Global air pollution—meteorological aspects

Global air pollution—meteorological aspects

Atmospheric Enrironment Pergamon Press 1971. Vol. 5, pp. 363402. Printed REVIEW in Great Britain. PAPER GLOBAL AIR POLLUTION-METEOROLOGICAL ASPEC...

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Atmospheric Enrironment Pergamon

Press 1971. Vol. 5, pp. 363402. Printed

REVIEW

in Great Britain.

PAPER

GLOBAL AIR POLLUTION-METEOROLOGICAL ASPECTS A SURVEY R. E. International

and B. BOLIN

MUNN*

Institute of Meteorology,

University of Stockholm, Swedent

(First receiced 2S October 1970 and in final form 11 December

1970)

1. INTRODUCTION

and time scales are important considerations in the study of air quality, as indeed they are in the investigation of many other geophysical processes. A plume of smoke carried forward by the wind expands and stretches into an increasing volume of air. The classical problem, the determination of ground-level concentrations a few hundred meters downwind from a source, encompasses only relatively small time and space scales: the usual assumptions of time independence (steady-state meteorological conditions) and space independence (homogeneous turbulence) are then often realistic. For distant transport, however, vertical diffusion through the entire troposphere may occur, while forward movement and cross-wind diffusion frequently carry the pollution from region to region and from continent to continent. On these space scales, the plume meanders in response to synoptic influences, and the centre-line is rarely straight, as assumed for the close-in models, even when averaged over a long time. There may in fact be discontinuities, associated for example, with wind shifts at fronts. No longer too on these space scales can the diurnal meteorological cycle be ignored, nor can the underlying surface be assumed to be homogeneous: after air moves from prairie to forest or from land to ocean, there are changes in the turbulence regimes and in the diffusion rates. The structure of the surface boundary layer is reasonably well understood but in the Ekman layer and above, there are few experimental measurements of wind fluctuations. For transport over less than a few kilometres (times of only an hour or so), the theoretical models of diffusion are often tested during ideal conditions by releasing tagged particles and monitoring the concentrations at various positions in the flow. Separate experiments are performed for each of a number of weather conditions, such as daytime lapse, night-time inversion and the neutral state. To construct an air pollution climatology for a specific location, then, the weather is assumed to change discretely at the end of each hour, and the separate results are averaged. For long-range transport, the time scales are too large to assume steady-state meteorological conditions during even a single experiment, because of the diurnal cycle of atmospheric stability. The classical Lagrangian experiment, in which a particle is released into a turbulent stream, with many repetitions in order to obtain meaningful statistics, is not possible because of non-stationarity in time and space. Another assumption implicit in most diffusion models is that all the pollution SPACE

* Visiting Scientist 1970. Present Address, Meteorological Service of Canada, Toronto, Canada. 7 Contribution

no. 228. 363

R. E. &Lib-xxand B. Born

361

remains in the atmosphere (perfect reflection at the interface, no precipitation jcavenging and no chemical reactions). The fractional depletions in the first few hundred meters of travel from a point source are usually within the limits of experimental error in mass balance determinations. The losses, however, may accumulate and become significant in calculations of distant-transport dilution rates. The World Meteorological Organization Panel on Meteorological Aspects of Air Pollution (WMO, 1970) has emphasized that virtually all dispersion formulations are only valid for stable pollutants, which is a great deficiency in distant-transport studies. In order to estimate regional and global budgets of air pollution, the investigator must therefore modify his methods, taking into account not only the processes of transport and of turbulent diffusion but also the source and sink mechanisms. Some success along these lines has been achieved, particularly for long averaging times and large areas. There are many published papers that relate to the problem of large-scale air pollution. Frequently, however, a particular substance is studied primarily as a tagged indicator of atmospheric motions and is used to infer some properties of the general circulation. In contrast, the problem is inverted in the following review. The general circulation is assumed known, and its effect on largescale transport of pollution from local chimneys and multiple urban sources is examined. Health officials and ecologists frequently seek information of this type concerning the extent of environmental degradation from distant emissions. The survey includes not only an examination of the relevant experimental data but also some suggestions regarding methodology and the design of meaningful experiments. In the first few sections, the emphasis is on the synoptic time scale. The review concludes with a consideration of some of the climatological aspects of the problem, and a discussion of monitoring networks. 2. TRANSPORT DISTANCES

AND DIFFUSION OVER UNDER STEADY-STATE

MODERATE TRAVEL CONDITIONS

The transport and diffusion of a passive pollutant, released in the surface boundary layer, has been widely studied theoretically and experimentally. For steady-state conditions over a uniform surface and for downwind distances ranging from a few hundred meters to a few kilometers, the Gaussian models give resonable predictions for averaging times of a half hour or so. Some progress has been made also in estimating the instantaneous peak concentrations, which can be many times greater than the mean values under certain conditions. For a continuous point source, the dilution rates are determined by the average wind, causing stretching of the plume, and by the turbulent wind components, causing lateral and vertical dispersion of the fluid elements. This latter process is usually parameterized by the standard deviations ov and u,, quantities which increase with downwind distance x, and which may be estimated from bi-directional vane observations, provided that the appropriate bandpass filters are applied. In the context of global pollution, this may be regarded as close-in transport, involving time scales of an hour or so and distance scales of less than a few kilometers. In recent years, the range of experimental sampling downwind from a point source has increased to a few hundred kilometers, and there is reasonable evidence that under steady-state meteorological conditions, prediction methods developed for a close-in diffusion can be extrapolated to these distances over a uniform surface. We call this the moclerate-travel scale. In this range, the chances of a high 10-s peak concentration

Global Air PoUution-~~eteoroio~~

365

Aspects

to the hourly mean value) decrease considerably because fluid elements have more time to mix thoroughly with their environment. As an example of moderate travel, PETERSON (1968) used an aircraft on one occasion to monitor the argon-41 emitted at ambient temperature from a 105-m stack at Brookhaven, NY. Winds were westerly and the plume was detected over the Atlantic to a distance of 300 km (the outer limit of aircraft sampling). This distance corresponded to a plume travel time of about 10-12 h. Because of the over-water trajectory, however, the diurnal cycle in the boundary layer was damped, and the temperature lapse rate remained near neutral. This was therefore a rather special case. Plume centroid concentrations, corrected for radioactive decay, decreased by almost an order of magnitude between 50 and 300 km, as wouid be expected from an extrapolation of close-in statistical models. Thus the lateral standard deviation of the plume could be fitted by extension of the Pasquiil D curve (PASQUILL, 1962), shown in FIG. 1. It is noted, however, that because aircraft sampling is rapid, the standard deviations do not include a significant contribution from low-frequency swings of the wind (relative

lkml /

1

/

/

/

./

/

/

/

/

/

/ ’

/

/

(n.mJ 0

I

~ -

0.1

Rc. 1. Lateral standard deviation (6,) of the Brookhaven argon41 plume as a function of downwind distance as measured by an aircraft over the Atlantic on the afternoon of Nov. 30, 1966. The Pasquiil~ifford Type D curve is given for comparison @TERsON, 1968)

366

R. E. MUM and B.

BOLIN

(meandering). A final feature of interest was that vertical cross-sections at downwind distances of 200 and 260 km revealed that the plume centroid had risen to heights of 220 and 240 m, respectively, somewhat above the chimney height of 105 m: data observed elsewhere yield qualitatively similar results (e.g., fhKURABh, 1969) and it is probable that the bottom of the plume was subjected to enhanced mixing in the surface boundary layer, resulting in an apparent rise in the height of the centroid. Absorption at the sea surface might be a contributing factor. The effect of wind shear on diffusion has been examined experimentally by PASQUILL (1962) using data obtained during almost steady state conditions over open country in England. Zinc cadmium sulphide was selected as tracer, and sampling was undertaken to downwind distances of 130 km. There were some releases from a continuous surface point source and others from an instantaneous elevated (usually at 300 m) line source. During conditions of good vertical mixing (mainly daytimz convection), the variation with height of the direction and speed of travel of the tagged particles was small, largely because the vertical wind shear was also small. The plume then moved with an effective velocity close to the mean wind velocity through the layer of vertical mixing. In contrast, the intensity of turbulence was low during a few trials, and the tracer cloud was sheared vertically at moderate travel speeds. Pasquill suggests that although this tilting of the vertical axis of the plume has no immediate effect on ground-level concentrations, lateral spread will be enhanced if at some subsequent time, the level of vertical turbulence increases. Subsequent analyses have been undertaken by SAFFMAN (1962), HGGSTR&I (1964), TYLDESLEY and WALLINGTON (1965), SMITH (1965) and CSANADY (1969). Experimental verification for moderate travel distances remains scanty, howei-er. One of the difficulties is that most of the models assume that the wind shear is constant with height and time, and that the eddy diffusivity is a well-behaved function. With diffusion into the middle troposphere, these assumptions are rarely realized. For close-in diffusion, turbulence in the surface boundary layer can be measured directly or can be estimated from similarity theory. For moderate distances, holyever, the mesometeorological time and space scales become important. Very little is known about the detailed structure of the Ekman layer and above, particularly during groundbased radiation inversions. SLADE (1969), for example, has described some cases of almost laminar flow at heights of a few hundred metres. Thus the assumption of a Pasquill-Gifford type D diffusion regime is not always valid. To overcome these difficulties, constant-level tetroons have been employed to estimate directly the Langrangian statistics of diffusion. The method has been described by ANGELL (1961) and by ISLITZER and SLADE (1968, pp. 179) and gives reasonable estimates of cross-wind dispersion under steady-state conditions. More studies of this type are recommended. The tetroon data, as well as vertical profiles of pollution, indicate the existence in the daytime of a well-mixed layer, extending upwards several kilometers above the surface on some occasions. The top of this layer is called the mixing height (or nzi.~in,o depth). Whenever this is well-defined, due to a capping inversion, for example, the problem of diffusion from a point source (within the layer) is simplified somewhat by assuming a uniform vertical distribution of the pollutants. The method, of course, is not valid for close-in diffusion (because the plume is still diffusing vertically) nor is it

