Climatological Aspects of the Composition and Pollution at the Atmosphere 9263103933

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WORLD

METEOROLOGICAL

ORGANIZATION

TECHNICAL NOTE No. 139

CLIMATOLOGICAL ASPECTS OF THE COMPOSITION AND POLLUTION OF THE ATMOSPHERE by G. C. Holzworth

•

WMO - No. 393 Secrétariat of the World Meteorological Organization - Geneva - Switzerland

THE WMO The World Meteorological Organization (WMO) is a specialized agency of the United Nations of winch 139 States and Territories are Members.

—

— — — —

I t was created: To facilitate international co-operation in the establishment of networks of stations for making meteorological and geophysical observations and centres to provide meteorological services and observations; To promote the establishment and maintenance of Systems for the rapid exchange of meteorological information; To promote standardization of meteorological observations and ensure the uniform publication of observations and statistics; To further the application of meteorology to aviation, shipping, water problems, agriculture, and other human activities; To encourage research and training in meteorology. The macliinery of the Organization consists of the following bodies:

The World Meteorological Congress, the suprême body of the Organization, brings together the delegates of ail Members once every four years to détermine gênerai policies for the fulfilment of the purposes of the Organization, to adopt Technical Régulations relating to international meteorological practice and to détermine the WMO programme. The Executive Committee is composed of 24 directors of national Meteorological Services and meets at least once a year to conduct the activities of the Organization and to implement the décisions taken by its Members in Congress, to study and make recommendations on matters affecting international meteorology and the opération of meteorological services. The six Régional Associations (Africa, Âsia, South America, North and Central America, South-West Pacific and Europe), which are composed of Member Governments, co-ordinate meteorological activity within their respective Régions and examine from the régional point of view ail questions referre d to them. The eight Technical Commissions, composed of experts designated by Members, are responsible for studying the spécial technical branches related to meteorological observation, analysis, forecasting and research as well as to the applications of meteorology. Technical commissions hâve been establishcd for basic Systems, instruments and methods of observation, atmospheric sciences, aeronautical meteorology, agricultural meteorology, marine meteorology, hydrology, and spécial applications of meteorology and cfimatology. The Secrétariat, located at Geneva, Switzerland, is composed of an international scientific, technical and administrative staff under the direction of the Secretary-General. It undertakes technical studies, is responsible for the numerous technical assistance and other technical co-operation projects in meteorology throughout the world aimed at contributing to économie development of the countries concerned. It also pubUshes specialized technical notes, guides, manuals and reports and in gênerai acts as the link between the Meteorological Services of the world. The Secrétariat works in close collaboration with the United Nations and other specialized agencies.

CUMO

WORLD

METEOROLOGICAL

ORGANIZATION

TECHNICAL NOTE No. 139

CLIMATOLOGICAL ASPECTS OF THE COMPOSITION AND POLLUTION OF THE ATMOSPHERE by G. C. Holzworth

U.D.C. 551.510.42: 551.58

WMO - No. 393 Secrétariat of the World Meteorological Organization - Geneya - Switzerland 1974

1974, World Metcorological Organization ISBN 92-63-10393-3

NOTE The désignations einployed and the présentation of the material in this publication do not imply the expression of any opinion wlmtsoevcr on the part of the Secrétariat of the World Metcorological Organization concerning the légal status of any country or territory or of its authorities, or concerning the délimitation of its frontiers.

CONTENTS

Foreword Summary (English, French, Russian, Spanish)

V VII

Introduction

1

Meteorological variables

4

Wind

4

Static stability

12

Mixing heights

18

Diffusion parameters

23

Fog

27

Précipitation

27

Stagnation

28

Photochemical oxidant

33

Air-quality data

37

Références

40

FOREWORD

At its fifth session (Geneva, 1969), the WMO Commission for Climatology (now Commission for Spécial Applications of Meteorology and Climatology) appointed Mr. G. C. Holzworth (U.S.A.) as Rapporteur on Climatological Aspects of the Composition and Pollution of the Atmosphère. In this capacity, Mr. Holzworth prepared a survey of climatological procédures for estimating the air-pollution potential of localities or areas. The présent Technical Note is the outcome of this survey. I am pleased to take this opportunity of expressing to Mr. Holzworth the sincère appréciation of the World Meteorological Organization for the time and effort he has devoted to the préparation of this Technical Note.

