante C (2014) Fire in ice: two millennia of boreal forest fire history from the Greenland NEEM ice core. Clim Past 10:1905–1924

Chapter 2

Introduction of the Zangsgangri Ice Core

In this section, we introduce essential information of the Zangsegangri ice core, and provide some necessary information of glacier snow and ice samples used in this thesis.

2.1 Basic Information of the Zangsegangri Glacier The Tibetan Plateau and its surrounding high-elevation regions (Fig. 2.1) covers an area over 5 million km2 with an average elevation higher than 3000 m above sea level (a.s.l) [9, 12, 13]. About 3 billion people live around and more than one billion people depend in part on meltwater from glaciers on the Tibetan Plateau [9]. Actually, the Tibetan Plateau and its surrounding high-elevation regions are home to around 100,000 km2 of glaciers (Fig. 2.1, and Ref. [11]), containing the largest volumes of ice outside the Arctic and Antarctic, and is thought as the Asian Water Tower [13]. Since the late twentieth century, temperature has increased as twice of the global average, making the Tibetan Plateau one of the world’s most risky regions with respect to its environmental, economic, and social consequences [13]. The equilibrium line altitudes of glaciers over the central Tibetan Plateau range from 5750 to 5900 m above sea level [14]. The Zangsegangri Glacier is with an area of more than 330 km2 and a volume of more than 40 km3 [5]. Although precipitation on the Zangsegangri Glacier (or even the central Tibetan Plateau) is reported deeply affected by local moisture recycle [1, 2, 10], and pollutants deposited on the glacier are alternatively influenced between the westerlies and the Indian summer monsoon [3, 17, 19]. Based on the climate records from meteorological stations (Gaize, Shiquanhe, Bange, Shenzha, Tuotuohe, Anduo, Naqu, and Wudaoliang) over the central Tibetan Plateau surrounding the Zangsegangri Glacier (Fig. 2.2), the local monthly mean temperature ranges from −1.70 °C in January to 1.38 °C in July, with an annual © Springer Nature Singapore Pte Ltd. 2021 C. You, Geochemical Behavior of Levoglucosan in Tibetan Plateau Glacier Snow and Ice, Springer Theses, https://doi.org/10.1007/978-981-15-7973-8_2

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2 Introduction of the Zangsgangri Ice Core

Fig. 2.1 Location site of the Zangsegangri ice core (red circle). The blue circles indicate the previously ice core sites, including Muztagh Ata, Guliya, Dasuopu, Puruogangri, and Dunde. The yellow delta indicates the meteorological stations nearby the Zangsegangri ice core, including 1—Shiquanhe, 2—Gaize, 3—Shenzha, 4—Bange, 5—Naqu, 6—Tuotuohe, 7—Anduo, and 8— Wudaoliang

average of −0.18 °C. Average precipitation is 284.04 mm per year, and 85% precipitation is concentrated between June and September over the central Tibetan Plateau.

2.2 Basic Information for the Zangsegangri Ice Core An ice core with a length of 208.6 m was extracted from the accumulation area of the Zangsegangri Glacier (34.32° N, 85.82° E, 6070 m a.s.l, Figs. 2.1 and 2.3) in the central Tibetan Plateau, during June 2013 [17, 19]. Ice core sections were kept frozen transported to Lhasa, and were stored in a cold storeroom at a temperature of −20 °C. Ice sections were cut at a sub-annual resolution of 3 cm in length. The outer part of each sample was scraped using pre-cleaned scalpel, and one portion of the inner part was stored in pre-cleaned polyethylene terephthalate bottles for levoglucosan analysis. Winter precipitation is less than 30 mm at meteorological stations across the central Tibetan Plateau (Fig. 2.2), and is lower than the sample resolution in the Zangsegangri ice core, potentially concealing the seasonal variation of water stable isotope signals. It has been shown that dust storms occur mostly in winter and spring over the central Tibetan Plateau regions [6, 8], forming dirty layers and insoluble particle peaks [6]. In addition, black carbon aerosols over South Asia are concentrated in the winter and pre-monsoon [4], yielding high pre-monsoon black carbon concentrations in Tibetan Plateau Glacier ice layers [7]. Ice core dating was thus

2.2 Basic Information for the Zangsegangri Ice Core

9

Fig. 2.2 Monthly temperature and precipitation amount variations at meteorological stations, including Shiquanhe, Gaize, Shenzha, Bange, Wudaoliang, Tuotuohe, Anduo, and Naqu over the central Tibetan Plateau around the Zangsegangri Glacier as shown in Fig. 2.1. Data are downloaded from the China Meteorological Administration (http://www.cma.gov.cn/)

confirmed by counting the annual variations of dirty layers and dust concentrations, and black carbon variations were used as references when the dirty layers or particle peaks were unclear (Fig. 2.4). The maxima of β activities and 137 Cs caused by the 1963 open-air nuclear weapon tests were used as the absolute layer for confirming the dating results [6]. The dating error is estimated about ±1 year for the past 50 years. However, dating result of the entire ice core still needs more evidences to approve, and is not used herein.

2.3 Additional Glacier Surface Snow and Ice Samples In order to understand the glacial-related geochemical processes for influencing levoglucosan variations on Tibetan Glacier surfaces, some snow and ice samples were collected. Those including Yala Glacier, Dasuopu Glacier, Zuoqiupu Glacier, Demula

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2 Introduction of the Zangsgangri Ice Core

Fig. 2.3 Overview of the Zangsegangri Glacier and ice core drilling site. Photo provided by prof. Tian Lide, Yunnan University

Fig. 2.4 Dirty layer levels (dark brown bars), dust number concentration (brown line), black carbon (black line) concentration profiles, and beta activity variations (green line) of Zangsegangri ice core in the upper 12 m. The green dash lines are annual layer boundaries, the red arrow indicates AD 1963, and the red stars stands for Cesium profiles. Adapted from Refs. [17, 19]

Glacier on the southern Tibetan Plateau; the Muji Glacier, Kuokuosele Glacier and Qiyi Glacier on the northern Tibetan Plateau [15, 16, 18]. The polyethylene terephthalate bottles were washed twice by soaking in ultrapure water for at least 24 h and rinsing three times with ultrapure water. These were dried in the shade in a clean room. Disposable polyethylene gloves were used for snow sample collections. Glacier snow and ice samples were compacted in pre-cleaned polyethylene terephthalate bottles and were kept frozen at a temperature of −20 °C before chemical analysis.

References

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References 1. An W, Hou S, Zhang Q, Zhang W, Wu S, Xu H, Pang H, Wang Y, Liu Y (2017) Enhanced recent local moisture recycling on the Northwestern Tibetan Plateau deduced from ice core deuterium excess records. J Geophys Res-Atmos 122(12):541–512, 556 2. Gao J, Yao T, Masson-Delmotte V, Steen-Larsen HC, Wang W (2019) Collapsing glaciers threaten Asia’s water supplies. Nature 565:19–21 3. Kang SC, Zhang QG, Qian Y, Ji ZM, Li CL, Cong ZY, Zhang YL, Guo JM, Du WT, Huang J, You QL, Panday AK, Rupakheti M, Chen DL, Gustafsson O, Thiemens MH, Qin DH (2019) Linking atmospheric pollution to cryospheric change in the Third Pole region: current progress and future prospects. Natl Sci Rev 6:796–809 4. Ramanathan V, Carmichael G (2008) Global and regional climate changes due to black carbon. Nat Geosci 1:221–227 5. Shi YF, Liu CH, Kang E (2009) The glacier inventory of China. Ann Glaciol 50:1–4 6. Wu GJ, Zhang CL, Xu BQ, Mao R, Joswiak D, Wang NL, Yao TD (2013) Atmospheric dust from a shallow ice core from Tanggula: implications for drought in the central Tibetan Plateau over the past 155 years. Quaternary Sci Rev 59:57–66 7. Xu B, Cao J, Hansen J, Yao T, Joswia DR, Wang N, Wu G, Wang M, Zhao H, Yang W, Liu X, He J (2009) Black soot and the survival of Tibetan glaciers. Proc Natl Acad Sci USA 106:22114–22118 8. Xu C, Ma YM, You C, Zhu ZK (2015) The regional distribution characteristics of aerosol optical depth over the Tibetan Plateau. Atmos Chem Phys 15:12065–12078 9. Yao T (2019). Tackling on environmental changes in Tibetan Plateau with focus on water, ecosystem and adaptation. Sci Bull 64 10. Yao T, Masson-Delmotte V, Gao J, Yu W, Yang X, Risi C, Sturm C, Werner M, Zhao H, He Y, Ren W, Tian L, Shi C, Hou S (2013) A review of climatic controls on δ18O in precipitation over the Tibetan Plateau: observations and simulations. Rev Geophys 51:525–548 11. Yao T, Thompson LG, Mosbrugger V, Zhang F, Ma Y, Luo T, Xu B, Yang X, Joswiak DR, Wang W, Joswiak ME, Devkota LP, Tayal S, Jilani R, Fayziev R (2012) Third Pole environment (TPE). Environ Develop 3:52–64 12. Yao T, Wu F, Ding L, Sun J, Zhu L, Piao SL, Deng T, Ni X, Zheng H, Ouyang H (2015) Multispherical interactions and their effects on the Tibetan Plateau’s earth system: a review of the recent researches. Natl Sci Rev 2:468–488 13. Yao T, Xue Y, Chen D, Chen F, Thompson L, Cui P, Koike T, Lau WKM, Lettenmaier D, Mosbrugger V, Zhang R, Xu B, Dozier J, Gillespie T, Gu Y, Kang S, Piao S, Sugimoto S, Ueno K, Wang L, Wang W, Zhang F, Sheng Y, Guo W, Ailikun X, Yang Y, Ma SSP, Shen Z, Su F, Chen S, Liang Y, Liu VP, Singh K, Yang D, Yang X, Zhao Y, Qian Y Zhang, Li Q (2019) Recent Third Pole’s rapid warming accompanies cryospheric melt and water cycle intensification and interactions between monsoon and environment: multidisciplinary approach with observations, modeling, and analysis. B Ame Meteorol Soc 100:423–444 14. Yao TD, Thompson L, Yang W, Yu WS, Gao Y, Guo XJ, Yang XX, Duan KQ, Zhao HB, Xu BQ, Pu JC, Lu AX, Xiang Y, Kattel DB, Joswiak D (2012) Different glacier status with atmospheric circulations in Tibetan Plateau and surroundings. Nat Clim Change 2:663–667 15. You C, Song L, Xu B, Gao S (2016) Method for determination of levoglucosan in snow and ice at trace concentration levels using ultra-performance liquid chromatography coupled with triple quadrupole mass spectrometry. Talanta 148:534–538 16. You C, Xu C, Xu B, Zhao H, Song L (2016) Levoglucosan evidence for biomass burning records over Tibetan glaciers. Environ Pollut 216:173–181 17. You C, Yao T, Xu C (2019) Environmental significance of levoglucosan records in a central Tibetan ice core. Sci Bull 64:122–127 18. You C, Yao TD, Gao SP, Gong P, Zhao HB (2014) Simultaneous determination of levoglucosan, mannosan and galactosan at trace levels in snow samples by GC/MS. Chromatographia 77:969– 974 19. You C, Yao TD, Xu C (2018) Recent increases in wildfires in the Himalayas and surrounding regions detected in Central Tibetan Ice Core Records. J Geophys Res-Atmos 123:3285–3291

Chapter 3

Determination of Levoglucosan in Tibetan Glacier Snow and Ice Samples

In this chapter, a method is developed for determination of levoglucosan at trace concentration levels in complex matrices of glacier snow and ice samples. Samples are analyzed using ultraperformance liquid chromatography system combined with triple tandem quadrupole mass spectrometry (UPLC-MS/MS). This method uses an injection mixture comprising acetonitrile and melt sample at a ratio of 50/50 (v/v). Levoglucosan is analyzed on BEH Amide column (2.1 mm × 100 mm, 1.7 μm), and a Z-spray electrospray ionization source is used for levoglucosan ionization. During the method validation, limit of detection (LOD), linearity, recovery, repeatability, and reproducibility were evaluated using standard addition method. The LOD of this method is about 0.11 ng mL−1 . Recovery higher than 90%, repeatability with relative standard deviation (RSD) lower than 10%, reproducibility with RSD lower than 10% at a concentration of 1 ng mL−1 . This method can be implemented using less than 0.50 mL glacier snow/ice sample volume in low and middle latitude regions like the Tibetan Plateau.