Global Air Pollution-Meteorological

367

Aspects

appropriate on any scale whenever the mixing height is poorly defined or is varying greatly in time and space. Nevertheless, the assumption of uniform concentrations in the vertical often is reasonable during daytime convection and has been verified experimentally on a number of occasions, e.g., by THOMPSON (1964) for downwind distances of 130 km from a line source of fluorescent particles released from an aircraft at a height of 300 m over rural England. In close-in diffusion studies, a chimney of finite width is usually idealized by a point source. Similarly for a sufficiently large scale, a city may be considered as a point source. CLARKE(1969) has presented experimental evidence for the existence of a thermal plume (presumably containing pollution) extending downwind from the city of Cincinnati during conditions of well-defined regional flow. The idealization is shown schematically in FIG. 2. The dimensions of the plume and its rate of dovvnwind expansion remain to be documented quantitatively for Cincinnati and elsewhere

RURAL

RURAL

INVERSION

INVERSION n

/

//////////////////

FIG. 2. Schematic

representation of the urban thermal and pollution downwind during clear nights.

plume that extends

but there is no reason to doubt that the pollution becomes well mixed to form a single urban plume after travelling a few hundred kilometers. Close-in, there will naturally be a separation of low-level emissions from those originating from tall chimneys. Detailed investigation of the urban plume can only be undertaken with an aircraft. Experimental probing will most likely be revealing as regards the possible application of point-source models if performed downwind of a rather isolated city surrounded by relatively uniform flat countryside during periods of clear skies. The time of sampling should preferably be just before sunrise, when the plume is disconnected from surface influences by a ground inversion over rural areas. In the daytime, the vertical distribution of concentrations is likely to be quite uniform through the entire mixed layer, extending up to the middle troposphere on many occasions. 3. TRANSPORT AND DIFFUSION OVER MODERATE TRAVEL DISTANCES DURING NON-STEADY-STATE CONDITIONS OR OVER INHO&IOGENEOUS SURFACES A problem that occurs even during close-in diffusion trials is that the wind cannot always be considered as a stationary time series during the sampling period, often simply because only part of a slow oscillation of the wind is included in the sampling

R. E. Mum

368

and B. BOUN

interval. This invalidates an important assumption, namely that there is a Gaussian form for the distribution of turbulent fluctuations and for the cross-wind spread of the tracer. The plume therefore fans out over a wider arc than is expected from the models. As travel distance extends beyond 100 km, the relevance of classical diffusion theory becomes more suspect for the following reasons : (a) There is a diurnal cycle of wind in the boundary layer (for 12 h of plume travel, only half of the daily oscillation is sampled). The diurnal cycle is particularly important wherever there are mesoscale circulations such as near coastlines or along slopes. Macroscale weather systems too may cause abrupt wind shifts and changes in the direction of plume travel. (b) There is a diurnal cycle in the thickness of the mixed layer. For steady-state clear-sky conditions, the variation is shown schematically in FIG. 3. By early afternoon at some distance from a point source, there is a uniform vertical distribution of pollution through the mixed layer. When the ground-based radiation inversion begins to form in the evening, however, the pollution emitted within it is trapped until convection begins again on the following morning. At certain times of day and also on occasions when the afternoon maximum mixing height is lower than it was on the previous afternoon, vertical profiles of pollution may show a rather complex layered structure. (c) In most cases the sources and sinks vary in ways which are not well known, and the resulting distributions models based on diffusion

of ground-level concentrations processes alone.

MIDDLE

MIXED

LAYER

AND

MIXED

UPPER

cannot

be used to verify

TROPOSPHERE

LAYER

MIXED

LAYER

MIXED

LAYER

” (HIGH OF

INTENSITY

/

TURBULEiCE)

/

(LOW OF

INTENSITY

TURBULENCE)

(HIGH OF

INTENSITY

TURBULENCE)

INVERSION

MIDNIGHT

I MIDNIGHT

NOON

TIME

NOON

_j

FIG. 3. Schematic representation of the structure of the lower troposphere during a period of

fine weather.

Global Air Pollution-~¶eteoroIog~calAspects

369

(d) The probability increases with time that pollution will participate in atmospheric chemical reactions or will be drawn into a cloud or a precipitation system. The preceding discussion illustrates the fact that it is not very useful to base a study of moderate-scale diffusion on a steady-state model as is done for close-in investigations. This would not be an important practical consideration if the assumption could be made that the most serious pollution incidents occur with “ideal” conditions, that all other physical mechanisms (such as stabifity changes in space and time} tend to increase diffusion rates: then, at least an upper limit for down-wind ground-level concentrations could be determined experimentally. Unfortunately, however, there is some evidence to the contrary. The morning break-up of an inversion causes a peak in ground-level concentrations downwind from an elevated source (the Hewson fumigation effect). As another example, convergent winds have been offered as an explanation for the formation of locust swarms in Africa (SCORER,1965) and for the wall of pollution along lake-breeze fronts in Chicago (LYONS, 1969). The classicai Gaussian point-source model does not apply in any of these cases. As an example of non-steady-state conditions, FIG. 4 displays isopleths of spore soncentrations over the North Sea as measured on July 16, 1964 by an aircraft of the meteorological Research Flight, England (FIRST et al., 1967). Winds were moderate westsouthwesterly, and the source region for the spores was the British Isles. The downwind sampling was to a distance of about 700 km from the English coast, i.e., to a point just north of Denmark. The source strength, of course, is not well defined but it is known to have a diurnal variation. The Cladosporium and pollens are emitted during the daytime whereas the damp-air types (bottom panel in FIG. 4) are liberated at night. The measurements were made in the late morning during fine weather and reveal a number of interesting features. {a) For CIadospor~um and pollens, there was a peak near the English coast-line (the edge of the cloud being generated on July 16), another cloud at a downwind distance of about 400 km (the July 15 release) and possibly a third peak, in the case of Cludosporium, at a distance of about 650 km (perhaps the July 14 release). (b) For damp-air types, the peaks occurred at downwind distance of about 150 and 500 km, out of phase with the other peaks, and presumabIy resulting from releases on each of the two preceding nights. (c) Although the releases were at ground-Ievel, the peak concentrations in FIG. 4 were at heights of from 500 to 1000 m. HIFST et al. (1967) suggest that the clouds were carried aloft by daytime convective cells, which tended in a few hours to produce a uniform distribution over the source region through the mixed layer. Over the North Sea, however, the bottom part of the cloud was eroded by sedimentation and impaction at the sea surface, The fact that the centroid of the pollen cloud was at a lower level than that of Cladosporium suggests that the former had a larger fall velocity. Visual observations indicated that the top of the haze layer (the mixing height) was about 1500 m near the coast of England, lowering to about 1000 m over the eastern part of the flight. These data therefore support the view that daytime convective mixing is quite efficient through this layer. There is a loss, of course, at the

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FIG. 4. Vertical temperature profiles (top) and isospore diagrams of Cludusporium, pollen and damp-air types. The abscissa scale is down-wind distance over the North Sea from the English coastline (HIIIRST et al., 1967).