D. A. Davies Secretary-General

SUMMARY

This report offers some practical guidelines on the processing and use of regular surface and upper-air observations in terms of their climatological influence on transport and diffusion of air pollutants. The concept of a meteorological potential for air pollution is discussed. An attempt was made to include examples of pertinent climatological data for various parts of the world but in fact the data are limited to temperate and northern latitudes of the northem hémisphère. There is a paucity of available meteorological studies relative to air pollution in the tropics, which, unfortunately, is where industrialization, energy consumption, and pollutant émissions are expected to increase rapidly. In addition to transport and diffusion, some attention is also devoted to climatological influences on the transformation of pollutants while they are airborne, especially the photochemical formation of oxidant. A brief summary of pertinent publications of the World Meteorological Organization is given and an effort is made not to repeat information unnecessarily therein. Meteorological variables that are discussed include wind speed and direction, static stability, mixing heights, diffusion parameters, fog, and précipitation. A more gênerai variable is atmospheric stagnation, which refers to meteorological conditions that are often associated with or conducive to épisodes with relatively high concentrations of pollutants, especially as experienced in cities. Various définitions and applications of stagnation are described for différent parts of the world. A brief description is given of considérations to be made in processing and summarizing measurements of air quality. The report includes almost 100 références.

RESUME

Ce rapport donne un certain nombre de directives pratiques pour le traitement et l'utilisation des données d'observations régulières en surface et en altitude aux fins de l'étude de l'influence des facteurs climatologiques sur le transport et la diffusion des polluants de l'air. Il présente une analyse du concept de potentiel météorologique en matière de pollution. L'auteur s'est efforcé de donner des exemples de données climatologiques pertinentes pour diverses parties du monde mais, en fait, les données citées ne portent que sur les latitudes tempérées et septentrionales de l'hémisphère Nord. L'on ne dispose actuellement que de très peu d'études météorologiques sur la pollution de l'air dans les zones tropicales où, malheureusement, du fait de l'industrialisation et de l'augmentation de la consommation d'énergie, on peut s'attendre à un rapide accroissement des émissions de polluants. En plus du problème du transport et de la diffusion des polluants, l'auteur a également accordé une certaine attention aux influences climatologiquei qui interviennent dans les transformations que subissent les polluants au cours de leur séjour dans l'atmosphère, notamment la formation d'oxydants sous l'action de processus photochimiques. L'auteur donne un bref résumé des publications pertinentes de l'Organisation météorologique mondiale et s'est efforcé de ne pas répéter inutilement les renseignements déjà contenus dans celles-ci. Les variables météorologiques considérées dans le rapport comprennent la vitesse et la direction du vent, la stabilité statique, les hauteurs de mélange, les paramètres de la diffusion, le brouillard et les précipitations. Une variable de caractère plus général est la stagnation atmosphérique qui traduit des conditions météorologiques qui sont fréquemment associées ou qui conduisent à des phases de concentrations relativement fortes de polluants, du genre de celles qui sévissent particulièrement dans les villes. Plusieurs définitions et applications de la stagnation, valables pour différentes parties du monde, sont données. L'auteur expose brièvement les considérations dont il y a lieu de tenir compte pour le traitement des mesures de la qualité de l'air et l'établissement de résumés de ces informations. Le rapport comporte près d'une centaine de références.