3.1 Background Information Levoglucosan can be adopted as specific molecular tracers for fire-related aerosols, because it can only be generated by the degradation of cellulose and hemicellulose when the burning temperature is higher than 300 °C [17, 18]. Levoglucosan can remain stable for long periods of time, with only negligible degradation in sediment conditions [3], which further extends its applicability in historical fire change studies. Glacier snow and ice can provide critical high-resolution information about past fire changes [10, 24, 25]. Levoglucosan can be used as ideal markers in fire activity studies of snow and ice [1, 14, 25, 26, 28, 29]. The gas chromatography-based methods need extreme dehydration conditions, which requires a long time for sample preparation

© Springer Nature Singapore Pte Ltd. 2021 C. You, Geochemical Behavior of Levoglucosan in Tibetan Plateau Glacier Snow and Ice, Springer Theses, https://doi.org/10.1007/978-981-15-7973-8_3

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3 Determination of Levoglucosan in Tibetan Glacier Snow …

[8, 15, 27]. The extreme dehydration conditions reduced the recovery and precision for handling trace concentration of levoglucosan in aqueous samples [15]. The high-performance liquid chromatography (HPLC)-based method was used as an alternative to overcome these issues [15]. The HPLC with triple quadrupole tandem mass spectrometry (HPLC-ESI-MS/MS) method with low LOD and high recovery ensured accurate results for detecting levoglucosan in Arctic and Antarctic ice [4, 9, 28, 29]. However, samples from middle and low latitude regions (e.g., the Tibetan Plateau) usually contain much more complex organic components like the glycose and sugar alcohols besides levoglucosan [8, 14, 21, 27]. The method for polar ice showed very poor performance for levoglucosan due to matrix interferences when applied to samples from Tibetan glaciers [22]. Besides, evidence showed that the levoglucosan concentration was at about 1 ng mL−1 level even in regions strongly affected by fire emissions [8, 11, 14, 27]. Due to the large quantity of insoluble particles in samples from low and middle latitude glaciers [7, 19, 20], direct injection without any pretreatment in previous methods [4, 22] is harmful to the HPLC system and chromatographic columns. Therefore, it is necessary to develop a rapid and effective method for determination of levoglucosan at trace concentration levels in complex matrix snow and ice for regions like the Tibetan Plateau. In this section, a method based on UPLC-MS/MS for determination of levoglucosan in snow and ice is reported. The preparation process and instrument conditions are specific for complex matrix samples from low and middle latitude glaciers.

3.2 Materials 3.2.1 Chemicals HPLC gradient grade acetonitrile was obtained from Fisher Scientific (U.S.A.). HPLC gradient grade methanol for standard stock solutions was obtained from J.T.Baker (USA). HPLC grade ammonium hydroxide (10%) was obtained from Mreda (USA). Ultrapure water was obtained from a Milli-Q ultrapure water system (USA). Standard levoglucosan (1,6-anhydro-β-d-glucopyranose, 99%) was obtained from Sigma-Aldrich (St. Louis, USA). Standard stock solution was prepared using methanol at a concentration of 1000 μg mL−1 , and progressively diluted by ultrapure water for use. Standard stock solution was stored in dark conditions at a temperature of 4 °C.

3.2.2 Sample Preparation Fresh snow and ice samples were compacted in pre-cleaned PET bottles and were kept frozen at a temperature of −20 °C before analysis. Previous studies indicated that

3.2 Materials Table 3.1 Detailed gradient flow program for the UPLC system

15 Time (min)

Mobile A/Mobile B

Flow rate (mL min−1 )

Initial

65/35

0.20

0.25

65/35

0.20

0.50

50/50

0.20

7.00

50/50

0.20

samples stored under these conditions displayed no apparent degradation of levoglucosan for at least several years [4, 9]. Samples were melted at a room temperature (about 15 °C) in a fume cupboard before analysis, and shaken for 5 min. A sample volume of 0.50 mL of each sample was extracted and filtered by polyether sulfone filter. The filtrate was collected in a 2.00 mL sample vial, and 0.50 mL acetonitrile was added to each sample before analysis.

3.2.3 Instrumentation Sample analysis was performed by a Waters Acquity UPLC system (USA) in reversed phase mode. The auto-sampler was thermostatically controlled at a temperature of 15 °C. A BEH VanGuard Pre-column (2.1 mm × 5 mm, 1.7 μm, Waters, USA) was used to protect the chromatography column. For the chromatographic analysis, 5.00 μL of each sample was injected onto a BEH Amide column (2.1 mm × 100 mm, 1.7 μm, Waters, USA). The column temperature was set to 40 °C. The mobile phase comprised ultrapure water with 0.1% NH3 H2 O (mobile A) and acetonitrile (mobile B). Gradient elution was employed at a flow rate of 0.20 mL min−1 , and the gradient program was shown in Table 3.1. The retention time of levoglucosan was 1.41 min, and the analytical process lasted 7.00 min in total (Fig. 3.1). The Acquity triple quadrupole mass spectrometer (TQD) equipped with a Z-spray electrospray ionization (ESI) source was used for determination of levoglucosan in this study. Data were collected in negative ion mode by multiple reactions monitoring (MRM), and the ESI source block temperature was 150 °C. The optimal MS conditions of the mass spectrometers were as follows: source voltage 3.00 kV; source desolvation temperature 500 °C; source gas flow desolvation 800 L h−1 ; cone gas 50 L h−1 . The ion transition m/z 161/101 was used for quantification of levoglucosan in samples. The data were collected and analyzed by Masslynx 4.1 software developed by Waters Company.

3.2.4 Method Validation The method was validated following recommendations of ICH and some recent scientific publications relating to the analytical process [2, 4, 6]. During the method

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3 Determination of Levoglucosan in Tibetan Glacier Snow …

Fig. 3.1 Chromatogram for levoglucosan under the optimal experimental conditions. a Procedural blank; b standard solution at a concentration of 2.00 ng mL−1 ; and c chromatogram for a sample from the Zangsegangri ice core

validation, LOD, linearity, recovery, repeatability, and reproducibility were evaluated using standard additions method.

3.3 Optimization of the Instrumental Performance

17

3.3 Optimization of the Instrumental Performance The ESI-MS conditions were optimized by infusing single standard in both positiveand negative-ionization modes. No signal was recorded under different ion source parameters in positive mode. Highly abundant analytes signals were detected in negative mode, and ion transitions of levoglucosan were selected using single standard solutions by direct infusion at a concentration of 100 ng mL−1 into the ion source of the mass spectrometer (161/71 and 161/101 for levoglucosan). The effect of various ESI-MS parameters such as ESI source temperature (100–200 °C), source voltage (2.5–4.5 kV), source desolvation temperature (300–650 °C), and desolvation gas flow rate (600–1000 L h−1 ) was studied. C18 columns have usually been used for HPLC determination of levoglucosan in snow/ice [4, 22]; however, very poor chromatographic response intensity was observed when the Acquity BEH C18 column (Waters, 2.1 mm × 100 mm, 2.1 mm × 75 mm and 2.1 mm × 50 mm, 1.7 μm) was introduced in the UPLC system in this study. Levoglucosan is strongly hydrophilic compounds, and previous studies even reported that levoglucosan could not be sufficiently retained on alkyl-bonded silica columns [5]. Due to extensive column bleeding, the carbohydrate column was not compatible with the mass spectrometer ion source for levoglucosan analysis reported in previous aerosol studies [2]. Polar compounds have been proven to achieve higher retention and selectivity on the BEH Amide stationary phase compared with BEH hydrophilic interaction chromatography (HILIC) stationary phase in the UPLC system [13]. Previous study indicated that BEH amide columns were with good performances for dealing with levoglucosan in aqueous matrix samples. Therefore, the Acquity BEH amide column (2.1 mm × 150 mm, 1.7 μm) was chosen for analysis of levoglucosan in snow and ice samples in this study. Pure water matrix injection is harmful to the amide column due to the severe depletion of the amide group by water. However, a higher proportion of acetonitrile can apparently dilute the concentration of levoglucosan in samples. An injection mixture comprising acetonitrile and melt sample at a ratio of 50/50 (v/v) was used. This ratio not only yields a good chromatographic peak of levoglucosan but also protects the BEH amide stationary phase. Furthermore, 50% acetonitrile can also prevent the potential microbial decomposition of levoglucosan in melted samples, noting the large number of bacteria reported in Tibetan Glacier snow and ice [23]. Result of repeated tests showed that levogluocsan was stable and displayed negligible degradation under the 50% acetonitrile during storage at a temperature of 4 °C for longer than one week. The recommended mobile phase for the Acquity BEH amide column includes a high acetonitrile ratio. However, a higher ratio of acetonitrile in the mobile phase can also lead to a broader chromatographic peak of levoglucosan. Different ratios of acetonitrile/water (30/70, 32.5/67.5, 35/65, 37.5/62.5, 40/60, 50/50, 60/40, and 70/30) were tested as mobile phases. The best signal to noise (S/N) was obtained using a mobile phase of 35% acetonitrile, with S/N = 263 at a concentration of 10.00 ng mL−1 for levoglucosan standard solution. However, when the isocratic

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3 Determination of Levoglucosan in Tibetan Glacier Snow …

program of 35% acetonitrile was applied to Tibetan Glacier samples, it showed poor levoglucosan separation. Considering the depletion of the BEH amide stationary phase and the impact of other unknown polar organic compounds on levoglucosan, a gradient method was used to achieve a better result (Table 3.1). The S/N under this gradient program was 287 at a target concentration of 10.00 ng mL−1 (3 injections) was 56 at a concentration of 2.00 ng mL−1 (Fig. 3.1), and was 73 at a concentration of about 3.00 ng mL−1 for a Tibetan ice sample (Fig. 3.1). Comparative experiments showed that signal intensity increased when temperature increased from 25 to 40°C, and then decreased with increasing temperature. Thus, the column temperature was chosen as 40°C in this study. Experimental results showed that the signal intensity increased a little with increasing flow rate of the mobile phase, but the retention time increased with lower flow rates. When the ratio of acetonitrile is lower than 50% in the mobile phase, a lower flow rate is helpful to protect the BEH amide column. Previous studies also indicated that a lower flow rate could reduce matrix effects [5]. Finally, a flow rate of 0.20 mL min−1 was used in this study. The excellent separation capacity of the BEH amide column can provide potential separation of levoglucosan and its isomers. Good chromatographic performance was obtained for single standard solutions of three isomers. However, the response intensity was very low for mixed standards even at a concentration of 100 ng mL−1 (about 20 times lower than that for a single standard). A similar phenomenon was reported in Antarctic ice samples [4]. Considering the extremely low concentration of the levoglucosan in snow and ice samples, we finally concentrated on increasing the instrumental sensitivity of levoglucosan at the cost of the chromatographic separation of the three isomers. This is acceptable because levoglucosan accounting for more than 90% of total monosaccharide anhydrides in environmental conditions [17, 18, 24], mannosan and galactosan were reported only minor contributors in the Tibetan Glacier ice [27].

3.4 Extraction of Levoglucosan in Samples Although previous studies have indicated that the filtration process might absorb and contaminate the samples [4], filtration was necessary for sample preparation when using the UPLC system due to the large quantity of insoluble particles in samples from Tibetan glaciers [19, 20]. In this study, comparative experiments were conducted with four commercially available 0.22 μm water system millipore filters: Teflon filter, polyamides filter, polypropylene filter and polyether sulfone filter (6 injections). Results showed that the PP filter could absorb levoglucosan significantly. However, repeatability tests (50 injections) showed a relative standard deviation (RSD) of 23% for the Polyamides filter, 16% for the Teflon filter and 6% for the polyether sulfone filter at a concentration of 5.00 ng mL−1 . All of the samples were filtered using 0.22 μm polyether sulfone filters.

3.5 Method Validation

19

3.5 Method Validation Results show that there are no apparent chromatographic peaks of levoglucosan in procedural blanks, and the response intensity is 51 ± 13 (based on 30 injections). The RSD was 0.036 ng mL−1 at a target concentration of 1.00 ng mL−1 (based on 30 injections). The LOD of this method was quantified as three times the averaged RSD, yielding a concentration of 0.11 ng mL−1 . The absolute mass LOD is 0.55 pg (0.11 ng mL−1 ×5 μL injection) in this study, compared with 0.3 pg (3 pg mL−1 × 100 μL injection) reported in Arctic and Antarctic ice [4, 9]. Results of three consecutive measurements show that recovery is 93% at a concentration of 1.20 ng mL−1 , and 99% at a concentration of 5.00 ng mL−1 . Repeatability of the method was evaluated by RSD, and varies from 15% at a concentration of 1.20 ng mL−1 to 3% at a concentration of 5.00 ng mL−1 . Reproducibility of the method was assessed by analyzing the same concentration over three consecutive days (three replications per day). RSD is 12% at 1.20 ng mL−1 , and 2% at 5.00 ng mL−1 . Three consecutive measurements of the selected samples from the Zangsegangri ice core also show high repeatability; RSD is 17.5% at a concentration of 1.49 ng mL−1 and 8.2% at a concentration of 3.41 ng mL−1 in authentic ice samples. The linearity was evaluated based on matrix-matched calibration curve, and daily calibration was performed. The target concentration of levoglucosan for the calibration curve varied from 0.50 to 10.00 ng mL−1 at five concentration levels (0.50, 1.00, 2.00, 5.00, and 10.00 ng mL−1 , see in Fig. 3.2), covering typical concentrations of levoglucosan in the snow/ice samples reported in previous studies. Good linearity was obtained, with an R2 value of 0.9997 (Fig. 3.2). The intercept of the calibration curve is 0.054 (intercept of the signal intensity is 42), indicating that the background is much lower than the method LOD. The coefficient of variation for the daily calibration curves was only 7.3% during the experimental period of 2019, which indicates that the matrix effect impact is small and has no apparent influences on snow and ice samples [12]. Levoglucosan concentration varied from below LOD to 26.67 ng mL−1 , and the sample-based average value was about 0.94 ng mL−1 in the Tibetan glaciers (Table 3.2). Levoglucosan concentration has been reported with an average value of about 0.1 ng mL−1 in Greenland and Antarctic ice samples [4], Kehrwald et al. [1, 9, 28, 29]. The concentration reached 0.75 ng mL−1 in an Ushkovsky ice core, due to the strong influence of wildfires over Siberia [8, 16]. The comparison of levoglucosan in glacier snow and ice among Tibetan Plateau and other regions is discussed in Sect. 3.4. Results indicated that Tibetan glaciers are more seriously contaminated by fire emissions when compared with high latitude regions.