Global Air Pollution-Meteorological Aspects

371

underlying surface and occasionally aloft through the mechanism of penetrative convection. HIRSTand HURST(1967) have found intermittent pockets of high concentrations of spores within a capping inversion, with an almost uniform afternoon distribution through the mixed layer below. As a second example, the Windscale reactor accident in Northern England lasting from about 16 GMT October 10 to 11 GMT October 11, 1957 provided an opportunity to examine the effect of disturbed synoptic patterns on transport. In this case, the principal tracer was iodine-l 31. Chimney height was 125 m but there was additional thermal rise. Winds at the site were light southwesterly at first, shifting to moderate northaesterly with the passage of a cold front about 01-02 GMT October 11. The close-in deposition has been described by CHAMBERLAIN (1959) and the moderate-scale transport by CRABTREE (1959). A maximum fallout occurred at a distance of 6 km with a secondary peak at 20 km (over a mountain, Black Combe: probably caused by washout in showers). The cold front that passed Windscale early on October 1Ith continued across England, crossing the southeast coast by mid-evening. Thereafter it moved into continental Europe but became very weak and difficult to follow. The data available for studying the motion of the cloud consisted only of 24-h average air concentrations from a network of European stations. These data were too coarse for detailed analysis but they did show that part of the cloud spread into Belgium, Holland, Germany and ultimately (October 14-15) into southern Scandinavia. That the analysis is so incomplete is due partly to the fact that no vertical profiles were available and partly to the fact that a cold front passed the station during the release, complicating the determination of subsequent trajectories. As a final example, PETERSON and DRURY(1967) observed from an aircraft the smoke produced by a series of small forest fires within the boreal forest near Yellowknife in the Canadian Northwest Territories. Winds were southsouthwesterly, carrying the smoke northward for a distance of 30 km or so over forested country until the treeline was crossed, after which the trajectory was over subarctic tundra. The plumes maintained their individual identities until they reached the tree-line. At that point, however, they merged abruptly, and the smoke extended from the surface to a height of about 1500 m. Measurements with a pyranometer on the aircraft indicated that at a height of 1200 m, the smoke caused an average attenuation of about 25 per cent in the downward solar flux. Several hours later on the return leg of the flight, the same discontinuity was observed at the tree-line. This is an example of the complexity of the diffusion process over non-uniform surfaces. To summarize Sections 2 and 3, there is some hope that for inert pollutants under steady-state meteorological conditions over homogeneous surfaces, the standard methods for calculating diffusion can be extrapolated to moderate travel distances (of the order of a 100 km or so). Because steady-state conditions are the exception rather than the rule on these time and space scales, however, the applicability of the Gaussian model is limited. In particular, mention should be made of the fact that the classical diffusion equations are not valid during light variable winds. These are the very conditions that create high regional pollution potential. In such cases, and particularly when there is a capping inversion, a simple box model may be useful. The depth of the box is the mixing height. The other dimensions vary with the type of problem under consideration but they could, for example, be the length and width of a polluted valley.

R. E. Mum

372

4. LONG-RANGE

and B. BOUN TRANSPORT

In Section 2 the suggestion was made that a city can sometimes be regarded as a point source (FIG. 2). Extending the analogy, a densely-populated industrial region may perhaps be considered as a point source on the global scale, under certain synoptic conditions. As an example, industrial haze over the Eastern United States has been detected on satellite photographs by CLODNAN and TAGGART(1969), during a period when a warm anticyclone caused high pollution potential. As the high pressure area moved slowly eastward, the large shroud of haze moved also, being still visible east of Bermuda several days later, when there was clean air over the Eastern United States in the wake of a cold front. The movement of regional pollution can often be approximately inferred by a trajectory analysis. In most relevant published papers, however, the problem is inverted. Trajectories are determined backwards in time to infer the source region for

DATE

FIG.5. Atmospheric CO

concentrations on thz Greenland ice cap, 23 July-3 Aug., 1967 (ROBNONand ROBBIXS, 1969).

an anomalously high ground-level concentration observed at an isolated background sampling station. This technique has been used, for example, to suggest the source regions for unusual invasions of pollen, spores, diamond-back moths, spruce aphids, locusts and birds. As another example, ROBIXSOX and ROBBINS(1969) have monitored the CO concentrations on the Greenland ice cap during the summer of 1967. Their results, given in FIG. 5, reveal a threefold increase above background on July 29. The associated trajectory analysis (FIG. 6) suggests that the contaminated air had originated in the Northeastern United States. In view of the fact that small- and moderate-scale diffusion processes are constantly at work, single trajectory computations like those in FIG. 6 are of course often only indicative. In some cases, there are considerable differences in the trajectories computed for the surface boundary layer, the 850 mb and the 700 mb levels, for travel times of 3-5 days. A firm estimate of trajectories must then be based on a statistical treatment of a number of cases at different levels. Mention should also be made of the study by PROSPERO(1968) of dust concentra-

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Aspects

373

G. 6. Air mass trajectories

for July 24, July 29, and Aug. 3, 1967 (ROBIS~~N and ROBBKS, 1969) (based on geostrophic winds at 700 mb).

tions at Kitridge Point, Barbados in the Northeast Trades. With this wind direction, the nearest source region for dust is North Africa. The data show large day-to-day variations in dust loadings (up to a maximum 2-day mean of 26 pg m-‘) and also a marked annual cycle, summer values averaging an order-of-magnitude larger than winter ones. Prosper0 believes that the dust has indeed originated in North Africa, the peak values being associated with dust storms, and he estimates that the average time for a cloud to travel across the Atlantic is about 5 days. An apparent anomaly is the fact that North African dust storms are most frequent in winter, whereas loadings at the Barbados site are highest in summer. Prosper0 suggests, however, that because the Bermuda-Azores subtropical high pressure area is farthest south in winter, most dust clouds pass to the south of Barbados at that time of year. As a final exampIe, the great smoke pall of September 1950 should be mentioned. Forest ties burned out of control in northwestern Alberta, Canada during the week of 17 Sept., reaching a peak in the period 22-24 Sept. WEXLER (1950), SMITH (1950)

371

R. E. MIJXX and B.

Ebrrs

and BULL (1951) have described the circumstances surrounding the transport of the smoke pall, which became visible along the eastern American seaboard by 25 Sept., in the British isles by 26 Sept. and at Gibraltar on 28 Sept. The mean 700-mb chart for 23-27 Sept. (FIG. 7) illustrates the flow pattern (WEXLER,1950): the streamline patterns at 500 mb were similar. In the eastern United States, the smoke was confined mainly to the middle troposphere (between 2.5 and 4.5 km) but muhipie layers were reported by some pilots. Horizontal visibility near the ground remained excellent, although the sun took on a bluish colour and became partly obscured. Subsequently on 26 Sept., an aircraft pilot reported that he had passed through thick smoke at an elevation of 5 km over the Atlantic Ocean (50” N, 3525”W). On 27 Sept. a pilot flying near Cambridge, England reported brown haze and a smell of burnt paper in the layer between 9 and 9.6 km. Farther north at Leuchars, Scotland, the brown cloud was found between the levels 9.3 and 11.4 km, presumably capped by the tropopause. A trajectory analysis (WESLER,1950; SMITH, 1950) indicated that the smoke moved from Alberta southeastward along isentropic surfaces. Part of the cloud became trapped for several days between subsidence inversions in a stationary anticyclone over the eastern United States. The remainder moved across the Atlantic in the westerly fiow (See FIG. 7). Due to the variability of the large-scale wind field and its diffluent character, the smoke reached Europe as far north as Scandinavia and as far south as Gibraltar. The initial injection of smoke into the atmosphere over Alberta appears to be at heights of from 3 to 4 km, due to the fact that the air-mass was

FIG. 7. The 5-day mean 7CKLmbchart for 23-27 Sept., 1950. The source region in Aiberta for forest fires is indicated (WEXLER, 1950).

Global Air Pollution-Meteorological Aspects

315

conditionally unstable and that the forest fires provided an intense source of heat. Subsequent motions were therefore not dependent in any way on conditions in the surface mixed layer. These several examples indicate that on the synoptic scale for periods of a few days, pollution transport is essentially controlled by the large-scale 3-dimensional atmospheric air motions. Particularly in regions of strong horizontal or vertical gradients of wind, however, we cannot readily predict the motion of pollution due to uncertainties in the trajectory analysis (a lack of knowledge of the 3-dimensional wind field and/or of the height of the centroid of the plume or cluster). Even when the general motion of a plume can be estimated with some assurance, there is no way of obtaining satisfactory predictions of the concentrations at downwind distances of a 1000 km or so. Part of the problem is that the regional source strength and the downstream sink strengths are difficult to estimate. In addition, the cross-wind dispersion rate cYis not easy to predict at these distances. In most theoretical and experimental investigations, the analysis is formulated around one or more of the following well-known Langrangian relations : (a) Small times [point sources or clusters):

uY2at2

(1)

(b) Intermediate time (clusters) :

uy2 at’

(2)

(c) Large times (Fickian diffusion) (point source or clusters) : uy2 = 2Kt

(3)

where K is the turbulent diffusivity and t is travel time. A homogeneous stationary field of turbulence is assumed. The regime represented by equation (2) only occurs with volume-source releases (BATCHELOR,1950). The principal uncertainty is in the interpretation of the words “small”, “intermediate” and “large”. Frequently the data contain so much scatter that any one of the exponents in equations (l-3) yields an equally good fit, provided that there is an associated adjustment in the value of the constant of proportionality. Another uncertainty arises in the conversion from the Langrangian reference frame, in which the motions of tagged fluid elements are followed, to the Eulerian coordinate system, in which the cross-section of a plume or cloud is sampled at given downwind distance. For steady-state conditions over a uniform surface, the Pasquill method (PMQUILL, 1962, p. 88) is widely used. For distant transport, however, the assumption of stationarity is rarely valid. The spectrum of atmospheric turbulence contains what is commonly called red noise, i.e., considerable energy in the largest wavelengths. There is some question, therefore, about the relevance of equation (3) out-of-doors in contrast to the controlled flows in a wind tunnel. Plume width in the atmosphere therefore depends on the averaging time as well as on the travel time. Values of uYfor any given travel time increase as the sampling interval is extended from minutes to hours to days, because mesoscale meandering and macroscale changes in the flow then contribute to diffusion. Thus the limiting condition represented by equation (3) is approached more rapidly as the Langrangian averaging time is lengthened. On some occasions when there are spectral gaps separating the micro-, mesoand/or macro-scales of turbulence, some progress may be possible by assuming that