•HOimoo oxo MXhou i w w d o ï ï o o HeirMoff ' B x £ t f e o a B3X09hBH HHH9d9W8H MHHBaodHWWiCo H 9HX0gBdg0 H d u qXBaNXHhÀ" X9À"tt9IfO OPldoXOH 'BMH9HiBdgOO0 nHBOHUO OHXBdj-I *BdHW JJ9X0Bh XHHfiBdgOOH8Bd BITtf BOX0B8 BHH9H9WHdu M BHII9ir9WaduO OIlHhHIfeBd WHBOHUQ "XBWodOJ S BOXOBtfOIIi'gBH OXOBh 0HH9g000 OXh 'ÀWOXe WHÏnorÀaXOgOOOUO HirH '#9If9XHH -8BdJB8 HHÏIBdxH9tlH0M XMHOOHa OHW9XH0OHXO W9HH9a0HHHH80a 0 WIHHHBEBaO OHÏÏodOH 'WBHaOIfOiî WHH09hHJOirod09X9W M BOXHOOHXO RHdoXOH lred9(|>00lMXB #0X0B8 B0X9BiraB ^OHMhMIfOa #01IH9WOd9H ff9tngO 9 8 I f 0 g 'HHtfBOO H HBWÀX 'HH8À\J) 4/10, use NR = - 1 in Table 7. 3. For daytime : Détermine insolation class number (IN) from Table 8. (a) If TC < 5/10, use NR = IN in Table 7. (b) If TC > 5/10, modify IN by the sum of the following applicable numbers: (i) If ceiling 7000 ft but 7000 ft, modification = - 1 , and let modified value of IN = NR in Table 7, except for day-time NR cannot be < + 1. Turner's System is easily adapted to high-speed data processing and has been used to détermine Pasquill stability catégories for a large number of locations throughout the United States. The input variables are time, longitudelatitude co-ordinates (for solar altitude), cloud conditions, and wind speed and direction. The output gives the per cent frequency of each stability category by wind direction and speed, as shown in Table 9 for stability category D TABLE 9 Frequency (per cent) of Pasquill stability category D; Baltimore, Maryland; based on ailregular3-hourly weather observations, June-August, 1968; 1 knot = 0.515 m/s (data source: U.S. National Climatic Center, Asheville, N.C.)

Wind speed (knots) Direction 0-3 N NNE NE ENE E ESE SE SSE S SSW SW WSW W WNW NW NNW

0.2

* * * * *

0.3 0.1 0.2 0.1

* 0.2 0.1

* *

0.0

4-6 0.3 0.5 0.3 0.4 0.8 0.4 0.3 1.1 0.9 0.1 0.4 0.8 1.1 0.3 0.1 0.0

7- 10

/ / - 16

0.3 0.1 0.1 0.1 1.5 0.4 0.5 0.8 1.4 0.7 0.4 0.4 1.6 0.5 1.1 0.4

0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.4 0.0 0.1 0.4 0.9 0.8 0.3 0.3

Total frequency of D stability = 23.1 % Frequency of calms distributed with D stability = 0.4 % •Indicates < 0.05

17-21 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0

> 21

Total

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.7 0.7 0.4 0.6 2.5 0.8 1.1 2-0 2.9 1.0 1.0 1.8 3.7 1.6 1.6 0.7

26

CLIMATOLOGICAL ASPECTS OF THE COMPOSITION AND POLLUTION OF THE ATMOSPHERE

(i.e. neutral). The occurrences of "calms" hâve been distributed among the directions by the method that was described earlier in this report. Table 10 shows the annual frequencies of stability catégories (ail wind directions and speeds combined) for a number of cities throughout the United States. Data like those in Table 9 hâve been used to generate the basic transport and diffusion inputs to a mathematical model that calculâtes the field of long-term average pollutant concentrations due to multiple source émissions in an urban area (Calder, 1973). TABLE 10 Annual per cent fiequency of Pasquill stability catégories for ail wind directions and speeds (data source: U.S. National Climatic Center, Asheville, N.C.)

Pasquill stability category

Birmingham, Alabama . Tucson, Arizona Los Angeles, California. Miami, Florida . . . Chicago, Illinois . . . New York, New York . Philadelphia, Pennsylvania

A

B

C

D

E

1 2 0 0 1 0 0

7 10 4 5 5 3 5

12 14 15 14 11 10 11

44 33 48 42 55 67 51

36* 41* 13 39* 12 13 14

F

19 17 6 18

*Indicates E and F catégories combined.

Wind data alone hâve been processed to obtain diffusion parameters. As discussed by Slade (1968), the standard déviation of horizontal or vertical wind direction fluctuations may be obtained from a continuous record of direction. The standard déviation, a, is obtained from the values of average direction for a relatively small averaging time, t (e.g. seconds), that occur during a much longer sampling time, T (e.g. tens of minutes). In gênerai it is found that a increases with increasing T up to some point, and a decreases with increasing t. Standard déviations may be determined by hand-calculations, by high-speed computers using digital data, or by electronic equipment that directly processes wind sensor signais. A much simpler estimate of a, though less précise, may be obtained from the range of wind direction over a specified sampling time. Markee (1963), using horizontal wind direction data, has shown that the average of many range values divided by about 6.0 gives a good approximation of the horizontal standard déviation, OQ, based on an averaging time, f, of roughly one to ten seconds. Thus, climatological data on horizontal wind direction range, or on Oa, may be obtained rather easily from continuous wind records. Summaries by hour and by month or season are of interest (e.g. Slade, 1968, p. 52). The usefulness of such OQ data is greatly enhanced by their relationship to the Pasquill stability catégories : Pasquill stability catégories