20

3 Determination of Levoglucosan in Tibetan Glacier Snow …

Fig. 3.2 Calibration curve during the experimental period of 2019 Table 3.2 Levoglucosan concentrations (ng mL−1 ) in Tibetan Glacier snow and ice Glacier name

Concentration

Mean

Sample number

Sample type

Dasuopu

BLD to 1.56

0.31

11

Muji

0.32–2.78

0.94

23

Demula

BLD to 1.13

0.28

6

Snow

Zuoqiupu

BLD to 6.07

1.41

20

Snow

Yala

21.94–32.90

26.67

4

Cuopugou

BLD to 3.95

0.98

63

Ice core

Zangsegangri

BLD to 7.56

0.88

157

Ice core

Snow Snow-pit

Snow

Notes BLD means below limit of detection

3.6 Summary A UPLC-MS/MS method for rapid determination of levoglucosan in glacier snow and ice has been validated in this study. This method is suitable for samples from Tibetan glaciers, and can be applied to other low- and middle-latitude regions with complex matrices. Result indicates that Tibetan glaciers are evidently affected by

3.6 Summary

21

fire emissions. This method will facilitate further studies of fire records on Tibetan glaciers.

References 1. Battistel D, Kehrwald NM, Zennaro P, Pellegrino G, Barbaro E, Zangrando R, Pedeli XX, Varin C, Spolaor A, Vallelonga PT, Gambaro A, Barbante C (2018) High-latitude Southern Hemisphere fire history during the mid- to late Holocene (6000–750 BP). Clim Past 14:871–886 2. Dye C, Yttri KE (2005) Determination of monosaccharide anhydrides in atmospheric aerosols by use of high-performance liquid chromatography combined with high-resolution mass spectrometry. Anal Chem 77:1853–1858 3. Elias VO, Simoneit BRT, Cordeiro RC, Turcq B (2001) Evaluating levoglucosan as an indicator of biomass burning in Carajas, Amazonia: a comparison to the charcoal record. Geochim Cosmochim Ac 65:267–272 4. Gambaro A, Zangrando R, Gabrielli P, Barbante C, Cescon P (2008) Direct determination of levoglucosan at the picogram per milliliter level in Antarctic ice by high-performance liquid chromatography/electrospray ionization triple quadrupole mass spectrometry. Anal Chem 80:1649–1655 5. Ghfar AA, Wabaidur SM, Ahmed AY, Alothman ZA, Khan MR, Al-Shaalan NH (2015) Simultaneous determination of monosaccharides and oligosaccharides in dates using liquid chromatography-electrospray ionization mass spectrometry. Food Chem 176:487–492 6. ICH (2005) Guideline Q2(R1)—validation of analytical procedures: text and methodology. ICH Secretariat, c/o IFPMA 7. Kang SC, Zhang QG, Qian Y, Ji ZM, Li CL, Cong ZY, Zhang YL, Guo JM, Du WT, Huang J, You QL, Panday AK, Rupakheti M, Chen DL, Gustafsson O, Thiemens MH, Qin DH (2019) Linking atmospheric pollution to cryospheric change in the Third Pole region: current progress and future prospects. Natl Sci Rev 6:796–809 8. Kawamura K, Izawa Y, Mochida M, Shiraiwa T (2012) Ice core records of biomass burning tracers (levoglucosan and dehydroabietic, vanillic and p-hydroxybenzoic acids) and total organic carbon for past 300 years in the Kamchatka Peninsula, Northeast Asia. Geochim Cosmochim Ac 99:317–329 9. Kehrwald, N., R. Zangrando, P. Gabrielli, J. L. Jaffrezo, C. Boutron, C. Barbante, and A. Gambaro. (2012). Levoglucosan as a specific marker of fire events in Greenland snow. Tellus B 64 10. Legrand M, McConnell J, Fischer H, Wolff EW, Preunkert S, Arienzo M, Chellman N, Leuenberger D, Maselli O, Place P, Sigl M, Schupbach S, Flannigan M (2016) Boreal fire records in Northern Hemisphere ice cores: a review. Clim Past 12:2033–2059 11. Li Q, Wang N, Barbante C, Kang S, Yao P, Wan X, Barbaro E, Del Carmen Villoslada M, Hidalgo A, Gambaro C, Li H, Niu Z Dong, Wu X (2018) Levels and spatial distributions of levoglucosan and dissolved organic carbon in snowpits over the Tibetan Plateau glaciers. Sci Total Environ 612:1340–1347 12. Matuszewski BK, Constanzer ML, Chavez-Eng CM (2003) Strategies for the assessment of matrix effect in quantitative bioanalytical methods based on HPLC–MS/MS. Anal Chem 75:3019–3030 13. Novakova L, Gottvald T, Vlckova H, Trejtnar F, Mandikova J, Solich P (2012) Highly sensitive fast determination of entecavir in rat urine by means of hydrophilic interaction chromatographyultra-high-performance liquid chromatography-tandem mass spectrometry. J Chromatogr A 1259:237–243 14. Pokhrel A, Kawamura K, Kunwar B, Ono K, Tsushima A, Seki O, Matoba S, Shiraiwa T (2020) Ice core records of levoglucosan and dehydroabietic and vanillic acids from Aurora Peak in

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15. 16. 17.

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28.

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3 Determination of Levoglucosan in Tibetan Glacier Snow … Alaska since the 1660 s: a proxy signal of biomass-burning activities in the North Pacific Rim. Atmos Chem Phys 20:597–612 Schkolnik G, Rudich Y (2006) Detection and quantification of levoglucosan in atmospheric aerosols: a review. Anal Bioanal Chem 385:26–33 Seki O, Kawamura K, Bendle JAP, Izawa Y, Suzuki I, Shiraiwa T, Fujii Y (2015) Carbonaceous aerosol tracers in ice-cores record multi-decadal climate oscillations. Sci Rep 5:14450 Simoneit BRT, Schauer JJ, Nolte CG, Oros DR, Elias VO, Fraser MP, Rogge WF, Cass GR (1999) Levoglucosan, a tracer for cellulose in biomass burning and atmospheric particles. Atmos Environ 33:173–182 Suciu LG, Masiello CA, Griffin RJ (2019) Anhydrosugars as tracers in the Earth system. Biogeochemistry Wu G, Yao T, Xu B, Tian L, Zhang C, Zhang X (2010) Dust concentration and flux in ice cores from the Tibetan Plateau over the past few decades. Tellus B 62:197–206 Xu J, Yu G, Kang S, Hou S, Zhang Q, Ren J, Qin D (2012) Sr–Nd isotope evidence for modern aeolian dust sources in mountain glaciers of western China. J Glaciol 58:859–865 Xu J, Zhang Q, Li X, Ge X, Xiao C, Ren J, Qin D (2013) Dissolved organic matter and inorganic ions in a Central Himalayan glacier—insights into chemical composition and atmospheric sources. Environ Sci Technol 47:6181–6188 Yao P, Schwab VF, Roth V-N, Xu B, Yao T, Gleixner G (2013) Levoglucosan concentrations in ice-core samples from the Tibetan Plateau determined by reverse-phase high-performance liquid chromatography–mass spectrometry. J Glaciol 59:599–612 Yao T, Liu Y, Kang S, Jiao N, Zeng Y, Liu X, Zhang Y (2008) Bacteria variabilities in a Tibetan ice core and their relations with climate change. Global Biogeochem Cycles 22:GB4017 You C, Xu C (2018) Review of levoglucosan in glacier snow and ice studies: recent progress and future perspectives. Sci Total Environ 616–617:1533–1539 You C, Yao T (2019) Fire records in glacier ice. Natl Sci Rev 6:384–386 You C, Yao T, Xu C (2019) Environmental significance of levoglucosan records in a central Tibetan ice core. Sci Bull 64:122–127 You C, Yao TD, Gao SP, Gong P, Zhao HB (2014) Simultaneous determination of levoglucosan, mannosan and galactosan at trace levels in snow samples by GC/MS. Chromatographia 77:969– 974 Zennaro P, Kehrwald N, Marlon J, Ruddiman WF, Brucher T, Agostinelli C, Dahl-Jensen D, Zangrando R, Gambaro A, Barbante C (2015) Europe on fire three thousand years ago: arson or climate? Geophys Res Lett 42:5023–5033 Zennaro P, Kehrwald N, McConnell JR, Schupbach S, Maselli OJ, Marlon J, Vallelonga P, Leuenberger D, Zangrando R, Spolaor A, Borrotti M, Barbaro E, Gambaro A, Barbante C (2014) Fire in ice: two millennia of boreal forest fire history from the Greenland NEEM ice core. Clim Past 10:1905–1924

Chapter 4

Spatio–Temporal Variations of Levoglucosan on Tibetan Glaciers

In this section, levoglucosan was detected in snow and ice samples from different Tibetan glaciers. Tibetan glaciers mainly acted as receptors of fire emissions from surrounding regions. Levoglucosan concentrations showed a roughly decreasing trend from west to east on glaciers on the southern edge of the Tibetan Plateau, while an opposite pattern was observed on glaciers along the northern edge. Significant differences of levoglucosan concentrations from two slopes on the same mountain range indicate that high mountains can act as natural barriers to block the transport of smoke aerosols to the Tibetan Plateau. The highest levoglucosan concentration was detected at an elevation near the equilibrium line altitude on two Tibetan Plateau glaciers. The post-depositional processes on the glacier surface determined the distribution patterns of levoglucosan concentrations at different altitudes. Intense wildfires can lead to extremely high concentrations (e.g. higher than 25 ng mL−1 ) of black carbon in ice near the surface of Tibetan glaciers and can therefore play an important role in glacial melt during the pre-monsoon season.

4.1 Background Information Fire emissions are one of the main sources of atmospheric aerosols [46, 53], contributing over two-thirds of the total carbonaceous aerosols in regions with intense fire sources (e.g., the Indian peninsula or South Asia) [14, 29, 46]. Fire emissions are considered to be a major cause of the South Asia Brown Clouds over the Indian peninsula [3, 14]. Smoke plumes from intense fire events can be transported to the upper troposphere and lower stratosphere by strong thermal convection processes [44, 45, 61], where they can potentially be transported to remote high elevation regions like the Tibetan Plateau [3, 61].

© Springer Nature Singapore Pte Ltd. 2021 C. You, Geochemical Behavior of Levoglucosan in Tibetan Plateau Glacier Snow and Ice, Springer Theses, https://doi.org/10.1007/978-981-15-7973-8_4

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4 Spatio–Temporal Variations of Levoglucosan …

Fig. 4.1 Spatial distribution of fire counts (2003–2012) and atmospheric circulations around the Tibetan Plateau and its surrounding regions

The main body of the Tibetan Plateau is enclosed by high-elevation mountains (e.g., the Himalayas to the south and the Kunlun to the north) [67], which can substantially interrupt the transport of atmospheric pollutants from surrounding regions [19]. However, the Tibetan Plateau is located very close to South and Southeast Asia (Fig. 4.1) which are considered as some of the world’s most intense fire sources [1, 43, 46]. Fire emissions could easily be transported to the high-elevation marginal regions of the Tibetan Plateau such as the Himalayas [5, 61, 84]. Smoke aerosols released from intense fire events could be even transported up and over the Himalayas [55, 62] and penetrate central Tibetan Plateau regions such as the Namco Lake Basin [33, 55] and the Qinghai Lake Basin [18]. Carbonaceous aerosols can significantly reduce snow albedo on glacier surfaces [6, 57, 60] and are considered as important contributors to the accelerating melt of Tibetan glaciers, besides rising temperature, in recent years [19, 25, 28, 57, 60, 68]. Evidence indicates that carbonaceous aerosols deposited on Tibetan glaciers are mainly from fossil fuel emissions [25, 60]. Other studies found that Tibetan glaciers were contaminated by fire smoke aerosols at different levels [25, 26]. As a matter of fact, more than two-thirds of the carbonaceous aerosols over the Indian peninsula were from fire emissions [14]. Wildfires (e.g., forest and grassland fires) mainly occurred during the pre-monsoon season, while crop residue burning peaked from late September to November over the northern Indian peninsula and Southeast Asia

4.1 Background Information

25

[43–45]. Fire smoke plumes can even be uplifted to the upper troposphere and lower stratosphere under thermal dynamic processes [44, 62], subsequently intruding into high elevation Tibetan Plateau regions through mountain valleys [5, 19, 61]. Levoglucosan can only be generated by combustion of cellulose and hemicellulose when the combustion temperature is higher than 300 °C [35, 37, 41, 72]. The atmospheric life time of levoglucosan varies from several hours to more than ten days under different conditions, in particular depending on the concentrations of the hydroxyl radical [15, 16, 30]. Experimental studies have reported that levoglucosan can react with hydroxyl radical when exposed in conditions of high relative humidity conditions [15], but it can normally remain stable for several days under most atmospheric conditions during the transport process [35, 37, 41, 72]. Studies have also reported that levoglucosan displays almost negligible degradation in snow and ice layers when under frozen deposition conditions [10, 72, 74]. Therefore, it was suggested that levoglucosan could be used as an ideal tracer for fire emissions in snow and ice records from polar and high-altitude sites [72, 74]. High levoglucosan concentrations were detected in aerosol samples at sites in the marginal regions of the Tibetan Plateau [5]. However, knowledge of the spatial distribution of fire aerosols on Tibetan glaciers is still unclear due to limited observations. Besides, factors which impact the transport and post-depositional processes of levoglucosan on Tibetan glaciers are poorly understood due to limited observations. In this section, the spatial distribution of levoglucosan is investigated using snow and ice samples from glaciers in different Tibetan Plateau regions. Based on these results, the sources and process of fire aerosols transported to Tibetan glaciers are discussed.