376

R. E. MUNNand

B. BOLIX

within each range, a plume may tend towards large-distance Fickian behaviour, with an associated typical value of K. When the plume grows to the point where the next larger scale of eddy energy begins to contribute significantly to diffusion, however, further expansion may revert to the rate given by equation (1) but with a Iarger constant of proportionality. After some time in the new regime, of course, Fickian diffusion may again be possible. This hypothesis quite naturaIly is tentative and wilf be very difficult to verify. The spectral gaps may be absent on occasion or they may not exist over a sufficiently broad range. In any event, interpretation of experimental results is complicated by lack of information concerning the concurrent spectrum of turbulence and by the great variety of averaging times. On the one hand, there are aircraft transects of plumes and clusters. On the other hand, dispersion is sometimes calculated from Langrangian experiments lasting several weeks or months using either constant-Ievel baIloons or numerical models in which the trajectories of fluid elements are foliowed at intervals of 12 or 24 h. HEFFTER,(1965) has summarized a number of estimates of o,, obtained from nuclear detonations, tetroons and transondes at heights ranging from 1.5 to 15 km. There is considerable scatter, as might be expected, but for a downwind distance of 1000 km, (sYis typically about 50 km, with extreme vaiues ranging from 1 to 100 km. Assuming a plume (or cloud) width of about 40,, diffusion must have been on the subsynoptic scale in many of these cases. The plumes were meandering in response to synoptic influences but with relatively small cross-wind spread. The numerical 500-mb experiments by MESINGERand MILOVANOVK~ (1963) on the two-dimensional rates of expansion of clusters of fluid elements, subjected only to macroscale flow deformations, also show this behaviour. After 8 days of travel, when a cluster was sometimes stretched halfway round the hemisphere, the cross-wind dimension was still rather small. Mesinger and MilovanoviC found, in fact, that equation (I) provided the best fit for their data. It should be noted, however, that the 500-mb wind field was assumed to be almost non-divergent. Thus the computed values of the variances were due entireIy to the large-scale deformation field, rather than to an increase in the initial area of the hypothetical cloud. PASQUILL(1962, p. 268) has noted that most of the experimentaf data do not support the idea of Fickian diffusion on the large scale. When observations of various kinds are combined (PASQUILL,1962, p. 169), the best fit seems to be given by an exponent of f that is intermediate between those of equations (1) and (3), but this may be due to the inclusion of data from different regimes. An estimate of Langrangian dispersion may be obtained by the geostrophic trajectory method. Fluid elements originating at the same point on consecutive upper-air charts are followed for travel times of several days. MLTGATROYD(1969), for example. has determined u.~and ov as a function of time, where a,, cryare the standard deviations of dispersion in the zonal (x) and meridional b) directions. He then examined the applicability of the relations: 1 Z 0.X = 2&t OY = 2K,,t (4 Some of his numerical calculations were for the troposphere (700 mb), for initial starting points of 40, 55 and 70”N, at 9O”W, and for subsets of 31 consecutive days in each of the four seasons. Murgatroyd found that the quantities oX2/2t and oY2/2t

Clobal Air Pollution-Meteorological Aspects

377

approached constant values after a few days in most cases. On these space and time

scales, therefore, Fickian diffusion, equation (3), seemed to apply. The resulting extrapolated estimates of diffusivity K ranged from 10'to lo7 rn’ s-l. The essence of the preceding discussion is that lateral dispersion cannot yet be predicted on the global scale. Similarly, the vertical extent of the polluted air cannot be inferred easiiy because of uncertainty in estimating the thickness of the surface mixed layer, which varies in space and time (see FIG. 3, for example) and because of our limited knowledge of vertical diffusion in the free atmosphere. We can only say that on the very largest scale, the tropopause may usually be assumed to act as an impenetrable upper lid. This is indicated by the aircraft measurements of CO, by BISCHOF (1965) and GEORGII and Josr (1969), which reveal a sudden change from quite variable conditions in the troposphere to almost constant values in the stratosphere. Similarly many visual observations from aircraft indicate that the tops of haze layers are at or below the tropopause, never above. For these reasons and because of the difficulty in estimating sink strengths (see Section 5), there is no possibility at the present time of estimating dilution rates on the global scale from classical diffusion theory. We are encouraged to believe, however, that some indication of the direction of motion of a plume or cloud can be obtained from a trajectory analysis in many cases. 5. POLLUTION

SINKS

In this and the next two sub-sections, we discuss in turn the three sinks for atmospheric pollution: losses due to tropospheric chemical reactions (including radioactive decay), precipitation scavenging, and depletion at the atmosphere-earth interface. No attempt will be made here to survey the very broad field of atmospheric chemistry: we restrict ourselves to only a few pertinent remarks. Additional information can be found, however, in a useful summary by ROBINSON and ROBBINS (1968). Many substances participate in atmospheric chemical reactions and isotope changes. Sometimes the rates are constant, and it is possible to determine a time constant for the process. PETERSON(196X), for example, was able to “correct” his downwind salues of argon-41 (obtained from aircraft sampling of the Brookhaven plume) (see Section 2) by using the value of 0.371 h-’ for the decay constant. More often than not, however, reaction rates are variable, depending not only on concentrations of the pollutant itself but also on those of other gases and aerosols in the same air-mass. Sulfur dioxide, for example, has a much shorter halflife in an urban area than in the stratosphere. Furthermore, conversions to new substances are not uncommon, e.g., from hydrocarbons and oxides of nitrogen to oxidants in the presence of sunlight, and from SO, to sulphuric acid during fog, There are three approaches that either avoid or exploit the fact that there are atmospheric chemical reactions. (a) A substance is studied that either is chemically inert or has a half-fife much longer than the time scaIe of interest. For downwind distances of a few hundred meters, SO2 is a suitable tracer while for global transport, COz and CO may be considered to be practically inert. (b) Instead of studying specific compounds, attention is directed to the chemical

R. E. Iclus~ and 3.

378

Botxs

elements themseives, thus by-passing the problem of atmospheric transformation. The gIoba1 elemental sulfur budget, for example, is easier to estimate than the separate ones for SOz and H,S. (c) When two or more compounds or isotopes with different reaction rates are monitored, the possibility often exists of inferring the principal transport and sink mechanisms. The larger the number of tracers, of course, the greater are the chances of obtaining a closed set of equations relating sources and sinks. This approach is of course well known in other fields of science such as medicine and biofogy, but has not been used widely in atmospheric studies simply because of the Iack of adequate data. Interesting results regarding tropospheric and stratospheric residence times have been obtained with the aid of cosmic-ray produced isotopes (LAL, 1963). The basic principle has also been exploited in studies of the oceans (KEELINGand BOLIN, 1967, 1968), and of the global seasonal atmospheric variations in COz and 0, (J~GE and CZEPLAK, 1968), for which the principal source regions are quite different. These kinds of studies will be considered further in Section 6. The study of atmospheric chemistry is interdisciplinary. Unfortunately, however, there is often a lack of communication between scientists who are separately interested in small segments of the same broad problem. 5.2

Precipitation

scarlenging on the synoptic scale

No one doubts that precipitation cIeans the air. This cannot be demonstrated, however, by asserting that air quality improves after a fall of rain or snow: what has happened in many cases is that a front has passed, the wind has shifted and the air has become more unstable. In fact, FLOWERSet al. (1969) have described a case in which heavy rain did not “clean” the troposphere. Their data were daily measurements of turbidity (as measured by a photometer at the half-micron waveiength) at Huron, SD., a Iocation far from industrial areas. The turbidity at this site climbed steadily during the period July 6-13, 1966. Although more than 2.5 cm of rain fen in air-mass showers on July 12, the upward trend in turbidity was not terminated until the passage of a cold front on JuIy 13. The showers, of course, may have scavenged mainly the large particles, those not contributing greatly to atmospheric turbidity: alternatively, the showers may have fallen through only a small fraction of the relevant tropospheric volume. BEILKEand GEORGII(1968) have studied the washout (below-cloud precipitation scavenging) of SO1 in the laboratory. For this gas, they have found an exponentiaf decrease with time in atmospheric concentrations, and their results may be summarized as foIlows :

(a) the rate of scavenging increases as the rainfall intensity increases, (b) for constant rainfall intensity, the rate decreases as the droplet size increases, although at intensities above 20 mm h- I, the rate becomes almost independent of droplet size, (c> the rate increases with increasing pH-value of the rain-water. Using a vertical distribution of SO, characteristic of the lowest 2 km in the vicinity of a Iarge city, BEILKEand GE~RG~I(I 968) have used their laboratory data to compute the relative contributions of washout and of rainout (in-cloud precipitation scaveng-

GlobaI

Xir

Po~l~~~on-meteorologicalhspects

379

ing) to the SO,-concentration in rainwater measured at ground-level. TABLE1 indicates that the principal mechanism is below-cloud washout. This result is due, however, to a postulated initially high concentration SO2 near the ground and a sharp decrease with height. For a well-mixed atmosphere, on the other hand, rainout undoubtedly predominates. TABLE 1. CO~~TR~BUTXONBY CASEOGS Am PARTICULATE SULFUR COMPOUNDS TO THE [email protected]~TIOX IN M-WATER FOR A WSTULATED IMTIAL ATMOSPHERE ~SWHICHTHES~~COSCE~~~-~~O?;S~E~IGHNEARTHEGUOUNDA~~LOWATA HEIGHT OF 2 km @EILKE and GEORGII 1968) Sulfur dioxide