°e

A, extremely unstable B, moderately unstable C, slightly unstable D, neutral E, slightly stable F, moderately stable

25 e 20° 15° 10e 5e 3°

Thèse relationships (Slade, 1968, p. 102) are based on expérimental diffusion data and on OQ values for short averaging times, t, and sampling times, T, of min. to 60 min. The same relationship has been specified by the U.S. Atomic Energy Commission (1972), except that for category F they give OQ = 2.5° and include a category G (extremely stable) for which a« = 1.3°; they also specify a température lapse rate (°C/100 m) for each stability category (A, B, ..., G) with values of >1.9, 1.9 to 1.7, 1.7 to 1.5, 1.5 to 0.5, 0.5 to - 1 . 5 , -1.5 to - 4 . 0 , < - 4 . 0 ,

METEOROLOGICAL VARIABLES

27

respectively. It should be appreciated that thèse relationships are only approximate, depending, e.g. for lapse rate, on the height interval over which température différence is measured, and for a^, on the instrumentation, exposure, sampling and averaging times.

Fog Fog can be an important factor in the meteorological potential for épisodes of air pollution because it atténuâtes incoming solar radiation and thereby inhibits the normal development of mixing during the day-time. As it is very well known, radiation fog has occurred during many of the severe air-pollution épisodes of the world ; it has been characteristic of London smog épisodes. In récent years, however, the frequency and intensity (i.e. in terms of visibility) of fog in London and vicinity hâve declined markedly (Jenkins, 1971 ; and Kelly, 1971). This improvement has been attributed in gênerai to a trend of decreasing smoke émissions. When such fog occurs, it is usually at night or in the early morning before being "bumed off" (i.e. evaporated) by solar heating. As Jenkins (1971) points out in his London study, a major effect of highly concentrated smoke is to delay clearance of the fog. Although radiation fog is a sufficient indicator of inhibited dispersion, its occurrence dépends on the moisture content of the air, and therefore, fog is not a necessary indicator of slow dispersion.

Précipitation Précipitation has an important effect on air quality because it is a major factor in cleansing the atmosphère. For example in considering the sulphur cycle on a global scale Kellogg et aL, (1972) estimated that 86 per cent of the atmospheric sulphur is deposited at the Earth's surface by précipitation. Sulphur and other pollutants are captured by précipitation éléments in two ways: rainout (or snowout) refers to capture within clouds (e.g. nucleation), whereas washout refers to capture below clouds. The important chemical and physical processes are quite complicated and dépend on detailed information about the précipitation and poilu tant éléments. In addition, after capture a pollutant may change its phase (i.e. state) or escape. For example, S0 2 gas that is absorbed into a raindrop may be converted to sulphate, depending on the acidity of the drop, or when the drop falls to a level where the atmospheric concentration of S0 2 decreases, the dissloved S0 2 may evaporate from the drop. Theoretical and expérimental approaches to the problem hâve been discussed by Munn et al. (1972), Slade (1968), and Haies (1972). But since the necessary detailed data are rarely available, the effect of précipitation on the meteorological potential for air pollution has not been considered in quantitative terms. The usual approach is to assume that précipitation has an overriding bénéficiai effect on air quality. Obviously, this is a great oversimplification although it is supported by the fact that in addition to the cleansing effects of précipitation, the meteorological processes associated with précipitation often favour enhanced dilution (e.g., convective storms, frontal passages, etc.). The fact that a large proportion of many airborne pollutants are removed in précipitation and deposited at the surface makes clear the rôle that meteorology can play in pollution of the Earth's surface. Such a large-scale effect is now being realized in northem Europe where acid précipitation, mainly from sulphur compounds in pollutant émissions, is having a deleterious effect on the soil and water of Scandinavia. Although the matter is not completely resolved, the transport of sulphur pollutants over long distances seems to be an important factor in this problem (Rodhe, 1972; F0rland, 1973; Nyberg, 1970).