4.2 Materials and Methods Snow and ice samples were collected from Yala Glacier, Dasuopu Glacier, Zuoqiupu Glacier, Demula Glacier, Cuopugou Glacier on the southern Tibetan Plateau; the Muji Glacier, Kuokuosele Glacier, and Qiyi Glacier on the northern Tibetan Plateau, within different climate control regions of the Tibetan Plateau (Fig. 4.2). The polyethylene terephthalate (PET) bottles were washed twice by soaking in ultrapure water for at least 24 h and rinsing three times with ultrapure water. These were dried in the shade in a clean room [71]. Disposable polyethylene gloves were used for snow sample collections [71]. Samples were stored in pre-cleaned PET bottles and were kept frozen at a temperature of −20 °C before chemical analysis. Some levoglucosan data reported in other glaciers including Zangsegangri Glacier [76], Yulong Glacier, Gurenhekou Glacier, Dongkemadi Glacier, Yuzhufeng Glacier, Meikuang Glacier, Muztagh Glacier, and Laohugou Glacier are collected to achieve a comprehensive evaluation ([26] and Fig. 4.2). Moderate resolution imaging spectroradiometer (MODIS) fire products (MODIS V6 data retrieved from http://modisfire.umd.edu/, onboard National Aeronautics and Space Administration’s Terra and Aqua satellites, Giglio et al. [11]) were used to

26

4 Spatio–Temporal Variations of Levoglucosan …

Fig. 4.2 Spatial distribution regimes of levoglucosan concentrations in Tibetan glacier snow and ice samples, data were from Refs. [26, 73, 75–77]

confirm fire regimes across the Tibetan Plateau and surroundings. Although fire counts differ between Terra and Aqua, detected fire variations are broadly consistent [11]. Fire spots with confidences lower than 50% were eliminated (12.75% of Terra and 13.88% of Aqua) in order to improve reliability [73, 76]. Wind fields at 500 hPa were obtained using the ERA-Interim monthly mean reanalysis data with 0.75° by 0.75° spatial resolution. These reanalysis data are products of the European Centre for Medium-Range Weather Forecasts (ECMWF). Tropical Rainfall Measuring Mission (TRMM) 3B43 (V7) data, with monthly time resolution and 0.25° by 0.25° spatial resolution, were used to investigate rainfall spatial features. ERA Interim and TRMM data were analyzed from March 2012 to February 2015. The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) of aerosol type and water phase were obtained from the Web site http://www-calipso.larc.nasa.gov.

4.3 Levoglucosan in Tibetan Plateau Glaciers Levoglucosan concentrations range from below LOD to 26.67 ng mL−1 in Tibetan glacier snow and ice samples. Apparent regional differences of levoglucosan concentrations were observed over Tibetan glaciers (Fig. 4.2). The average levoglucosan concentration in glaciers on the northern Tibetan Plateau (>35° N) was 1.06 ng mL−1 , while that on the southern Tibetan Plateau (≤35° N) was 4.43 ng mL−1 (or

4.3 Levoglucosan in Tibetan Plateau Glaciers

27

2.21 ng mL−1 if excluding Yala Glacier). Levoglucosan concentrations revealed a roughly decreasing trend from the central Himalayas to the southeastern Tibetan Plateau on the southern edge, but displayed a roughly increasing trend from the western Pamir Plateau to eastern Qilian Mountains (Fig. 4.2). Central Tibetan glaciers (including Zangsegangri, Dongkemadi, Meikuang, and Yuzhufeng) were with levoglucosan concentrations lower than 1 ng mL−1 . The highest levoglucosan concentration was detected in samples from the Yala Glacier, which is located on the southern side of Mt. Xixiabangma (Fig. 4.2). Here, levoglucosan concentration reached 26.66 ng mL−1 based on four samples collected at around 5400 m a.s.l. This concentration is significantly higher than those in samples from the other glaciers. Levoglucosan concentration at Yala was much higher than the highest level in the Kamchatka ice core [21] and was even comparable to seriously contaminated winter snowfall samples in urban Beijing [70]. This result indicates that glaciers on the southern side of the Himalayas are seriously contaminated by fire emissions even at an altitude higher than 5400 m. In contrast, the average levoglucosan concentration was 0.31 ng mL−1 in snow samples from the Dasuopu Glacier on the northern side of Mt. Xixiabangma in the central Himalayas. Significant differences of levoglucosan concentrations were also observed between samples from the Zuoqiupu and Demula glaciers, situated on different sides of the same mountain in the southeastern Tibetan Plateau (Fig. 4.2). Levoglucosan concentration was 0.94 ng mL−1 at Cuopugou Glacier and 2.41 ng mL−1 at Yulong Glacier on the eastern edge of the Tibetan Plateau. On the northern Tibetan Plateau, levoglucosan concentrations were detected in the Muji (1.13 ng mL−1 ) and Kuoluosele (1.97 ng mL−1 ) glaciers on the eastern side of the Pamir Plateau, about 0.75 ng mL−1 at Muztagh Glacier. The highest levoglucosan concentration was found on Qiyi Glacier (2.56 ng mL−1 ) in the Qilian Mountain on the northeastern Tibetan Plateau. Levoglucosan concentrations in Tibetan glacier samples are significantly higher than those in samples collected from the Arctic [22, 81, 82] and Antarctic [2, 34], but are comparable to samples obtained from the Kamchatka peninsula [21, 32] and European Alps [24] ice cores (Fig. 4.3). The Arctic and Antarctic samples are mainly contaminated by long-range transport smoke aerosols and lack of local emissions [2, 17, 22, 38, 79–82]. It usually takes several days for smoke plumes to reach the remote polar region sites [17, 38, 79, 80]. Levoglucosan that adheres to the aerosol particles could be potentially deposited by gravitational settling during the long-distance transportation process [17]. Besides, wet deposition can effectively wash out levoglucosan [17], and photochemical degradation can consume some of the levoglucosan during the transport process [7], resulting in the extreme low concentration in snow and ice samples from the Arctic and Antarctic. Unlike the polar regions, the Tibetan Plateau [73–77], the Kamchatka peninsula [21, 32], and the European Alps [24] are located directly downwind of intense fire source regions. Stronger fire emission sources and shorter transport distances together caused much higher levoglucosan concentrations in snow and ice samples from the Tibetan glaciers when compared with high-latitude regions.

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4 Spatio–Temporal Variations of Levoglucosan …

Fig. 4.3 Levoglucosan concentrations in glacier snow and ice worldwide. The short lines show the maximum and minimum; the cross shows the 99th and 1st percentiles; the box shows the first and third quartiles; the square indicates the arithmetic mean. Data were from Refs. [10, 21, 22, 32, 72–77, 81, 82]

4.4 Impact Factors for Spatial Distribution of Levoglucosan Atmospheric environment over high-altitude Tibetan glaciers is hardly affected by local sources [28, 57, 73], and surrounding emissions usually act as major sources for pollutants deposited on Tibetan glaciers [72, 73]. Levoglucosan concentrations in central Tibetan Plateau are much lower than those in boundary regions, which possibly indicates the strong blocking role of the extreme high mountains (like Himalayas) around the Tibetan Plateau. The spatial distribution of levoglucosan concentrations over Tibetan glaciers is apparently different from those of anthropogenic pollutants such as carbonaceous aerosols [25, 28, 57, 60] and polycyclic aromatic hydrocarbons (PAHs) [51, 52]. As indicated in previous studies, the carbonaceous aerosols and PAHs over Tibetan glaciers generally displayed increasing trends from the marginal region to the inner part of the Tibetan Plateau [51, 52, 59]. Results of our study indicate that Tibetan glaciers mainly act as receptors of fire emissions from surrounding regions, while the contribution from sources within the inner Tibetan Plateau might be limited. The southern edge of the Tibetan Plateau is located adjacent to South and Southeast Asia, which are regions often characterized by intense fire emissions [43–45]. Wildfires mainly occur from March to May in the pre-monsoon season, and crop residue burning peaks from late September to early November in the post-monsoon season over the northern Indian peninsula [45]. Fire is rare during the monsoon season possibly due to intense precipitation. Therefore, levoglucosan deposited on

4.4 Impact Factors for Spatial Distribution of Levoglucosan

29

glaciers along the southern edge of Tibetan Plateau was mainly sourced during the non-monsoon seasons. Results based on MODIS observations reveal that fires are concentrated during the pre-monsoon season along the southern side of Tibetan Plateau [44, 45, 62]. The Yala and Zuoqiupu glaciers are both located along the southern side of the Himalayas, and fire emissions are observed much stronger in upwind regions of Zuoqiupu Glacier (Fig. 4.4 region e) than those of Yala Glacier (Fig. 4.4 region d). However, detected levoglucosan concentration in Yala Glacier was about 20 times higher than that in Zuoqiupu Glacier. Considering that the Yala and Zuoqiupu glaciers are both dominated by the southern branch of the westerly winds during the nonmonsoon season (Figs. 4.1 and 4.4), deposition during the transport process is thought to be a controlling factor in the significant differences in levoglucosan concentrations between these two glaciers. Elsewhere, great differences in levoglucosan concentrations were observed between the northern and southern slopes of the Himalayas (Fig. 4.2). The magnitude of fire counts during the post-monsoon in region d over the northern Indian peninsula was comparable to that of the pre-monsoon season during 2012 (Fig. 4.4 region d and e), which suggests that the marked difference in levoglucosan concentrations between the two sides of the Himalayas was not primarily caused by the strength of fire sources. The highest levoglucosan concentration (1.56 ng mL−1 ) in the Dasuopu Glacier snow samples was observed at an altitude of 6392 m a.s.l, while other lower-altitude sampling sites are with lower levoglucosan concentrations. It is notable that there were almost no fire emissions in regions on the northern side of the Himalayas according to the fire data (Fig. 4.4). Therefore, smoke emissions from

Fig. 4.4 Seasonal variation of fire counts for different regions, a for region 20–50° N, 50–70° E, b for region 35–50°N, 70–90° E, c for region 35–50° N, 90–110° E, d for region 20–35° N, 70– 85° E, e for region 20–35° N, 85–100° E, and f for region 20–35°N, 100–110° E. The black dotted line represents the monthly fire emissions of 2012, and the red dotted line represents the monthly fire emissions of 2014

30

4 Spatio–Temporal Variations of Levoglucosan …

Fig. 4.5 a CALIPSO retrieved aerosol type and b water phase information on May 15th, 2012. It is noticeable that great differences are observed in smoke aerosol levels on the two sides of the Himalayas near the Yala and Dasuopu glaciers. Large smoke plumes are transported to, and accumulated on, the southern side of the Himalayas; this transport and accumulation decreases sharply on the northern side. Figures are downloaded from http://www-calipso.larc.nasa.gov

strong fire events over the northern Indian peninsula can evidently penetrate into the upper troposphere (Fig. 4.5a, and Ref. [62]) and can then be directly transported and deposited at these high altitudes over the Himalayan region (Fig. 4.5a). Furthermore, pollutants can be directly transported to high-altitude Tibetan glaciers by the valley breezes [3, 5]. Snowfall and the drifting snow can effectively clear the smoke aerosols carried by the air mass when passing over the glacier surface [38]. These processes can also scavenge the particle-bound water-soluble organic components including levoglucosan [17, 73]. However, the highest levoglucosan concentration on Dasuopu Glacier was still significantly lower than those in samples from Yala Glacier on the southern side of the Himalayas. The CALIPSO observation (http://www-calipso.larc.nasa.gov) results revealed that large amounts of water droplets were concentrated on the windward side of the Himalayas over regions near the Yala Glacier during the sampling time (Fig. 4.5b), ultimately causing the high wet deposition rate of levoglucosan on the windward side. High wet deposition rate can reduce levoglucosan remained in the air masses [17], leading to less levoglucosan transported to the leeward side. On the other hand, large amount of water

4.4 Impact Factors for Spatial Distribution of Levoglucosan

31

droplets was also observed on the leeward side of the Himalayas near the Dasuopu Glacier (Fig. 4.5b), which could possibly result in low levoglucosan concentrations on Dasuopu Glacier surfaces due to the dilution of abundant precipitation. Results suggested that the high mountain peaks along the Himalayas, located on the southern edge of the Tibetan Plateau, can act as a natural barrier for the transport of smoke particles from South Asia to the main body of the Tibetan Plateau. Levoglucosan concentrations in the Cuopugou Glacier and Yulong Glacier on the eastern Tibetan Plateau were much lower than those in glaciers along the Himalayas. Cuopugou Glacier is located further away from the northern Indian peninsula fire emission sources than the Himalayan glaciers (Fig. 4.4a). In fact, a high spring precipitation rate was observed at meteorological stations on the windward direction of the Yarlung Zangbo Grand Canyon (Fig. 4.6 and Ref. [65]), which can effectively wash out atmospheric water-soluble organic compounds such as levoglucosan. Meanwhile, smoke aerosols transported by the southern branch of the westerlies during the premonsoon season might be rapidly deposited by the blocking effect of the Himalayas and the parallel Hengduan Mountains before reaching to the eastern Tibetan Plateau [84]. Smoke aerosols transported by the northern branch of the westerlies could be obstructed by the extreme high mountains like Kunlun Mountains along the air mass pathways. On the other hand, levoglucosan deposited on Cuopugou Glacier