Rainout Washout

5 70

(%)

Sulfate aerosols (%) 20 5

In order to study scavenging on the synoptic scale, one woufd Iike to measure at consecutive intervals of time, the vertical prof?les of pollution (below and within the clods), the precipitation intensity, the cloud and raindrop size distribution, and the concentrations of pollutants in the rain water. Even then, a budget caIcuIation for the vertical column might not be meaningful because of the possibility of an inflow of dirty air into the column during the sampling period. Initial experiements should be undertaken with stratiform rather than convective clouds. EDDIE (1962) used an aircraft to sample precipitation over Southern England at heights of from 450 to 2700 m. The experimental method was designed to capture precipitation rather than cloud droplets. Chemical analysis for a number of trace elements showed that even at the higher elevations, there were often significant amounts above background of suffur and other industrial pollutants (4.1 mg 1-r of S at a height of 1950 m on one occasion when the aircraft was down-wind of the Birmingham area). Although Oddie’s data demonstrated that pollution is carried into the middle troposphere, the observations were not sufficiently numerous to study synoptic developments in time and space. Such theory as exists has been summarized by ENGELMAN(1968, pp. 208-221) and is based on simplified models and smallscale experiments of washout (and snowout) from tracer plumes. The methods used operationally by air poflution meteorologists are largely those of CHAMBERLAIN (1953): they yield reasonable order-of-magnitude estimates for close-in scavenging of a plume. Where pollution is distributed over a large fraction of the troposphere, and where there are significant vertical motions, however, the prediction of washout becomes difficult. Even if the vertical distribution of pollution is assumed constant, the dropsize distribution and the resulting washout coefficients may vary with height. In addition, there will often be significant horizontal gradients due to orographic influences. Despite these various experimental difficulties, there is convincing evidence that pollution concentrations in precipitation are indeed largest near man-made sources. Stevenson’s maps (STEVENSON, 1968) for the British Isles reveal a maximum of sulfur near Manchester while ANDERSON(1969) has shown that the amount of sulfur in rain-water is higher within Uppsala than it is over the surrounding countryside.

380

R. E. Mum

and

B. BOLIN

In conclusion, we understand reasonably well the physical processes associated with precipitation scavenging. We do not yet know, however, how to deal with this “sink” quantitatively. This is a serious deficiency for those pollutants that are particularly susceptible to washout, e.g., SO,, NO1 and particulates. 5.3

Depletion of pollution at the atmosphere-earth interface

The mechanism of depletion of pollution at the surface of the earth is a microscale phenomenon. The rate depends on two factors: (a) the efficiency with which gas molecules and particles are captured or exchanged at the interface, (b) the rapidity with which the pollution can be delivered to the interface, through the Ekman layer, the surface layer and the viscous sublayer, each of which presents a resistance and results in a vertical gradient of pollution. In many cases, the problem may be simplified by the reasonable assumption that particles are in the sub-micron range and that gravity is not an important consideration, except possibly above a very smooth surface. Some kinds of surfaces are almost perfect sinks for some gases and particles, the substances being irreversibly absorbed. Tritiated water vapour and iodine-131 act in this way over a snow surface (BARRYand MUNN, 1967). These special cases have been used to study the relations between meteorological resistances and vertical fluxes (CHAMBERLAIN and CHADWICK,1966; THOM, 1968). The other limiting case, perfect reflection, is assumed in close-in diffusion models. More often than not, however, the efficiency of the interface exchange is either poorly understood or else quite variable. The CO2 flux to and from vegetation, for example, depends inter alia on stomata1 and mesophyll resistances, which vary with time of day. In the case of the oceans (BOLIN, 1960), an increase in dissolved CO, increases the equilibrium CO, partial pressure by an amount that is about 10 times as much relatively as is the increase in CO2 concentration in the water: this is due to a change in pH, which results in a rise in the number of CO, and H2C03 molecules in the water. The effect on chemical equilibrium of the boric acid in sea water has also been considered. There is thus a “buffering” effect which slows down the flux of CO, into the seas. Similar complexities exist for other gases and particles and for other types of surfaces, and there is a great need for research concerning interface effects. The scavenging effect of a forest, for example, is not yet predictable. A few centimeters to a few meters above an interface, vertical fluxes may be derived from measurements of vertical gradients together with estimates of the atmospheric diffusivity, during steady-state conditions over a uniform surface when and where vertical flux divergences can be assumed to be insignificant. F=

where,

AX KG

F is vertical flux, K is diffusivity (cm’ s-l),

4X is the vertical difference in concentration

X over thickness 4~.

Global .-XirPollution-Meteorological Aspects

Xl

Of course when the earth is a perfect reflector, hXis zero, and F = 0. In all other cases, the possibility exists in principle of estimating vertical fluxes through equation (5) without any information on the efficiency of capture or exchange of gases and particles at the interface. Sometimes equation (5) is rewritten in terms of a transfer velocity Y (Cm S-l) or a resistance r (5 cm-‘). F/AX=

V = r-I

(61

The latter two quantities can be seen to be related to the diffusivity in the following way : V = r- 1 s $J&

(7)

Equation (7) shows that although V has the dimensions of a veiocity, it is in fact the d~ffusivity per unit thickness (or i5.1~per unit vertical gradient.). When the interface acts as a perfect sink for the poIiution, i.e., X = 0 at z = 0, F/X=

V--r-l

03)

where X is the concentration at a convenient height such as 1 m. Using this relation, transfer velocities of about 0.15-2 cm s- l have been calculated from experimenta data, e.g., CHAMBERLAIS and CHKIWICK (1966). The quantities V and r vary with sampling height but far from a source, the principal gradient in X is within a few meters of the interface. At higher elevations, therefore, X is almost constant with height and V changes little with further increases in z. These concepts are used frequently in studies of regional and global atmospheric balances of particular pollutants, applying “reasonable” published vaIues of Y over various types of surfaces. Given the air concentration X, equation (8) is then used to infer the flus F. There is no doubt, however, that the experimental estimation of vertical fluxes deserves far more attention than it has received. PASQUILL(1962, p. 235) estimates that for a long-grass surface acting as a perfect sink for pollution from a distant source, the resulting increase in concentrations between 1.5 and 2.5 t-n would be only about 10 per cent. Great precision is therefore required in the measurement of vertical profiles. A final topic requiring special comment is the measurement of dry deposition with gauges. These instruments yield estimates of wet deposition that are no worse and no better than estimates of precipitation amount: the usual precautions regarding exposure and height apply. The deposit gauge cannot be used, however, to estimate regional dry “fall-out”. This is because the collector is a very unnatural receptor, not at all like an ocean or a vegetative surface, and the following difficulties arise: (a) The interface area per unit horizontal area is not the same for a gauge as for many natural surfaces. (b) The turbulence regime around the lip of a gauge is quite different from that found, for example, within a forest canopy. (c) The rate of deposition often depends on the nature of the surface. For example, the SO2 uptake by barley is much greater when the stomata are open than when they are closed (SPEDDISG,1969).

R. E. hlun

382

and B. Borrx

The gauge might be expected to be most successful for the collection of particulate matter. However, the same difficulties arise as occur with flat-plate samplers for pollen (HAFWSGTON et al., 1959; HAGE et al., 1960). The turbulent boundary layer that develops over a flat plate or gauge has a considerable influence on the collection efficiency, as compared with that of a natural surface. 6. CLIMATOLOGICAL 6.1

CONSIDERATIONS

Introduction

The emphasis up to this point has been on the synoptic time scale, i.e., on the transport of a plume on a particular occasion. Of equal importance is the climatological analysis of regional and global pollution. Over periods of a year or longer, the Gaussian point-source diffusion equation is usually irrelevent but other methodologies may be employed. By way of introduction, it is worth noting that a well-designed climatological study of pollution contains three ingredients: (a) A carefully formulated objective, e.g., the determination of global trends of a specific pollutant over decades, (b) A predictive model based on physical principles, e.g., based on balance equations that include both source and sink strength terms, (c) Observational data designed to yield answers to (a) and to provide a test for (b). As an example, there is merit in determining the global trends over decades from a network of background stations. Without a physical model, however, there is no way of extrapolating the trends safely into the future because of non-linear relations between source and sink terms. The model is a powerful tool not only in this positive sense, but also in a negative way: some insight into the physical behaviour of a system is gained whenever a model is tested and is shown to be over-simplified or incorrect. The point should be made also that the observational requirements for merely documenting trends may not be the same as those for verifying a model. 6.2 Climatological characteristic times The evaluation of characteristic time scales is often a helpful first step towards understanding complicated physical systems. Two such scales are used in the analysis of global pollution. The residence time TR is the average period spent by a pollutant in a given volume. The turnouer time T,- is given by the ratio of the mass of a particular substance in a given volume to its net loss per unit time out of the volume. Thus for the troposphere, Tr = M/FA (9) where Mis the total mass of a particular pollutant in the troposphere, Fis the interface flux per unit area and A is the surface area of the earth. Fluxes across the tropopause are assumed to be negligible in this simplified formulation. If the global annual net assimilation of CO, by vegetation is O.O3M, the turnover time is 1M/0.03M, i.e., about 35 yr. This relation may be expressed formally as,

(10) where the time derivative diM,/dt, expresses the assimilation by vegetation per unit time.