STAGNATION

Atmospheric stagnation may be defined as slow and/or limited dispersion that occurs over a large area for at least a day. When it occurs over a city, the dispersion of much of the poilu tan ts emitted by various and diverse sources is generally suppressed, resulting in a wide-spread occurrence throughout the city of higher pollutant concentrations than are normally experienced. Such occurrences of stagnation are often associated with a warm-type anticyclone that can affect a particular area for several successive days. The undesirable features of warm anticyclones are that characteristically they generate slow winds through a deep layer ; they are relatively dry and without précipitation, favouring the formation of low-level radiation-type inversions ; they produce subsidence that suppresses the vertical extent of day-time mixing, and they migrate slowly. Such features for the United States hâve been discussed by Niemeyer (1960), Boettger (1961), and Holzworth (1969); for the U.S.S.R. by SonTcin (1968) and Bezuglaja (1968); and for Britain by Meade (1959). The effect of a warm anticyclone on ground-level concentrations of slow-reacting pollutants over a city dépends on the interplay among time and space variations of pollutant émissions and of atmospheric transport and diffusion. For example, consider only the interplay of the diurnal variation in vertical mixing on pollutants emitted near ground-level and emitted from tall chimneys. At night when lowlevel température inversions are common, the émissions from tall chimneys may rise above the inversion top or may rise only to some height within the inversion layer. In either case, with the meteorological conditions at the time of the example, the chimney émissions would not be contributing significantly to the ground-level concentrations, although ground-level émissions could be contributing very significantly to the ground-level concentrations. However, at a later time (e.g. after sunrise) when in the course of its usual diurnal variation the top of the mixing layer reaches the height of the chimney plumes, those chimney émissions may then contribute significantly to groundlevel concentrations (e.g. see Garnett, 1971 and Holzworth, 1972b). But the contribution of ground-level sources to ground-level concentrations then would be reduced accordingly. Thus, as discussed by Berlyand et al. (1972), the appropriate définition of atmospheric stagnation for each city may dépend upon the relative amounts of émissions from high- and low-level pollutant sources, their diurnal variation, and the diurnal variations of other meteorological factors, e.g. heights of inversion base and top, patterns of air flow, etc. In the United States, however, even during air-pollution épisodes, the afternoon mixing heights ordinarily hâve reached the heights of the pollutant plumes from ail but perhaps the very largest chimneys. For example, in the middle of the very severe air-pollution and stagnation épisode in the eastern United States of 27 November to 5 December 1962 (Lynn et ai, 1964), the aftemoon mixing heights were mostly above 500 m, though less than 1000 m (Holzworth, 1969). Similarly, during the Thanksgiving week 1966 épisode in New York City, the afternoon mixing heights exceeded 500 m except that on one unusual afternoon they reached only about 150 m (Holzworth 1972b). In any case it is clear that the définition of atmospheric stagnation is a complicated matter that has been approached climatologically from a number of aspects, although each deals with rather simple meteorological parameters due to the nature of available data. Korshover (1971) has determined the frequency in the eastern United States of areas experiencing épisodes of slow winds in association with stagnating anticyclones. Using the United States Daily Weather Maps for 1936-1970, he determined those locations (grid points at two-degree intervais of longitude and latitude) where the geostrophic wind speed was less than 15 knots (7.7 m/s) for at least four consécutive days, where there were no frontal zones, no précipitation, and no troughs in the flow patterns at either the 700-mb or the 500-mb level (the former used in the earlier part of the study period and the latter in the later part for convenience). Korshover found that generally it was a warm anticyclone that met his stagnation criteria. His study was confined to the United States east of the Rocky Mountains because in areas of irregular and high terrain the réduction of atmospheric pressure to sea-level often results in fïctitious or artifïcial pressure gradients. Korshover présents various summaries of his data, including maps with isopleths of the total number of stagnation cases (four days or longer) and the total number of stagna-

29

STAGNATION

tion days (e.g. see Munn et ai, 1972). The area of greatest frequency is centred over the states of Georgia and South Carolina, and amounts to about 90 cases and 350 days in 35 years, which average two and one-half stagnation cases and ten stagnation days per year. Further west, over the plains area, stagnation (by Korshover's définition) never occurred in the 35 years reviewed. Stagnation was most fréquent in October with a secondary peak in MayJune. The longest case of stagnation lasted 17 days. Using five years of daily values of morning and afternoon mixing heights and wind speeds averaged through the mixing layers for each of 62 rawinsonde stations in the United States, Holzworth (1972a) has objectively determined the frequency of épisodes of various durations during which various specified limiting values on mixing height and wind speed were not exceeded and during which signifîcant précipitation did not occur. Figure 15 shows the frequency of episode-days for two-day or longer épisodes with mixing heights >

W \ 18*42 i

j 28*71 \

24*54 | A Z_A., • .