Fig. 4.6 Seasonal precipitation regimes and wind field at 500 hPa for the Tibetan Plateau and surrounding regions from 2012 to 2014: a spring (March to May), b summer (June to August), c autumn (September to November), and d winter (December to February)

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4 Spatio–Temporal Variations of Levoglucosan …

could be potentially affected by the East Asia summer monsoon, besides the westerlies (Fig. 4.1). Wildfire and agriculture residue burning in Yunnan province and Myanmar may also act as potential sources (Figs. 4.1 and 4.4). Levoglucosan concentrations on Yulong Glacier was higher than the Cuopugou Glacier. High fire aerosols have been reported at the Tengchong site in Yunnan province [31], but the physical blocking effect of the Hengduan Mountains and the strong wet deposition process during transport would hamper levoglucosan transport to Cuopugou Glacier. On the contrary, the prevailing wind direction is from west to east over the northern edge of the Tibetan Plateau during the entire year (Fig. 4.6). Previous studies have noted that fire emissions from central Asia and the Middle East, and sometimes even from regions further away, can be transported and deposited on glaciers distributed around the Pamir Plateau. High fire levels were also observed from June to September in 2012 over the windward regions mentioned above (Fig. 4.4b). However, the high mountains around the Pamir Plateau (Fig. 4.1) on the northwest Tibetan Plateau can block most of the smoke particles, as well as particle-bound levoglucosan carried by the westerlies from source regions in the windward direction. Consequently, relatively lower levoglucosan concentrations were observed at Muji and Kuoluosele glaciers on the eastern side of the Pamir Plateau. On the other hand, previous studies reported that firewood and crop residue burning was prevalent in southern Xinjiang and northern Gansu provinces [4, 40] along the air mass trajectories. High levoglucosan concentrations were reported in aerosol samples from the Hetian and Tazhong station, two remote sites in the Taklimakan desert, China [8]. As a result, levoglucosan deposited along the westerly pathway reached a maximum from the Muji and Kuokuosele glaciers in the west to the Qiyi Glacier in the east. Local fire emissions from regions such as the southern Xinjiang and northern Gansu provinces may act as considerable sources and may explain the much higher levoglucosan concentration in the northern Tibetan Plateau than in the southern Tibetan Plateau. It is notable that lower fire emissions coupled with much higher levoglucosan concentrations on the northern Tibetan Plateau were observed when compared with the southern Tibetan Plateau (except Yala Glacier). Higher fire emissions were observed in regions d, e, and f adjacent to the southern Tibetan Plateau (Fig. 4.4), but much lower levoglucosan concentrations were detected in glaciers distributed nearby (Fig. 4.2). The discordance between the spatial distributions of fire emissions and levoglucosan concentrations was possibly caused by the different deposition and degradation levels of levoglucosan during transport. The southern Tibetan Plateau is impacted by the Indian summer monsoon and usually experiences greater precipitation [56, 61] and higher relative humidity [64, 78] than the northern Tibetan Plateau during fire seasons. Levoglucosan is a highly hydrophilic compound [35, 37], and therefore wet deposition processes can effectively scavenge hygroscopic smoke particles in the atmosphere [27], as well as the particle-bound levoglucosan [17, 72]. Therefore, the scavenging process can effectively reduce the amount of levoglucosan remaining in the atmosphere for long-distance transport toward the southern Tibetan Plateau. High snowfall rates over the southern Tibetan Plateau glaciers [57, 64, 65, 73] can also dilute the levoglucosan concentrations to some extent. Higher relative humidity could lead to a much higher degradation rate of

4.4 Impact Factors for Spatial Distribution of Levoglucosan

33

levoglucosan during the transportation process [15, 17] over the southern edge of the Tibetan Plateau, potentially reducing the lifetime of levoglucosan under atmospheric conditions [15, 16, 30]. As a result, airborne levoglucosan concentrations would have decreased, thereby reducing the deposition of levoglucosan on glaciers in leeward directions. Due to high relative humidity conditions [78], the photochemical reactions can be possibly responsible for the differences of levoglucosan on southern and northern sides of Himalayas. However, the degradation rate cannot be quantitatively calculated using the limited data in this study, and it will be addressed in our future work. On the contrary, dry deposition of smoke aerosols is more prevalent over the northern Tibetan Plateau due to that region’s lower precipitation [56] and lower relative humidity [78], and levoglucosan can thus be transported further away from the emission sources.

4.5 Levoglucosan at Different Elevations The injection height is an important factor for understanding the environmental effect of fire emissions [23, 47, 48]. Smoke aerosols can be transported far away from the emission sources only when the aerosols can penetrate through the atmospheric boundary layers, and the meteorological conditions are conducive to the spread of aerosols. The atmospheric conditions over high-altitude Tibetan glaciers are such that this area is mainly affected by long-range transport pollutants, while local emissions only play weak roles [28, 57, 73]. However, at present there is almost no direct evidence for the distribution patterns of fire aerosols at different altitudes on Tibetan glaciers. To address this, surface snow samples were collected on Zuoqiupu Glacier and Muji Glacier in this study (Fig. 4.7). The highest levoglucosan concentrations were detected in the surface snow sample collected at an elevation of 5412 m a.s.l. on the Zuoqiupu Glacier [75] and at an elevation of 5291 m a.s.l. on the Muji Glacier [77]. Levoglucosan concentrations decreased sharply above and below these elevations (Fig. 4.7). The elevations of the levoglucosan concentration maxima correspond roughly to that of the equilibrium line altitude of Tibetan glaciers. Our previous studies have reported that the equilibrium line altitude of the Parlung NO.4 Glacier (located at 29.21° N and 96.92° E, on the leeward site of the same mountain of the Zuoqiupu Glacier) varies from about 5340 to 5450 m [64]. Interestingly, the elevation of the levoglucosan concentration maximum was also very close to that of the black carbon concentration maximum observed during the post-monsoon season on a southeastern Tibetan Plateau glacier adjacent to the Zuoqiupu Glacier [57]. Previous studies found black carbon concentrations reaching about 20 ng mL−1 at a height of 5400 m a.s.l. on the glacier surface during the post-monsoon period, due to the melting and refreezing processes in surface snow [57]. Melting and refreezing processes occur alternately in the accumulation areas of Himalayan glaciers during the melt season [9] and could therefore redistribute levoglucosan on glacier surface after deposition.

34

4 Spatio–Temporal Variations of Levoglucosan …

Fig. 4.7 Levoglucosan variations at different altitudes on Zuoqiupu Glacier and Muji Glacier

Considering that levoglucosan is a strongly water-soluble organic component [36], it could be washed out by meltwater during the summer, resulting in very low levoglucosan concentrations across the ablation area after each melt season. In contrast, the melting and refreezing processes in the accumulation area may concentrate levoglucosan in the glacier surface. The melting and refreezing rates are similar (almost equally) around the equilibrium line altitude, which could lead to the observed peak in levoglucosan concentration. Noticeably, more theatrical changes of levoglucosan around the equilibrium line altitude were observed on the Zuoqiupu Glacier compared with the Muji Glacier (Fig. 4.7), and this may be also possibly caused by the different melting rate on those two glaciers [69]. Actually, strong leaching rate was reported on southeastern Tibetan glaciers during summertime due to intense melt [64–66]. In comparison, observed air temperature on the Muji Glacier was even lower than 0 °C during the summertime [63], thus leading to lower melting rate. As a result, low glacial melt leaded to low leaching rate and finally displayed a wide distribution range of levoglucosan around the equilibrium line altitude on the Muji Glacier. The elevation of the levoglucosan concentration maximum is also consistent with the vertical distribution of smoke aerosols over the Tibetan Plateau surrounding regions. Smoke aerosols from pre-monsoon wildfires were reported with an average injection altitude of about 5.35 km [44]. High concentrations of smoke and polluted dust aerosols (generally mixed dust and smoke) have been reported during the nonmonsoon season, especially during the pre-monsoon and wintertime [62]. Smoke aerosols are concentrated within a layer reaching about 5–6 km altitude during the pre-monsoon season, falling to below 4 km during the post-monsoon season

4.5 Levoglucosan at Different Elevations

35

and wintertime. Smoke aerosol loadings decrease sharply at elevations higher than the maximum injection height [52, 75], perhaps causing the low levoglucosan concentrations above that height on Tibetan glaciers.

4.6 Temporal Variations in Levoglucosan and Black Carbon Carbonaceous aerosols are mainly transported to the southeastern Tibetan Plateau regions during the pre-monsoon [83] and can lead to the high black carbon depositions on southeastern Tibetan glaciers [50, 57, 60]. Levoglucosan variations were analyzed in samples from the Zuoqiupu snow-pit and the Cuopugou ice core to reveal the temporal variations of fire records on southeastern Tibetan glaciers. Our Zuoqiupu snow-pit samples may reveal an annual fire emission record. Higher black carbon concentrations during the non-monsoon seasons than those in the monsoon season in Zuoqiupu ice core have been reported previously [50, 60]. Average levoglucosan concentrations in Zuoqiupu snow-pit samples were 2.45 ng mL−1 during the pre-monsoon season, which is about three times that of the monsoon and about two times that of the post-monsoon seasons. Meanwhile, levoglucosan and black carbon displayed similar variations in the Zuoqiupu snow-pit samples (Fig. 4.8).

Fig. 4.8 Temporal variations of levoglucosan and black carbon concentrations in the Zuoqiupu snow-pit samples

36

4 Spatio–Temporal Variations of Levoglucosan …

These results support our assertion that fire-related aerosols deposited on southeastern Tibetan glaciers are mainly during the non-monsoon seasons, especially the pre-monsoon. Levoglucosan levels displayed similar patterns to those of black carbon in both Zuoqiupu snow-pit and the Cuopugou ice core samples (Figs. 4.8 and 4.9). However, these two proxies were poorly correlated with each other (Spearman’s correlation coefficient r = 0.126, p > 0.10 for the Zuoqiupu snow-pit samples; and r = 0.257, p > 0.05 for the Cuopugou ice core samples). These differences may indicate that levoglucosan and black carbon deposited on southeastern Tibetan glaciers do not always share the same sources. Although fire emission was considered to be a major contributor for black carbon aerosols over the northern Indian peninsula [14, 49], geochemistry results showed that black carbon deposited on Tibetan glaciers is mainly derived from fossil fuel emissions [29]. The different chemical behaviors of levoglucosan and black carbon during the transport process (e.g., different

Fig. 4.9 Visual stratigraphy of ice core characteristics and variation of levoglucosan and black carbon concentrations in the Cuopugou ice core

4.6 Temporal Variations in Levoglucosan and Black Carbon

37

scavenging rates and photochemical reactions) might also be responsible for the differences between levoglucosan and black carbon in Tibetan glaciers. Furthermore, levoglucosan can only be generated from fire emissions [35, 37], and only emissions from strong fire events can be transported far away from source regions [23, 47, 48]. Continuous use of fossil fuels throughout the year in northern Indian peninsula [14, 39] could potentially act as a stable emission source for black carbon deposited on southeastern Tibetan glaciers. Therefore, the different emission sources and chemical behaviors in the atmosphere lead to a relatively stable pattern of black carbon in the glacier snow and ice layers, when compared to levoglucosan concentrations that are mainly influenced by inputs from individual fire events (Fig. 4.9). It is notable that levoglucosan and black carbon both peaked at a depth of 90– 135 cm in the Cuopugou ice core (Fig. 4.9), possibly indicating the impact of strong fire events. Previous studies found that dust was detected mainly during the spring in air samples from the northern Indian peninsula [62]. Dust storm events occur most frequently in winter and spring over the southern Tibetan Plateau regions [54, 62], and dust layers are mainly formed in the pre-monsoon seasons in Tibetan glaciers [20, 42, 54]. Levoglucosan and black carbon maxima both appeared near the dust layers, possibly indicating significantly concentrated levoglucosan and black carbon during the pre-monsoon season. However, differences are observed between the levoglucosan and black carbon maxima (Fig. 4.9). The black carbon concentration maxima lags behind the levoglucosan concentration maxima, indicating asynchronous post-depositional processes affecting levoglucosan and black carbon. The post-depositional melt and refreeze processes may redistribute black carbon and organic carbon on Tibetan glacier surfaces [25, 60, 63]. Generally, enrichment of black carbon and organic carbon on glacier surfaces occurs in the accumulation area during the melt season [58]. Unlike the insoluble particles (e.g., black carbon and dust), levoglucosan is strongly water soluble [36, 37, 72], and may be easily leached during the melt–refreeze process. Previous studies found that soluble species had a greater affinity for meltwater than for ice and partition preferentially into the liquid phase [12, 13]. Consequently, they are removed at the beginning of the leaching process [12, 13, 75]. Therefore, the leaching process can reshape the postdepositional distribution of levoglucosan concentration in snow/ice layers on the glacier surface. This assumption is supported by the synchronous appearance of the levoglucosan maxima and firn/ice layers in the Cuopugou ice core (Fig. 4.9). Firn and ice layers are formed in the accumulation area of southeastern Tibetan glaciers by the melt and refreeze processes during melt season. Snow particle size increased during the melting and refreezing processes [9, 65]. Spaces between snow particles can act as the major pathways for meltwater, as well as for pollutants carried by meltwater [58]. However, when the meltwater reaches a depth where the temperature is lower than the freezing point, the meltwater refreezes and forms an ice layer [9]. Void spaces are reduced during the freezing process and become closed after the formation of ice layers, potentially preventing any further infiltration of meltwater and pollutants. The superimposed ice under the snow pack can prevent further infiltration of the meltwater and black carbon [58, 75].