Global Air Polf~~io~-Met~orolo~calAspects

383

For a substance that is undergoing chemical change in the atmosphere as well as loss at the interface, the denominator on the right side of equation (9) must be rewritten as (F = F1 A + F2), where F1 is interface flux and F2 is an additional loss due to chemical decay. In the case of a radioactive material with decay constant (1, the second sink is given by, F

_

2--

dLf = ZUl i dt ? decay

(11>

Both F, and Fi are assumed to be proportional to the mass &1. ERIKSSON(1961) has shown that only in a well-mixed reservoir are the residence and turnover times identical. Most of the troposphere can in fact be assumed to be well mixed when global averages are taken over periods of a year or Ionger. There is, however, a boundary resistance in the lowest 10 m or so, slowing down the loss of pollution at the interface and creating a vertical gradient near the ground. This results in an increase in both characteristic times, but TV is then always larger than TV (ERIKSSON,1961). From equations (6) and (9) TV = r~~lA~X

(12

where r is the boundary-layer resistance and hX is the difference in concentration between the surface and some reference height such as 10 m. The model implies that ‘r varies linearly with r. Turning now to the residence time TV, no simple relation exists between it and the quantities in equation (9). In fact T~ is not an easy quantity to estimate experimentally, even for radioactive species, since the “radioactive age” is not necessarily the same as the time spent by the particle in the reservoir. Because of mixing and the necessity of sampling finite volumes, a true frequency distribution of “age” cannot be calculated : instead, the method yieIds only an apparent distribution, which necessarily is narrower than the true one, mixing aIways tending to produce a fluid of intermediate age. By considering the apparent residence time (Tag) of particles close to the boundary across which exchange occurs, ERIKSSON(1961) showed that TAR

=

b-T

(13)

On this basis he deduced, for example, that ~~ is 800 yr for the world oceans relative to the ocean surfaces. No information can be deduced, however, about the true residence times. For the above-noted reasons, the quantity T= (rather than 711)is a widely-used index in global pollution studies. An illustrative example is given in the following section. 6.3

The global budget for CO2 : An illustratice study

BOLINand BISCHOF(1970) have estimated the annual tropospheric CO, budget in the Northern Hemisphere north of about 30”N for the period December 1962September 1968. Their study illustrates a useful methodology and their results are important. For COz, there is no significant atmospheric loss through precipitation. The industrial input must therefore balance the storage in the atmosphere and the interface fluxes over land and sea.

384

BOLIX

R. E. MUNX and B. 13 IIll

I

I

12

I i NORTHERN

II/i . . -1

TROPOPAUSE -_-_-___~~~~~-----------

I

II

i

I

/

HEMlSPttERE

I____

TROPOPA.SE_

I I

_

\ I

\

IO I

9

‘ii8 f APRIL--+

FLUX

7-

~

I3 4

R FLUX

sS-

:

43k

2-

!

I o -10

Ill -9

I

t -6

-7

-6

-5

I -4

DEPARTURE

I -3

OF

I

I -2

-I

I

I

t

Ill

0

I

2

3

CO2 CONCENTRATION

(ppm)

4

FROM

5

!

I

Ii

6

7

8

f 9

10

MEAN

FOG. 8. Vertical profiles of CO2 concentrations @pm) in September and April averaged over the Northern Hemisphere north of about 30’N (BOLIN and BISCHOF,1970).

From a total of 701 aircraft observations of CO2 concentrations, Bolin and Bischof first established the seasonal cycles for each of 5 different Iayers, 1-3 km, 3-5 km, 5-7 km, 7-9 km and 9 km-tropopause. The annual range was 15 ppm in the lowest layer decreasing to 7 ppm just below the tropopause. In summer, CO2 values increased upward due to the vegetative sink while in vvinter, the gradient was reversed. Mean profiles for September and April shown in FIG. 8 reveal a sharp change at the tropopause.” After normalizing the data for the seasonal cycle, Bolin and Bischof found a welldefined annual trend of 0.7 rt 0.1 ppm yr- I. The remaining probIem then was to relate this to the increase in industrial output. If a physical model could be developed and verified, the CO, concentration trend could be extrapolated to future years, given an estimate of the rate of increase of industria1 emissions. In an earlier paper, BOLN and ERKSSON(1959) found that the CO, reteased to the atmosphere by combustion processes could be described approximately by the equation : Y = cl exp [c2 (t -

to)]

* The existence of vertical gradients in the troposphere itself suggests that this part of the atmosphere is not entirely a well-mixed reservoir on time scales of about a month. At any particular place and on any particular day, constant values of CO2 are to be expected through the surface mixed layer. Because this Iayer varies in thickness (FIG. 3) from place to place and from day to day, however. global monthfy average CO> vertical gradients must exist, their magnitude depending on the frequency distribution of mixing depths.

Global Air Pollution-meteorological

355

Aspects

where (t - to) is the time in years after t, = 1880, and cl, c2 are empirical constants. Thus the accumulated amount of CO, at any time t is given by, G = cl r-l[-

1 + exp c2. (t -

to)]

(15)

Not all of G has remained in the atmosphere by any means: there has been increased biosphere assimilation while some has gone into the oceans. If the fraction remaining in the atmosphere is given by CL,then the changes with time are shown schematically in FIG. 9. Employing equation (lo),

where TVis the tropospheric turnover time relative to assimilation by vegetation. Thus, jGdt $0 I

.

Hence the fraction of the increased CO, that has found its way into the biosphere is given by the ratio a jGd* L. 7T.G The constant cL of equation (15) disappears from this ratio. Reasonable values of the other variables and parameters are, c2 = 0.05 yr- ‘, a = 3.5%, or = 35 yr, t--to = 90 yr. Substituting, the ratio is found to have a value of about f. Thus about 25 per 1

c?.

AT~OSPHERtC MASSm,

Iuuw FIG. 9. Schematic representation

TIME

(YEARS)

IN

1880

-_3

of time change in the globat mass of CO2 since 1550.

R. E. .CIu>x and B. BOLIN

386

cent of the increased CO1 emissions seem to have been assimilated by vegetation, although Bolin and Bischof emphasize that this is undoubtedly an upper limit: some of the CO, is already returning to the atmosphere via the decomposition of dead organic material. An impiicit assumption in the model is that assimilation increases linearly vvith increasing CO2 content of the atmosphere: because there are other growth-limiting factors, this is not necessarily so. This order-of-magnitude argument leads to the conclusion that about one-half of G is in the oceans. Assuming that the sea can be subdivided into tvvo reservoirs, the surface mixed layer (about 75 m in thickness) and the deep ocean, and using the CO, equilibrium relations derived by BOLIS and ERKSSON (1959) for the air-sea interface, an examination of surface-layer turnover time reveals that part of the deep ocean (as much as 25-30 per cent) must have participated in the global balance over time intervals of the order of 30-50 yr. The remainder of the deep waters may then have the turnover time of about 1000 yr as indicated by other studies. These results on the global budget of CO1 are necessarily qualitative but they are internally consistent and they do provide guidelines for the design of additional experiments. For example, the increase in biosphere assimilation may already have been sufficient to be detected in an increase in normalized tree-ring widths, when averaged over global “background” stations. One might, as another example, find experimentally that the predicted uptake by the oceans is physically unrealistic: some of the assumptions in the model would then require modification. Finally, Bolin and Bischof have estimated future trends based on the assumptions, (a) that the emission increase is as forecast by OECD (1966), i.e., an annual increase of 4 per cent, with a possible rise to 5 per cent after 1980. (b) that 35 per cent of the emission increase remains in the atmosphere. To test the sensitivity of the estimates to these assumptions, repeated, using 45 per cent rather than 35 per cent for the amount atmosphere. The results are shown in TABLE 2.

the analysis was remaining in the

TABLE 2. PREDICTED ATMOSPHERIC CO2 COSCENTRATION (pprn), BASED ON AN EMISSION INCRiASE OF 4 PER CENT PER YEAR. FOR COMPARISOS, ESTIXIATESOBTAINED OS THE BASIS OF A 5 PER CENT ESlISSIONISCREASE AFTER 1980 ARE ALSO SHOWN

(Borrr and BISCHOF,1970) Year

Percentage of emission assumed to remain in the atmosphere 35% 45%

1970 19so

320 332.