,_,

( S

Y/

'

f( / 12*28'

k£V2

NOTE: 1S0PLETHS FOR DATA AT SAN DIEGO, CALIF0RNIA WERE LEFT INCOMPLETE FOR CLARITY

Figure 15 - Isopleths of total number of episode-days in five years with mixing heights < 1500 m, wind speeds < 4.0 m/s, and no signifîcant précipitation - for épisodes lasting at least two days. Dots indicate rawinsonde stations ; numerals on left and right give total number of épisodes and episode-days, respectively. Season with greatest number of episode-days indicated as winter, spring, summer, or autumn (after Holwzorth, 1972a)

30

CLIMATOLOGICAL ASPECTS OF THE COMPOSITION AND POLLUTION OF THE ATMOSPHERE

The persistence of slow wind speeds in Canada has been presented by Shaw et al (1971) as a factor in the meteorological potential for air pollution (i.e. stagnation). They performed a sequential analysis of ten years of hourly wind speeds for each of 111 stations (located mostly in southem and western Canada) to détermine the seasonal frequencies of two non-overlapping periods, 24-47 hours and longer than 47 hours, during which the wind speeds did not exceed seven miles/hr (3.1 m/s). In gênerai the minimum and maximum seasonal frequencies occurred in the spring and winter, respectively ; the frequencies tended to be larger in the western mountains than elsewhere. Considering both persistence periods together, the largest frequencies in the mountains averaged about twenty per winter season, while to the east the highest frequencies were about half that number. Stagnation conditions throughout the U.S.S.R. hâve been studied by Bezuglaja (1968), based mainly on an analysis of five years of surface wind-speed observations for January, April, July, and October at 220 well-exposed stations. Stagnation was defined as a 24-hour period during which ail wind speeds did not exceed 1.0 m/s. Such instances were generally found to be associated with the central parts of stationary anticyclones or with fields of weak pressure gradient. Bezuglaja summarized his investigation as follows : "In the western part of the European U.S.S.R., including the western Ukraine, interiors of the Baltic région, and Byelorussia, stagnation averaged 1-5 days/month in each of the mid-season months; the same frequency occurred on the eastem and western slopes of the Ural Mountains and in the forest zone of western Siberia, except that in the latter région stagnation was very rare in months other than January. Coastal zones, steppe and forest-steppe zones of the European U.S.S.R., and northern Kazakhstan expérience stagnation very rarely, i.e. less than one day in three to four years for ail the months that were studied. In the mountains of Transcaucasia and central Asia the frequencies of stagnation are complicated, although some cities expérience high frequencies (e.g. Erevan 18, Fergana 17, Alma-Ata 13, and Horog 18 days/month). Eastern Siberia and non-coastal régions of the Far East are dominated by an anticyclone in winter, which is highly favorable to stagnation. Frequencies are generally greater than five days/month but in several towns they average 20-25 days/month in winter, and 10-13 days/month in the other mid-season months." Using the aforementioned stagnation data, wind speeds at 500 m, and inversion data, Bezuglaja's (1968) estimation of the distribution of hazardous pollution conditions does not generally appear to be significantly différent from that for stagnation alone. According to Jost (1970) stagnating high-pressure régions with poor air exchange (i.e. slow wind speeds and stable atmospheric stratification) lasting several days over Central Europe had the following frequencies for the period October — March over a ten-year span: 35 times, lasting three to five days; 15 times, lasting five to seven days; and four times lasting seven to ten days. Jost made a very valid point that under such stagnation conditions the realization of high poilu tant concentrations dépends upon the amounts of pollutants emitted. He compared two stagnation situations with similar meteorological conditions, except that the ground-level air températures were considerably différent, and ascribed the higher S0 2 concentrations in the colder stagnation espisode to the impact, at least in part, of température on fuel consumption and S0 2 émissions. For both Tokyo (Anonymous, 1970a) and Osaka (Anonymous, 1970b), Japan, a considérable effort has gone into the analysis of relationships between city-wide S0 2 pollution and meteorological variables, including classifications of synoptic-scale, sea-level pressure distributions. Such classifications are dépendent, of course, upon the synoptic-scale climate of the area. Briefly, Japan is in a région that frequently expériences migrating anticyclones and, alternately, subtropical cyclones and weather fronts. Migratory anticyclones are particularly common during the winter season when they frequently originate as an extension of the Siberian anticyclone. As they move across Japan, they often resuit in a few cold days followed by a few warm days. If the pressure gradient is weak, the potential for air pollution is heightened. One of the higher potential situations occurs with high pressure Systems that produce warm and balmy weather in Japan, analogous to the American "Indian summer" weather, both being most common in late autumn. Subtropical cyclones ordinarily move north-eastward or eastward as they cross Japan. Pollutant concentrations tend to increase when cities are in the area which is being approached by a front. In such