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Annual precipitation amount ranges from 497 to 765 mm, with an average of about 630 mm at meteorological stations near Cuopugou Glacier (Fig. 4.10; Table 4.1). Considering the much greater precipitation rates at high-altitude glaciers around southeastern Tibetan Plateau [64–66], we therefore estimate that the annual accumulation rate is about 1000 mm on Cuopugou Glacier. The upper 145 cm above the first ice layer in the Cuopugou ice core could thus have been formed within one year (Fig. 4.9). The highest levoglucosan concentrations were observed in the dust layers (average of 2.79 ng mL−1 ), followed by the course firn layers (average of 0.98 ng mL−1 ), then the ice layers (average of 0.89 ng mL−1 ). The lowest levoglucosan concentrations were observed in the upper 10 cm (average of 0.54 ng mL−1 ), which are characterized by fine-grained firn after slight aging of fresh snow (Fig. 4.9). The

Fig. 4.10 Meteorological stations for regions around the Cuopugou glacier (by Google Earth)

Table 4.1 Annual precipitation amount from meteorological stations as in Fig. 4.10 near the Cuopugou Glacier (1981–2010), data were obtained from the China Meteorological Administration Meteorological Information Center (CMA, http://data.cma.cn) Station name

Latitude (° N)

Longitude (° E)

Altitude (m)

Precipitation (mm)

Batang

30

99.06

2589

497

Litang

30

100.16

3949

765

Xinlong

30.56

100.19

3000

636

Baiyu

31.13

98.5

3260

627

Ganzi

31.37

100

3394

646

4.6 Temporal Variations in Levoglucosan and Black Carbon

39

meltwater can therefore remove levoglucosan from the glacier surface, from where it becomes concentrated in layers close to the superimposed ice. Similar process by which solute concentrations are enhanced for soluble organic molecules has been previously reported [12, 13]. However, further details regarding the redistribution of levoglucosan during the post-depositional processes on the glacier surface cannot be revealed in this work due to lack of observations. This will be addressed in future studies. Although our results indicated that levoglucosan can be redistributed by the melting and refreezing processes on the glacier surface, it does remain as a proxy that can represent the primary characteristics of fire-related aerosols deposited in the accumulation area at seasonal time scales over the southeastern Tibetan glaciers. Our results demonstrated that strong fire events could lead to a substantial increase in black carbon concentration on the southeastern Tibetan glaciers to some extent (Figs. 4.8 and 4.9). Averaged black carbon concentration in the pre-monsoon season is about seven times that of the monsoon seasons in the Zuoqiupu snow-pit samples (Fig. 4.8). The black carbon concentrations from 90 to 135 cm are about three times that of the concentration in the total upper 3.5 m in the Cuopugou ice core, and the highest black carbon concentration even reached 25.24 ng mL−1 (Fig. 4.9). Such a high black carbon concentration can significantly reduce the snow albedo on the glacier surface. Studies found that black carbon concentration in the pre-monsoon season was a key factor affecting glacier mass balance in the southeastern Tibetan Plateau region [50, 57, 60]. Black carbon concentrations exceeding 10 ng mL−1 can increase the absorption of visible radiation, depending on the size and shape of snow crystals [6, 57]. Our results demonstrate that fire emissions can significantly contribute to black carbon deposited on southeastern Tibetan glaciers and may play an important role in glacier melting during the pre-monsoon season.

4.7 Summary This section provides the spatial–temporal variations of levoglucosan concentrations on Tibetan glaciers. Tibetan glaciers are contaminated by fire emissions more seriously than polar and high-latitude regions, and mainly act as receptors of fire emissions from surrounding regions. High mountains can act as natural barriers to block the transport of smoke aerosols to the Tibetan Plateau. The emission sources, the controlling climate system, as well as deposition and degradation during transport determined a roughly decreasing trend of levoglucosan from west to east on glaciers on the southern edge of the Tibetan Plateau, but an increasing trend was observed on glaciers along the northern edge. The highest levoglucosan was observed in sample collected near the equilibrium line altitude. Similarities in seasonal variations of levoglucosan and black carbon concentrations indicated that fire aerosols on southeastern Tibetan glaciers were mainly deposited during the pre-monsoon seasons. The post-depositional melting and refreezing processes in the accumulation area reshaped the temporal distribution patterns of levoglucosan and black carbon. Strong fire events can lead to extremely high black carbon loading (>25 ng mL-1 ) on southeastern

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Tibetan Plateau glacier surfaces, consequently impacting on glacier mass balance during the pre-monsoon season. Knowledge of this section will be helpful for better understanding geochemical behavior of levoglucosan and its climatic significance on Tibetan glaciers.

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35. Simoneit BRT (2002) Biomass burning—a review of organic tracers for smoke from incomplete combustion. Appl Geochem 17:129–162 36. Simoneit BRT, Elias VO, Kobayashi M, Kawamura K, Rushdi AI, Medeiros PM, Rogge WF, Didyk BM (2004) Sugars—Dominant water-soluble organic compounds in soils and characterization as tracers in atmospheric particulate matter. Environ Sci Technol 38:5939–5949 37. Simoneit BRT, Schauer JJ, Nolte CG, Oros DR, Elias VO, Fraser MP, Rogge WF, Cass GR (1999) Levoglucosan, a tracer for cellulose in biomass burning and atmospheric particles. Atmos Environ 33:173–182 38. Stohl A, Berg T, Burkhart JF, Fjaeraa AM, Forster C, Herber A, Hov O, Lunder C, McMillan WW, Oltmans S, Shiobara M, Simpson D, Solberg S, Stebel K, Strom J, Torseth K, Treffeisen R, Virkkunen K, Yttri KE (2007) Arctic smoke—record high air pollution levels in the European Arctic due to agricultural fires in Eastern Europe in spring 2006. Atmos Chem Phys 7:511–534 39. Streets DG, Bond TC, Carmichael GR, Fernandes SD, Fu Q, He D, Klimont Z, Nelson SM, Tsai NY, Wang MQ, Woo JH, Yarber KF (2003a) An inventory of gaseous and primary aerosol emissions in Asia in the year 2000. J Geophys Res-Atmos 108 40. Streets DG, Yarber KF, Woo JH, Carmichael GR (2003b) Biomass burning in Asia: annual and seasonal estimates and atmospheric emissions. Global Biogeochem Cycles 17 41. Suciu LG, Masiello CA, Griffin RJ (2019) Anhydrosugars as tracers in the Earth system. Biogeochemistry 42. Thompson LG, Yao T, Mosley-Thompson E, Davis ME, Henderson KA, Lin PN (2000) A highresolution millennial record of the South Asian Monsoon from Himalayan ice cores. Science 289:1916–1919 43. Vadrevu K, Ohara T, Justice C (2017) Land cover, land use changes and air pollution in Asia: a synthesis. Environ Res Lett 12:120201 44. Vadrevu KP, Ellicott E, Giglio L, Badarinath KVS, Vermote E, Justice C, Lau WKM (2012) Vegetation fires in the Himalayan region—aerosol load, black carbon emissions and smoke plume heights. Atmos Environ 47:241–251 45. Vadrevu KP, Giglio L, Justice C (2013) Satellite based analysis of fire–carbon monoxide relationships from forest and agricultural residue burning (2003–2011). Atmos Environ 64:179–191 46. van der Werf GR, Randerson JT, Giglio L, van Leeuwen TT, Chen Y, Rogers BM, Mu M, van Marle MJE, Morton DC, Collatz GJ, Yokelson RJ, Kasibhatla PS (2017) Global fire emissions estimates during 1997–2016. Earth Syst Sci Data 9:697–720 47. Veira A, Kloster S, Schutgens NAJ, Kaiser JW (2015) Fire emission heights in the climate system—part 2: impact on transport, black carbon concentrations and radiation. Atmos Chem Phys 15:7173–7193 48. Veira A, Kloster S, Wilkenskjeld S, Remy S (2015) Fire emission heights in the climate system—part 1: global plume height patterns simulated by ECHAM6-HAM2. Atmos Chem Phys 15:7155–7171 49. Venkataraman C, Habib G, Eiguren-Fernandez A, Miguel AH, Friedlander SK (2005) Residential biofuels in south Asia: carbonaceous aerosol emissions and climate impacts. Science 307:1454–1456 50. Wang M, Xu B, Cao J, Tie X, Wang H, Zhang R, Qian Y, Rasch PJ, Zhao S, Wu G, Zhao H, Joswiak DR, Li J, Xie Y (2015) Carbonaceous aerosols recorded in a southeastern Tibetan glacier: analysis of temporal variations and model estimates of sources and radiative forcing. Atmos Chem Phys 15:1191–1204 51. Wang X, Wang C, Zhu T, Gong P, Fu J, Cong Z (2019) Persistent organic pollutants in the polar regions and the Tibetan Plateau: a review of current knowledge and future prospects. Environ Pollut 248:191–208 52. Wang XP, Sun DC, Yao TD (2016) Climate change and global cycling of persistent organic pollutants: a critical review. Sci China Earth Sci 59:1899–1911 53. Ward DS, Kloster S, Mahowald NM, Rogers BM, Randerson JT, Hess PG (2012) The changing radiative forcing of fires: global model estimates for past, present and future. Atmos Chem Phys 12:10857–10886

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74. You C, Yao T (2019) Fire records in glacier ice. Natl Sci Rev 6:384–386 75. You C, Yao TD, Xu BQ, Xu C, Zhao HB, Song LL (2016) Effects of sources, transport, and postdepositional processes on levoglucosan records in southeastern Tibetan glaciers. J Geophys Res-Atmos 121:8701–8711 76. You C, Yao TD, Xu C (2018) Recent increases in wildfires in the Himalayas and surrounding regions detected in Central Tibetan Ice Core Records. J Geophys Res-Atmos 123:3285–3291 77. You C, Yao TD, Xu C, Song LL (2017) Levoglucosan on Tibetan glaciers under different atmospheric circulations. Atmos Environ 152:1–5 78. You Q, Min J, Lin H, Pepin N, Sillanpää M, Kang S (2015) Observed climatology and trend in relative humidity in the central and eastern Tibetan Plateau. J Geophys Res-Atmos 120:3610– 3621 79. Zangrando R, Barbaro E, Vecchiato M, Kehrwald NM, Barbante C, Gambaro A (2015) Levoglucosan and phenols in Antarctic marine, coastal and plateau aerosols. Sci Total Environ 544:606–616 80. Zangrando R, Barbaro E, Zennaro P, Rossi S, Kehrwald NM, Gabrieli J, Barbante C, Gambaro A (2013) Molecular markers of biomass burning in arctic aerosols. Environ Sci Technol 47:8565– 8574 81. Zennaro P, Kehrwald N, Marlon J, Ruddiman WF, Brucher T, Agostinelli C, Dahl-Jensen D, Zangrando R, Gambaro A, Barbante C (2015) Europe on fire three thousand years ago: arson or climate? Geophys Res Lett 42:5023–5033 82. Zennaro P, Kehrwald N, McConnell JR, Schupbach S, Maselli OJ, Marlon J, Vallelonga P, Leuenberger D, Zangrando R, Spolaor A, Borrotti M, Barbaro E, Gambaro A, Barbante C (2014) Fire in ice: two millennia of boreal forest fire history from the Greenland NEEM ice core. Clim Past 10:1905–1924 83. Zhao ZZ, Cao JJ, Shen ZX, Xu BQ, Zhu CS, Chen LWA, Su XL, Liu SX, Han YM, Wang GH, Ho KF (2013) Aerosol particles at a high-altitude site on the Southeast Tibetan Plateau, China: implications for pollution transport from South Asia. J Geophys Res-Atmos 118:11360–11375 84. Zheng J, Hu M, Du ZF, Shang DJ, Gong ZH, Qin YH, Fang JY, Gu FT, Li MR, Peng JF, Li J, Zhang YQ, Huang XF, He LY, Wu YS, Guo S (2017) Influence of biomass burning from South Asia at a high-altitude mountain receptor site in China. Atmos Chem Phys 17:6853–6864

Chapter 5

Levoglucosan Records in the Zangsegangri Ice Core

In this chapter, a continuous levoglucosan record was reconstructed in the Zangsegangri ice core. The levoglucosan record was classified into two categories: background levels and extreme events. Annually resolved levoglucosan record and background levels in the ice core were strongly correlated with satellite observations of fire variations. In addition, peaks in levoglucosan concentrations may also represent extreme fire events occurred in Central Asia. Levoglucosan in the Zangsegangri ice core indicated a rapid increase in wildfires at the beginning of the twenty-first century. Decreasing precipitation in pre-monsoon has prolonged the dry seasons across Himalayan and its surrounding regions affected by the Indian summer monsoon; meanwhile, increasing precipitation over the arid and semiarid Indus River Plain promotes plant growth and thereby increases biofuel availability. These trends have induced increased frequencies of strong wildfires. Increasing strong wildfire events can potentially enhance black carbon deposited on Himalayan glaciers during the pre-monsoon wildfire seasons. Under these consideration, levoglucosan records in Tibetan ice cores are suggested as a creditable tool for calibrating past fire changes.