320 335

4 % annual emission increase

1990 2000 2010

34s 371 403

355 388 430

5 % annual emission increase

1990 xOo 2010

349 37s 418

356 395 450

Global Air Pollution-Meteorological 6.4

Aspects

387

The study of local and re.qional secular trends in air quality

Given an adequate observations network for a particular pollutant, her~~spheri~ budgets are easier to estimate than regional ones: the problem of horizontal advection out of the “box” is avoided. Nevertheless, there is considerable interest in computing and explaining regional secular trends in air quality. This is partly because historical series of pollution observations do exist in a few regions, e.g., the precipitation chemistry data of the European network. Two separate but refated topics will be discussed in this section:

(a) The search for additional indirect indicators of past trends. (b) The interpretation of available pollution time series. Direct measurements of pollution do not in general extend back over many years. There is therefore a continuing search for indirect indicators, to recover as much information about past trends as possible. One such indicator is visibility, or the frequency of haze and/or smoke, as measured hourly at synoptic weather stations over many years (HOLZWORTH,1961). An interpretive difficulty is that most long records of this type come from airports, and the resulting trends reflect the growth of the adjacent urban area rather than regional or global changes. CHARLSON(1969), however, believes that with a careful selection of stations and with a restriction to hours when the relative humidity was less than 70 per cent, some useful information may be derived. MCCOR~‘~ICKand LUDWIG(1967) have used some early measurements of turbidity at Washington, D.C. and at Davos, Switzerland to yield the results given in TABLE3. The Davos station is at an elevation of more than 1600 m, and it may approximate a globat background location. At both stations there has been an increase in turbidity over the last few decades. TABLE3. MEAN VALUES OF TURBILXTY (MCCORMICK

Location Washington, D.C. Davos, Switzerland

and LUDWIG, 1967)

Years

Mean turbidity

1903-1907 1962-1966 1914-1926 19.57-1959

0.098 0.154 0.024 0.043

Another indirect indicator of past trends is the variation with depth of the chemical and physical compositions of ice and snow cores in glaciers. DAVITAYA(1969) has sampled the particulates in a glacier in the Caucasus, in layers corresponding to each decade from 1793 to 1962. Concentrations averaged about 10 mg 1-l of water between 1793 and 1922. Thereafter, there has been a substantial increase, with a value of more than 225 mg I-’ in the 1953-1962 decade. Traces of DDT have been found recently in Antarctica, while in Greenland, the lead concentrations in snow have increased ten-fold since IS50 (MUROZUMIet aZ., 1967). This technique has hardly been exploited yet, and there is considerable merit in developing a systematic program of sampling, using standardized analytical methods. The establishment of long time-series of annual pollution concentrations within various glaciers around the world might yield useful

R. E. MUNN and B. BOLIX

35s

information concerning the effects of anomalies in the atmospheric general circulation on the spatial variations in fallout patterns. We turn now to the equally important question of the interpretation of pollution time series. Long-term trends at a single station or over a region are caused both b\changes in emissions and by secular fluctuations in the atmospheric general circulation. LAMB (1969), for exampie, has shown that the frequency of westerly-type days in the British Isles has decreased since 1950, and this must surely have had an effect on local air quality wherever the effective emissions of pollution are greater in some upwind directions than others. SCHXHDTand VELDS (1969) have examined the trend in the mean winter SO, concentrations averaged over 7 stations in the vicinity of Rotterdam for the years 19621963 to 1967-1968. Their results are shown in TABLE 4, together with the frequencies TABLE 4. MEAN SO, CONCESTRATIONS AXD FREQUEKIES

TO CALSE

OF PARTICULARGROSSWETTERLAGEN KSOWS HIGHSO1 VALUESIN ROTTERDAM,FOR THI WWJERS 1962-1963 TO 1967-1968 (SCHMIDTand VELDS,

Winter

Mean SOr (r-rgm-Y

1962-1963

1969)

Number of days with H HM S:

1963-1964

261 258

32 35

1964-1965 1965-1966 1966-1967 1967-1968

229 216 203 183

13 26 9 8

of the three types of Grosswetterlagen associated with high SO1 values:

10 13 7 3 0 0

shown

Total Ko. with HAM, HNa and Sz

55

13

51 22 29 11 8

3 2 0 2 0

previously

by VELDS (1965) to be

HA4 anticyclone over Central Europe HNa anticyclone over the northern part of the Atlantic & cyclonic southerly flow over Western Europe. TABLE 4 shows that although the winter mean SO2 concentrations decreased during the 6-yr period, so also did the frequencies of weather patterns that were associated with high SO, values. The data therefore indicate that although there may have been a reduction in man-made emissions, trends in the atmospheric general circulation. contributed significantly to the decrease in mean SO1 values. For long series of hourly values of pollution at a single station, thz effect of climatic fluctuations may be removed partly in the following way:

(1) X local pollution wind-rose is prepared based on a number of years of records. e.g., MUXX (1969). (2) One year is selected as a standard year. (3) The frequencies of Lvind speeds and directions in other years are adjusted to the frequencies of the standard year. (4) The associated pollution concentrations obtained from the pollution rvind rose are summed, separately for each year of record.

Global Air Pollution-Meteorological

359

Aspects

The resulting normalized mean annual concentrations should be relatively independent of climatic variation. Alternatively, at a location where one or a few point sources predominate, the hourly pollution concentrations at a sampling location may first be estimated directly from some appropriate plume-rise and downwind diffusion equations. Given then the hourly weather observations over the years of interest, a series of predicted annual mean concentrations or annual frequency distributions may be calculated and compared with the observed values. Differences between the two sets of data may yield at least some clues about trends. CLARENBURG (1969) has proposed a rather similar approach for removing the effect of weather variability from year to year on air quality variability. Extrapolation to the regional scale is possible by using a pollution mixing-height wind rose as suggested by BALL (1969). The surface wind is replaced by the mean wind through the afternoon mixing depth. LMUNN and RODHE (1971) have examined the monthly depositions of sulfur in precipitation at the Pldnninge and Flahult stations in southern Sweden for the period July 19.51-June 1969. The two lower curves in FIG. 10 display the median monthly depositions for consecutive 2-yr periods. This is to be compared with the uppermost curve in FIG. 1Uwhich shows the percentage frequencies of precipitation associated with surface winds from the southeast clockwise to northwest. The man-made sources of

10

I

0 1952

1954

I

I

1956

1958

I 1960

TIME

I 1962

I 1964

I 1966

I ,968

1

s 2

197oOz

(YEARS)

FIG. 10. Median depositions of sulfur in precipitation for consecutive 2-yr periods at two stations in southern Sweden (lower two curves and left-side ordinate axis): frequencies of precipitation-bearing SE-S-SW-W-NW surface geostrophic winds for southern Sweden and for consecutive 2-y periods (uppermost curve and right-side ordinate axis) (MUNNand RODHE, 1971).

R. E. ML?;N and

390

B. Born

sulfur are to the southeast, south and southwest while there are natural sources to the southwest to northwest. The secular variations in FIG. 10 are very similar, indicating that the atmospheric general circulation exerts a considerable control on sulfur deposition patterns at these two stations. If an adequate monitoring network were available for studying global secular trends, the effects of variations in the general circulation would mostly disappear. although source strengths of natural constituents, and sink strengths for both natural and man-made pollution, might vary somewhat. For regional and local air quality studies, however, the results of SCHMIDT and VELDS(1969) and of ~IUSN and RODHE (1971) indicate that observed air quality trends require careful interpretation. A rise or fall in pollution level does not necessarily mean that there has been a change in emissions. 7. ATMOSPHERIC 7.1

MONITORING

FOR POLLUTION

Introduction

Network design is of central importance in the study of global pollution. Although there is an understandable desire to develop a network around existing stations in order to maintain continuity and to reduce costs, sound physical principles rather than expediency should prevail. There are two primary considerations: (a) The time and space variability of each pollutant, (b) The purposes to be served by the network. Because pollution variability is known imperfectly in most cases, no final decision can be made yet on how to optimize the spacing of stations. There is no doubt. for example, that the grid size will vary with the type of contaminant and the region. These uncertainties should not be permitted io delay the establishment of the network now being organized by the World iMeteorological Organization. Paralleling such activities, however, research on network design must be encouraged. The data generated from pollution networks will be used for many purposes, and it is important that the objectives be defined rather carefully. Because each application will have associated with it a particular requirement for data, several types of stations are no doubt necessary. The data requirements for examining the effects of pollution on climate are rather different from those needed for correlation with biological observations of a forest ecosystem. In the latter case there is merit in sampling within the forest rather than over open countryside. Simply because there already exists a global network of fully-staffed meteorological observing stations at airports does not necessarily mean that these are the best locations for all types of background pollution measurements. 7.2

Surface sampling stations

To begin, the term background station must be defined carefully. If J4 is the mean concentration of a pollutant over a given hour and P is the peak concentration measured over a few seconds within the hour, then we suggest that a background station is one at which the peak-to-mean ratio P/M remains close to unity.