STAGNATION

31

situations the advection of warm air aloft and the occurrence of clouds, which reduce the insolation received at the ground, tend to enhance the atmospheric stability. Sea-level pressure patterns that are associated with high pollution in Osaka hâve been classified as follows (Anonymous, 1970b): 1. Japan is near the centre of an anticyclone or in a ridge (or lobe) of high pressure. 2. Japan is covered by an extension of a Siberian anticyclone with pressure in the extension weakening due to a trough in the pressure pattern aloft. 3. When clouds form on the back (i.e. western) side of a migrating anticyclone with a cyclone approaching from the west. 4. When there is a cold front to the west of Osaka and warm air advection aloft is occurring. 5. When there are centres of small cyclones in the Sea of Japan and in the Pacific Océan with Osaka between them and with a very small pressure gradient; or when Osaka is in a local (i.e. small) anticyclone or pressure trough." Similarly, for high pollution in Tokyo the synoptic patterns (see Figure 16 hâve been classified as follows (Anonymous, 1970a): Aj.

Centre of a drifting high-pressure zone located in central Japan.

A2 • Central Japan is located on the back side of a drifting high-pressure area. B.

Pressure is high to west of Japan and low to east (winter, snow type).

Ci.

Pressure is high to south of Japan and low to north with an accompanying front.

C2.

Pressure is high to south of Japan and low to north but occurs only in summer.

F,.

A strong front to the south of Japan and a south wind at Tokyo.*

F 2 • A strong front to the south of Japan without a south wind at Tokyo From the définitions and examples in Figure 16 it appears that considérable expérience is required to classify confidently the synoptic patterns associated with high pollution in Japan. Yet much practical use, including forecasting, is being made of this approach. For example, days during 1966-1968 when the S0 2 concentration in Tokyo exceeded 0.2 ppm for more than six hours were summarized according to the synoptic classification as follows : A1 = 17, A2 = 19, B = 14, Cj = 44, C2 = 11, Fi = 18, and F 2 = 11. Thus, it is clear that in Japan the stagnation anticyclone is not the common weather situation associated with high pollution as in other countries. It is of interest in passing that the above-mentioned high-pollution days in Tokyo occurred seasonally as follows : winter = 57, spring = 31, summer = 30, and autumn = 16. Furthermore, of thèse 134 pollution days, 45 % occurred on two or three consécutive days. Although this section has focused on the gênerai, large-scale features of atmospheric stagnation, it should be appreciated that such features may at times be overwhelmed by local or meso-scale phenomena. For example, Lyons and Olsson (1972) hâve described the very significant effect of the lake breeze on the recirculation of pollutants in the vicinity of Chicago, noting that during summer the lake breeze occurs on at least 35 % of the days. Such effects are, of course, most conspicious in areas of non-uniform or irregular terrain, which characterizes the locations of many of the prominent cities of the world.

*In connexion with this classification it is of interest that an air-pollution incident in New York City has been attributed largely to the effects of a stationary front (Nudelman and Frizzola, 1974).

32

CLIMATOLOGICAL ASPECTS OF THE COMPOSITION A N D POLLUTION O F THE ATMOSPHERE

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