5.1 Background Information Fire, including both wildfires and anthropogenic crop residue burning [1, 2, 22], exerts a crucial influence on global ecosystems and human society [1, 7, 19, 22]. Large-scale wildfires can cause disastrous economic losses [7, 15, 22, 39]. Fire emissions are important causes of serious environmental and health issues [10, 15, 17, 18, 26, 39]. Each year, about 340,000 deaths were caused by fire emissionrelated cardiovascular and respiratory diseases [10]. Globally, annual fire emissions have been estimated at about 2000 Tg Carbon [38], but display strongly inter-annual variations [17, 38, 44]. For instance, more than 1000 Tg carbon was released from

© Springer Nature Singapore Pte Ltd. 2021 C. You, Geochemical Behavior of Levoglucosan in Tibetan Plateau Glacier Snow and Ice, Springer Theses, https://doi.org/10.1007/978-981-15-7973-8_5

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5 Levoglucosan Records in the Zangsegangri Ice Core

wildfires in Equatorial Asia during the strong El Niño in 1997/1998 [38]. However, with only about 20 Tg carbon was released during the La Niña in 2000 [17, 38]. Fire variations over subtropical Eurasia (e.g., Tibetan Plateau and surroundings) are poorly understood due to insufficient evidence [19, 20, 38], especially under the recent warming and drying trends [6]. The Tibetan Plateau is located adjacent to subtropical Asian regions such as South Asia and Central Asia [5, 49, 54], which are considered among the most intense fire emissions sources across the northern hemisphere [1, 51]. The Tibetan Plateau experienced evident climate changes in the late twentieth century, which have changed its atmospheric and hydrological cycles and thus reshaped environmental changes in the Tibetan Plateau surrounding regions [45]. The Tibetan Plateau has been characterized by different climatic patterns since the 1990s [49]. There is a wetting trend in the westerly dominated northern part and a drying trend in the monsoon-influenced southern part of the Tibetan Plateau [6, 49]. Such climatic patterns are potentially leading to complex variations in fire activity over the upwind regions. Fire smoke can be uplifted to the free troposphere and can sometimes even penetrate the lower stratosphere under strong thermal convection [37, 43] and can subsequently be transported and deposited on these high-elevation Tibetan glaciers [51], especially during the major wildfire seasons [53]. Geochemical evidences showed that Tibetan glaciers have been substantially contaminated by levoglucosan at different grades [51, 53, 55] and are thought to mainly act as receptors of fire emissions released from surrounding regions [51]. The chemical compositions of the individual components of smoke aerosols provide multiple lines of evidence for characterizing fire emissions [14, 31, 50]. Levoglucosan can only be generated from the degradation of cellulose and hemicellulose, and accounts for more than 90% of the total dehydration monosaccharides in fire emissions [31]. The lifetime of levoglucosan varies from several hours to more than ten days, under different atmospheric conditions, therefore enabling its long-range transport and global distribution [14, 31, 50]. Levoglucosan in different environmental matrices, including atmospheric aerosols, precipitation, soil, lake/marine sediment, and glacier snow/ice, has been extensively used for indicating fire emission changes [50]. Previous studies have revealed that levoglucosan records in glacier ice layers can provide information about fire emissions changes on time scales ranging from the past several millennia to the last few decades [3, 54, 56] and can sometimes even capture strong fire event signals [13, 50, 53]. However, strong melting and leaching processes occur on Tibetan Glacier surfaces, most notably during summer [25], and may substantially modify the post-depositional levoglucosan distribution in glacier ice layers [53]. These processes must be addressed when assessing the environmental significance of levoglucosan in Tibetan Glacier ice and its suitability as a calibration tool for indicating past fire changes, especially on sub-annual to annual time scales [53, 54]; however, this issue has received little attention and has not yet been resolved. In this section, we firstly verified whether levoglucosan records in Tibetan Glacier ice layers can represent regional fire changes; then, we reconstructed a rapid fire increasing since 1990 by using a continuous levoglucosan record extracted from a central Tibetan ice core.

5.2 Identify the Possible Fire Sources

47

5.2 Identify the Possible Fire Sources Moderate Resolution Imaging Spectroradiometer fire products (MODIS V6 data, retrieved from http://modis-fire.umd.edu/, on-board NASA’s Terra and Aqua satellites) [8] were used to confirm fire regimes across the Tibetan Plateau and surroundings from 2003 to 2012 (Figs. 4.1 and 5.1). Although fire counts differ between Terra and Aqua, detected fire variations are broadly consistent [8]. Fire spots with confidences lower than 50% were eliminated (12.75% of Terra and 13.88% of Aqua) in order to improve reliability. Fire spots with a fire radiative power higher than 100 MW were adopted as the strong fire events and accounted for 1.02% of the total fire counts. MODIS observations revealed that fires mainly occurred from March to May and October to November over the northern Indian peninsula (20–35° N, 65–90° E, Fig. 5.1) [37, 51], and from June to September over Central Asia (35–50° N, 60– 90° E, Fig. 5.1) [51]. Although fire counts are comparable between pre-monsoon (March to May) and post-monsoon (September to November), the pre-monsoon season accounted for more than 75% of the total strong fire events over the northern Indian peninsula. There were no detected fire sources on the Tibetan Plateau within 300 km of the Zangsegangri Glacier from MODIS observations (Fig. 5.1), suggesting that levoglucosan records reconstructed in this ice core are mainly affected by long-range transport of fire emissions. Although moisture carried by the Indian summer monsoon can reach the central Tibetan Plateau during summer [35, 46], pollutants deposited on the Zangsegangri Glacier are mainly affected by the westerlies. Actually, previously work based on water stable isotopes and geochemical analysis has provided evidence that air masses from Central Asia could have an important influence on central Tibetan glaciers [34, 41, 46]. This phenomenon is more noticeable in the upper troposphere (e.g., above 500 hPa, [43, 53]). On the other hand, moisture carried by the westerlies can be substantially intercepted due to physical blocking by the very high mountains (e.g., the Pamir Plateau) along the transport pathways and formation of precipitation on the windward slope [35]. Levoglucosan released from West Asia and regions further away (e.g., Eastern Europe) could be substantially influenced by wet deposition during its long-range transport. Noting that levoglucosan is a water-soluble component [30], which can be effectively scavenged by wet deposition [50, 51, 53]. Fire emissions from the southern Indian peninsula and Southeast Asia can contribute little to the central Tibetan Plateau glaciers, because the prevailing winds impair the transportation of smoke aerosols throughout the year [51]. In fact, MODIS observations only detected a few fire points across the Indian peninsula and the Southeast Asia during the monsoon months from June to September due to abounding monsoon rainfalls (Fig. 5.1, and Refs. [36, 51]), though fire smokes were reported to occasionally penetrate to an altitude of over 6 km on the southern slope of the Himalayas [43]. Considering the cold environmental conditions [47, 49] and low biomass loading capacity [28, 29], fire emissions from the interior Tibetan Plateau play a negligible role. As a result,

48

5 Levoglucosan Records in the Zangsegangri Ice Core

Fig. 5.1 a Spatial distribution of fire counts around the Tibetan Plateau and surrounding regions. b Monthly variations of fire counts over Central Asia (35–50° N, 60–90° E) and North Indian peninsula (20–35° N, 65–90° E) between 2003 and 2012

5.2 Identify the Possible Fire Sources

49

Central Asia and northern Indian peninsula are the main sources for levoglucosan records in the Zangsegangri ice core.

5.3 Levoglucosan Records in the Zangsegangri Ice Core About 30% of the entire Zangsegangri ice core samples were with levoglucosan concentrations lower than the method LOD. The arithmetic average of levoglucosan is lower than 1.00 ng mL−1 in the Zangsegangri ice core based on more than 3000 ice samples. Although the stability of levoglucosan in ancient sediment conditions (e.g., hundreds to more than tens of thousands years ago) is still a controversial issue as mentioned in previous studies [3, 32, 56], levoglucosan concentrations in the Zangsegangri ice core do not decrease vs increasing ice core depth (Fig. 5.2). It is notable that the near bottom section (205–206 m) has higher levoglucosan concentrations than in the upper sections (0–11 m). It is worth noting that the net accumulation rate of the Zangsegangri ice core is only about 220 mm ice equivalent per year [52, 54], and the ice age of a depth 205–206 m is thus at least exceeding the past several millennia. This suggests that levoglucosan can remain stable in the enclosed and refrigerated glacial conditions. However, because the final dating results of the Zangsegangri ice core still need more evidences to approve, levoglucosan records in the entire ice core are not shown and discussed herein. In the following part of this section, we mainly focus on the significances

Fig. 5.2 Variation of levoglucosan concentrations in the Zangsegangri ice core at different depth. Noting that levoglucosan concentrations did not declined with increasing ice core depth

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5 Levoglucosan Records in the Zangsegangri Ice Core

Fig. 5.3 Primary levoglucosan concentration variations with depth in the Zangsegangri ice core

of levoglucosan in the Zangsegangri ice core and reveal a rapid increasing of fire activities since 1990 based on our results. The primary levoglucosan concentration results showed an increasing trend (p < 0.01) between 1990 and 2012 over the study period (Fig. 5.3). Samples had levoglucosan concentrations higher than the mean + 1stdev (1.96 ng mL−1 ) and were considered to represent strong events; samples had levoglucosan concentrations higher than the mean + 2stdev (3.04 ng mL−1 ) and were considered extreme events (Fig. 5.3). The highest levoglucosan concentration was 8.49 ng mL−1 at a depth of 1.70 m, which is more than nine times the arithmetic mean (0.88 ng mL−1 since 1990). All these strong levoglucosan events are concentrated in the upper 2.0 m of the ice core, corresponding to the time period from 2006 to 2012, indicating strong events have occurred more frequently in recent years.

5.4 The Annually Resolved Levoglucosan Records The annually based levoglucosan arithmetic mean was 0.92 ng mL−1 and displayed an increasing trend from 1990 to 2012 [54] (Fig. 5.3). The annually resolved levoglucosan concentrations and mass fluxes in the Zangsegangri ice core are highly correlated (both with p < 0.01) with fire counts over the northern Indian peninsula (Table 5.1). However, high levoglucosan concentrations in some years are mostly controlled by sporadic levoglucosan extreme events (Figs. 5.3 and 5.4). For instance, the year 2006 had a mean levoglucosan concentration of 1.83 ng mL−1 , about twice

5.4 The Annually Resolved Levoglucosan Records

51

Table 5.1 Correlation confidence between fire counts over the northern Indian peninsula and annually based levoglucosan variations in the Zangsegangri ice core from 2003 to 2012 Correlation coefficient (r)

Annually levoglucosan variations Concentration

Flux

Background

Total fire counts

0.76**

0.67*

0.84***

Fire counts with FRP > 100WM

0.83***

0.77**

0.86***

FRP > 100WM during pre-monsoon

0.79**

0.72*

0.85***

***p < 0.001, ** p < 0.01, * 0.01 < p < 0.05

Fig. 5.4 Annually resolved levoglucosan concentration and mass fluxes between 1990 and 2012 in the Zangsegangri ice core

the annual average during the study period, mostly due to one sample at 1.70 m with a levoglucosan concentration of 8.49 ng mL−1 . This time corresponded with low fire counts over the northern Indian peninsula in 2006 (Fig. 5.3). In order to reduce the impacts of these extreme levoglucosan events, the annually resolved levoglucosan background level was calculated after removing extreme levoglucosan values (those higher than mean + 2stdev, equal to 3.04 ng mL−1 ). The annual levoglucosan background level was strongly correlated with fire counts (p < 0.01) over the northern Indian peninsula (Table 5.1). Results indicated that levoglucosan records in the Zangsegangri ice core were strongly influenced by fire events, and therefore could be used for calibrating regional fire changes at least on sub-annual to annual time scales.

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5 Levoglucosan Records in the Zangsegangri Ice Core

5.5 Possible Influences of Strong Fire Events Results showed that the preliminary levoglucosan records displayed event-based characteristics in the Zangsegangri ice core (Fig. 5.3), indicating that postdepositional processes on Tibetan Glacier surface did not completely alter the levoglucosan distribution in ice layers. For instance, the levoglucosan concentration in the sample at a depth of 1.70 m was 8.49 ng mL−1 , yet samples above and below this level all showed levoglucosan concentrations fluctuating close to the mean value (Fig. 5.3). This isolated extreme value contributed more than half of the annual levoglucosan concentration for 2006 (Fig. 5.5). Some similar levoglucosan extreme values can also be seen in the Zangsegangri ice core and may be used to assess the impacts of extreme or strong fire events. The extreme fire event identified at 1.70 m (year 2006) may be traced back to the Central Asia regions. There were eight fire counts with FRP > 3000 MW detected in an area near the lakefront of Balkhash Lake, Central Asia, during August 2006 (approximately 47.06° N, 73.24° E, Table 5.2). Those fire counts were the most concentrated extreme fire events in Central Asia from 2003 to 2012. Smoke aerosols from Central Asia carried by those air masses can be transported and deposited onto Tibetan Glacier surfaces, possibly causing the extreme high levoglucosan events in the ice core records. Nevertheless, the influences of those extreme fires should be evaluated using fire-related climate models in future work, and such results do not imply a one-to-one relationship between fire events in the source region and the detected levoglucosan records, because many factors (e.g., atmospheric deposition

Fig. 5.5 Fire changes derived from the levoglucosan record from 1990 to 2012 in the ZSGR ice core: relative contribution of fire events, variations of strong fire events and background

5.5 Possible Influences of Strong Fire Events

53

Table 5.2 Information on MODIS detected extreme fire events over the Central Asia regions during August 2006 (data downloaded from Fire Information for the Resource Management System (FIRMS) at https://firms.modaps.eosdis.nasa.gov/) Latitude

Longitude

acq_date

Satellite

Instrument

Confidence

FRP (MW)

47.043

73.207

2006-8-24

Aqua

MODIS

100

3681.1

47.044

73.209

2006-8-24

Aqua

MODIS

100

3796.9

47.06

73.262

2006-8-24

Aqua

MODIS

100

3084.8

47.06

73.265

2006-8-24

Aqua

MODIS

100

3051.1

47.061

73.202

2006-8-24

Terra

MODIS

100

3510.9

47.062

73.234

2006-8-24

Terra

MODIS

100

4589.9

47.066

73.242

2006-8-24

Terra

MODIS

100

6749

47.067

73.275

2006-8-24

Terra

MODIS

100

3244.4

during atmospheric transport; ice core sample resolution; and other processes) could impact levoglucosan records in glacier ice [13, 50, 53]. Annually based levoglucosan concentrations and flux records in the Zangsegangri ice core indicate a recent increase in fires in Tibetan Plateau and its surroundings from 1990 to 2012, in terms of both the long-term trend and fire events. Strong and extreme fire events represented by samples with levoglucosan concentrations higher than the mean + 1stdev (1.96 ng mL−1 ) were all concentrated in the period from 2006 to 2012 (Figs. 5.2 and 5.5), indicating that strong fire events have occurred more frequently in recent years. In addition, the annually resolved levoglucosan background level displayed a significantly increasing trend (p < 0.01, Fig. 5.5), indicating that the increasing frequency of fire events has also strongly affected background levels.

5.6 Increasing Fire Activities Since 1990 An apparent increase in annual levoglucosan concentration and flux was revealed in the Zangsegangri ice core after AD 2000 (Figs. 5.5 and 5.6). MODIS observations showed an apparent decrease in fire counts across Central Asia from 2003 to 2012 (Figs. 5.3 and 5.6), while northern Indian peninsula showed a significant increase in fire counts (Fig. 5.6). Meanwhile, strong fire events displayed similar increasing trends along the Himalayas and surroundings (Fig. 5.6). Noticeably, the levoglucosan arithmetic average after 2006 was about four times greater than that before 2006 (Figs. 5.4 and 5.5), indicating fire displayed an accelerating increase after 2006 and that levoglucosan maxima occurred more frequently (Fig. 5.5). Levoglucosan records in the Zangsegangri ice core evidently revealed increasing fires in the Himalayas and their surroundings during the first decade of the twenty-first century.

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5 Levoglucosan Records in the Zangsegangri Ice Core

Fig. 5.6 Annual variations of fire activity from 2000 to 2012 as indicated by a levoglucosan concentration, b levoglucosan mass flux, and MODIS observations of fire counts over northern Indian peninsula from 2003 to 2012 plotted as c total fire counts and fire counts with fire radiative power (FRP) > 100 MW d annual totals and e pre-monsoon totals

5.7 Possible Linkages to Recent Climate Changes Fire activities were low during the monsoon and winter over northern Indian peninsula (Fig. 5.1 and Ref. [37, 51]). Although anthropogenic crop residue burnings were reported as major post-monsoon fire sources over the Gangetic Plain [12, 27, 37], these sources were considered relatively small scales, when compared with wildfires [37]. Furthermore, stable post-monsoon atmospheric conditions over northern Indian peninsula impair the uplift of smoke to the free troposphere [12], hence hampering the long-range transport of smoke plumes. Studies recommended that human activity should be considered as a major influencing factor in fire activity over the past few decades [1, 20]. Andela et al. [1] even indicated that anthropogenic agricultural intensification might increase fire activity in densely populated India, where crop residue burning was suggested as the dominant fire type. However, there was no apparent increase in the agricultural acreage during the first decade of the twenty-first century over the Indian peninsula [27, 33]. Pre-monsoon fires accounted for more than 70% of the total strong fire events with fire radiative power >100 MW and displayed a significant increasing trend from 2003

5.7 Possible Linkages to Recent Climate Changes

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Fig. 5.7 Annual variations of levoglucosan concentration and levoglucosan/refractory black carbon in the Zangsegangri ice core

to 2012 (Fig. 5.6). Moreover, Levoglucosan/black carbon ratio in the Zangsegangri ice core increased dramatically over the study period (Fig. 5.7), indicating that the increasing levoglucosan records in the Zangsegangri ice core were mainly caused by wildfires rather than anthropogenic burnings. Emissions of black carbon resulted from incomplete combustion during the burning of biofuel and fossil fuels [21], and anthropogenic fossil fuel burning was thought to be the main source of black carbon on Tibetan glaciers [42]. The rapid increasing trend of levoglucosan/black carbon (Fig. 5.7) also indicates that anthropogenic fossil fuel burnings were not the main factor related to increasing fires across the northern Indian peninsula in recent years. Instead, the climate system has been considered as the principal driver of increasing wildfires in the Himalayas and surroundings over the past decade, despite that human activity has been deemed as a major influencing factor in fire activity worldwide over the past 50 years [20]. Modeling results have indicated that climate will play the key role, rather than human influence, in twenty-first century fire activity [24]. Generally, compared with cold periods, warm intervals usually result in higher net primary productivities and can yield more biofuel available for burning [19, 23, 24]. Modeling results also indicated areas of enhanced fire activity in the high-altitude Himalayas and surroundings under warming and drying scenarios [24, 44]. However, precipitation rather than temperature acts as the crucial factor regulating terrestrial ecosystems across regions around the Tibetan Plateau [28, 58] and controls the quantity of available biofuels.

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Decreasing precipitation has been reported in regions around the Himalayas since 1979 (Fig. 5.8), and with the decreasing trends being more significant during the premonsoon and winter seasons [40]. Similar decreasing precipitation patterns were also observed in northern Indian sites such as Punjab, Haryana, and some others. Decreasing precipitation can lead to prolonged dry seasons in the Himalayas and surroundings, ultimately resulting in increased wildfires in recent years. On the other hand, decreasing Indian summer monsoon rainfall does not apparently influence

Fig. 5.8 Annual contour of precipitation trend for regions around the Tibetan Plateau from 1979 to 2012 based on Global Precipitation Climatology Project data (https://www.esrl.noaa.gov/psd/ data/gridded/data.gpcp.html), a precipitation amount (mm/year) and b percentage (%/year). Positive values indicate increasing trend, and negative values indicate decreasing trend. The red circle indicates the site of the Zangsegangri ice core

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the available biofuels in the humid regions [40]. Unlike the humid Himalayas and Gangetic Plains, precipitation is the dominant factor controlling available biomass fuels across the semiarid Indus Plain [40] and Central Asia [23, 57, 58]. Notably, significant increases in precipitation were observed on the Indus Plain (Fig. 5.8), which may have yielded increased biofuels. The northern Indian peninsula has therefore suffered increasing wildfires over the past decade. Although an overall increase in precipitation was detected over Central Asia since 1979, slight decrease in precipitation was observed in the northern part of Central Asia (Fig. 5.8) covered by grassland and forest, especially during major growing seasons (spring and summer) (e.g., [16, 57] and Fig. 5.8). Decreasing precipitation can reduce the net primary productivity [16, 57, 58] and result in reduced yields of biofuels [28] during fire seasons. Agriculture is major sector in the economy of Central Asia countries. Agricultural area was estimated about 7 × 105 km2 in 2009 in Central Asia, accounting for less than a quarter of the natural vegetation; in addition, substantial conversion of farmland to natural vegetation was reported after the collapse of the former Soviet Union, and farmland area decreased more than 30% from 1990 to 2009 [4]. Although increasing precipitation was reported over northwestern China (Fig. 5.8b), the extreme low productivities over the Taklimakan desert [28, 57] cannot provide sufficient biofuels for wildfires; consequently, this region is characterized by extreme-low fire levels. Ultimately, decreasing precipitation and decreasing agricultural area have resulted in the decrease in fires across Central Asia during the study period. On the other hand, there is an overall increasing trend in precipitation over the Central Asia and a roughly declining trend in precipitation over the South Asia, at least since 1940s (also in Fig. 5.9). Under such a climatic situation, those regions are prone to more fire activities. These changes will cause higher greenhouse gas and carbonaceous aerosol emissions and result in serious societal and environmental problems over the densely populated Asian regions [15, 26]. Furthermore, although fossil fuel burning was thought to be the main source of black carbon deposited on Tibetan glaciers [11, 42], our previous studies have indicated that strong fire events can enhance black carbon deposition on Himalayan glaciers during the pre-monsoon wildfire seasons [53]. Increasing fire activities over the upwind regions thus can impact the melting of Tibetan glaciers during the major fire seasons [52, 54] and finally can aggravate the instability of the Asian Water Tower at some extent [47, 48].

5.8 Summary In this section, the annually based levoglucosan record extracted from a Tibetan ice core was found to be highly correlated with satellite-observed fire count variations from 2003 to 2012. Fire activities over the northern Indian peninsula were revealed to be a major factor regulating annual levoglucosan records in the Zangsegangri ice core. Background fire levels showed an overall increasing trend and were accompanied by

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Fig. 5.9 Annually resolved precipitation changes since 1900 CE over Central Asia (upper, data from Ref. [9]) and India (bottom, data from Indian Institute of Tropical Meteorology, https://www. tropmet.res.in/Data)

a greater frequency of fire events in recent years. Extreme fires during August 2006 in Central Asia could be responsible for the highest levoglucosan concentration, at a depth of 1.70 m, in the 1990–2012 ice core record. High occurrences of strong and extreme fire events may be responsible for the observed rapid increase in levoglucosan concentration since 2006. Results demonstrate that continuous levoglucosan records in the Tibetan Plateau ice cores can capture the signals of discrete fire events as well as the long-term trend and can therefore provide fire event information for reconstructing past fire changes. Besides, our results have revealed recent increases in wildfires in the Himalayas and surrounding regions based on levoglucosan records and provided an important basis to extend the record of levoglucosan in the Tibetan Plateau back in time. Recent climate change has exceeded the natural variability of the past millennia, resulting in complex variations in fire activity on regional scales. Results suggest that, under the recent warming and drying trends, high wildfire risks are likely across the Himalayas and their surroundings. This has important implications for glacier melting on the Tibetan Plateau and its surrounding high mountains, ultimately impacting on water resources available in major Asian rivers.

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Summary and Conclusions

Understanding fire dynamics can provide insights into the interactions between fire, human beings, and components of the earth system. High-resolution reconstructions of ancient fire changes in glacier ice are crucial for modeling predictions and policymaking. In this work, we focus on the geochemical behaviors and high-resolution records of levoglucosan in Tibetan glacier snow and ice core. The topics include a novel method for determination of levoglucosan at trace concentration level in Tibetan glacier snow and ice by using UPLC-MS/MS (Chap. 3); the geochemical processes for influencing levoglucosan records in Tibetan glacier snow and ice (Chap. 4); and the recent increasing fire activities since the 1990s revealed in the Zangsegangri ice core (Chap. 5). This work has approved that levoglucosan can be used as a powerful tool for calibrating and reconstructing past fire changes in glacier ice, and the knowledge of this work thus improved our understanding of the geochemical behaviors of levoglucosan in earth system. For future work, we should pay more attention on the following issues. Firstly, seasonally to annually resolved reconstruction of levoglucosan records is in Tibetan snow layers and ice cores. Those high-resolution ice core records can provide essential information for more detailed characteristics of fire changes from the recent few decades to the past millennia over the densely populated Asian regions, which are necessary for integrating and understanding of global fire changes. Secondly, the geochemical processes of levoglucosan in precipitation. Precipitation is known as a major process for scavenging levoglucosan in atmosphere and plays an important role in connecting atmosphere and sediment (e.g., glacier ice). However, current work does not note the importance about the geochemical significance of levoglucosan in precipitation. Thirdly, the interface processes among various environmental matrixes of earth surfaces, including atmosphere–snow interface, snow–ice interface, ice–water interface, water–sediment interface, and so on. These processes are directly related to the variability and stability of levoglucosan in various environmental matrixes on the earth surface system and are important to the usage of levoglucosan as a tracer for calibrating fire changes over different time scales.

© Springer Nature Singapore Pte Ltd. 2021 C. You, Geochemical Behavior of Levoglucosan in Tibetan Plateau Glacier Snow and Ice, Springer Theses, https://doi.org/10.1007/978-981-15-7973-8

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