Global Air Pollution-Meteorological

Aspects

391

Secondly, it is highly desirable to distinguish amongst several types of background stations. Two classifications come immediately to mind: (a) according to the nature of the underlying surface, e.g., forest, grassland, ocean, tundra, etc., (b) according to scale, e.g., urban, regional and global background stations. We shall consider classification (a) later. Under (b), the objective is to use location the effects of scales smaller than that of interest. An urban background station, for example, is one located in the centre of a large city park relatively far from chimneys and automobiles. Sites in Central Park, New York City and Kensington Gardens, London meet this criterion. Monitoring at such locations over decades permits study of long-term trends in the background air quality of cities, although no information is provided about the magnitude of peaks occurring, for example, near busy highways. The urban background loadings will, of course, include pollution that has come from other cities and regions, but this will be an insignificant fraction in most cases. On the next larger scale, background stations in the countryside permit study of regional patterns and trends. Measurements made in rural England or rural New York State are in this category. Although a particular region may become more (or less) polluted, this change may have only a very small effect on the global loading. Thus there is a requirement for a third type of background station (called a baseline station by the WMO) (WMO, 1970) in isolated arctic, oceanic or mountain locations. With increasing scale, concentrations of pollution become smaller and often more difficult to monitor. At the same time, however, contributions from relatively small local or regional sources assume increasing importance. Two examples will illustrate this point. First, concentrations at an arctic station may increase over the years simply because the local population has risen from, say, fifteen to thirty people with an attendant increase in local emissions. Secondly, concentrations at a mountain station may rise because an adjacent valley is becoming more polluted and there are occasional intrusions of upslope winds (or of subsiding air that has participated in a circulation cell extending to the valley floor). KEELING (1970) notes that the atmospheric CO, concentrations at Mauna Loa Observatory (above the surface mixed layer) in Hawaii occasionally rise for a few hours due to locally produced volcanic COz, or dip in the afternoon due to vegetative uptake 20 km away. Keeling mentions that before an analysis for seasonal or secular trends is undertaken, the record is scanned and non-steady-state periods are removed. These examples suggest that the choice of a groundbased background station is not without difficulty. With increasing remoteness, operating costs rise and more sensitive sampling equipment is often required to monitor the lower concentrations that prevail. In addition, special attention must be paid to the micro-environment. In many cases, the same guidelines as those recommended for siting meteorological sensors such as anemometers and rain-gauges are applicable. Measurements should preferably be made at a height of 10 m, i.e., above that part of the surface layer containing the principal vertical gradient of pollution. In addition, there is merit in monitoring in the early afternoon when the mixing depth is largest, and concentrations as a band-pass filter, removing

391

R. E. MU~Xand B. BOLIN

are likely to be representative of a substantial fraction of the troposphere. During inversions, on the other hand, air quality is likely to vary over small distances. The WMO Panel on Meteorological Aspects of Air Pollution (WMO, 1970) has established criteria1 for baseline stations, including the following: (a) “No significant changes should be anticipated in land-use practices within 100 km of the site (in any direction) for at least the next 50 yr after the stations are established.” (b) “The observing staff should be small in order to minimize self-contamination of the local environment by their presence and living requirements.” (c) “All requirements for space heatinb,0 cooking, etc., should be accommodated by electric power generated off the site.” (d) “Access to the station should be limited to electric-powered vehicles on an infrequent, but regular schedule. Visits by persons not necessary to the operation of the facility should be discouraged.” (Including summer campers particularly.) (e) “Sites should be located away from major air routes (to avoid jet contrails), preferably on isolated islands or on mountains above the timber lines.” (f) “Sites should experience infrequent effects from natural phenomena such as volcanic activity, forest fires, dust and sand storms.” We agree with these recommendations, suggesting further that feasibility studies be undertaken regarding the use of satellites for interrogating automatic sampling stations. The requirements for a regional station, as distinct from a baseline site, are not so stringent. In this case, for example, mountain peaks are perhaps not the most suitable locations because they are not representative of the surrounding countryside. Quite naturally, however, sites near dusty roads or ploughed fields should be avoided, and no major changes in land use practices should be anctipated. We return now to a consideration of the other classification scheme for background stations, i.e., according to the nature of the underlying surface. Observations should be made over snow/ice, tundra and, in both the tropics and temperate zones, over forest, grassland and ocean. At representative locations within each region, an attempt should be made to obtain routine measurements of vertical fluxes of the most important pollutants. There are two reasons for making this recommendation. In the first place, the global budgets for many pollutants depend critically on the strengths of the interface sinks, and the surface vertical fluxes are often much more important than the losses due to precipitation scavenging. Secondly, there are a number of biometeorological applications for which the flux of pollution at the surface is more important than the concentration in the air. The latter quantity may on occasion be low simply because the exchange rate with the interface is rapid, as has been found by MAKAYX~IAL (1965) in his investigation of oxidant damage to vegetation. Every effort should therefore be made at background stations to monitor vertical fluxes. This is admittedly not an easy thing to do, even for water vapor and COz, but research along these lines should be encouraged. To avoid boundary-layer readjustment edge effects, background stations should be near the centre of large areas having rather uniform underlying surfaces and climates. Even then, there will be occasional peaks in pollution, as demonstrated by the CO values on the Greenland Ice Cap (ROBINSONand ROBBINS,1969) and the dust loadings at Kitridge point, Barbados (PROSPERO,1968), both locations being thousands of

Global Air Poliu~ion-~~eteorologic~Aspects

293

kilometers from source regions. Because such sites are sometimes difficult to find, a compromise solution is to choose an exposed coastal station, on the west coast of Europe for example, analyzing air quality on only those days when the wind is off the water. There is difficulty, however, in deciding objectively whether the air sample has had a long maritime trajectory or whether it has participated in a recent mesoscale, land-sea circulation. In some cases, the investigation of edge effects is of particular interest, e.g., in the determination of the rate of decrease of concentrations downwind from a large industrial region. A few welt-designed case studies of edge effects using mobile equipment might be more meaningful, however, than an analysis of data from a permanent monitoring network. Finally, there are the questions of the density of the surface network and the types of measurements to be made at each station, The Commission on Atmospheric Chemistry and Radioactivity (CACR, 1970) has suggested that a distinction be made between two classes of pollutants, those with relatively long atmospheric residence times and those with short ones. In the former class are CO2 and hydrocarbons: these have lifetimes of more than half a year and they spread over the entire hemisphere. About 10 global stations are recommended, and the majority on a Noah-south line running from the north coast of Alaska, through the Aleutians, Hawaii, the Line Islands, a South Sea island or New Zealand to Antarctica (CACR, 1970). Short-lived pollutants include many aerosols and some gases such as SOz, NO, and fluorine. An important sink mechanism for this class is precipitation. Since rainfall varies widely in space, a denser global network of precipitation chemistry stations is required than that for long-lived pollutants. The recommendation has been made (CACR, 1970) that about 100 provisional stations be established, including some on oceanic weather ships. This spacing may, of course, prove to be inadequate because of strong mesoscale variability in precipitation, particularly in hilly country and over coastal strips. Additional research on this problem is undoubtedly required. Finally ROBINSON(1970) has recommended that condensation and ice nuclei concentrations be monitored with a global network of up to 10 stations. These measurements are needed for investigations of long-term effects of pollution on cloudiness and precipitation. Since the residence time for particulate matter in the atmosphere is short (weeks), ten stations hardly suffice for such studies. Network density depends to a large extent on the purposes to be served by the network. Because not all of the purposes are clearly defined yet, and because the data requirements for testing physical models will change as the models become more realistic, no final guidelines can be given for the spacing of surface stations. If a large network already existed, we might be able to decide which stations were redundant, through a study of spatial correlation coefficients or through principal component analysis. In the absence of such a network, however, only an “educated” guess is possible, based on a knowledge of the atmospheric general circulation, the source and sink regions and residence times for pollution. In this connection, there is no reason to expect a priori that optimum network design will be the same for each pollutant, 7.3

Upper-air sampling stations

A surface network of stations is sufficient for some investigations such as studies of health effects, uptake of pollution by vegetation, and corrosion of materials. A.&51&c

394

R. E. hlu~ and B. BOLIN

Frequently, however, there is a need to sample the entire troposphere, particularly when attempting to develop models of secular trends in air quality, and when search ing for links with climate. Some indication of the total particulate loading can be obtained from ground-based turbidity measurements, and these are properly included in the global network proposed by the World Meteorological Organization. Although such observations are biased towards fine weather (a direct siting of the sun is required), they nevertheless do provide information on long-term turbidity trends (as shown in TABLE 3, for example). These measurements should be supplemented by Laser observations of vertical profiles of particulate matter (ROBINSON,1970, p. 39). In addition, satellites are already capable of detecting large-scale dense haze, dust and smoke. The study by CLOD~IANand TAGGART(1969) was mentioned in Section 4. Other examples have been given by WOBBER (1969) and GORSCHBOTHet al. (1968). These latter papers include satellite photographs of a smoke plume from a tall chimney in Arizona, smoke plumes from forest fires in Florida, the urban haze over the Los Angeles basin, the urban plume downwind from Houston, Texas, and, on the subcontinental scale, dust clouds over Iran and Iraq. Although the satellite is a qualitatively valuable tool, ROBIXSON(1970) believes that “quantitative assessment with any useful precision is a little beyond current techniques”. There is no doubt that satellite technology will develop rapidly. For many years to come, however, the aircraft and the balloon are likely to be important devices for obtaining detailed three-dimensional synoptic and time-averaged values of tropospheric pollution. Studies like those by BOLINand BISCHOF (1970) on CO2 and GEORGIIet al. (1969) on SO2 should be continued and extended to include other gases. Despite the costs of such programs, the conclusion is inescapable that vertical sampling of the troposphere is absolutely essential in order to provide answers to many theoretical and practical pollution problems on both the regional and global scales. NEIBURGER(1969) has portrayed the quality of the air as it crosses the United States from west to east (FIG. 11). The diagram indicates that the air mass has not had time to purify itself before arriving at the next large urban complex. The model is highly simplified, and there is no doubt that three-dimensional sampling is required in order to obtain reliable quantitative estimates. Network design is often coloured by the fear that present measurement techniques may soon become obsolete, that remote sampling by satellites and improved chemical and physical methods will become available in the next decade or so. Instrument developments are to be welcomed, however. Furthermore, the experience in synoptic meteorology has been that technological advances have generally supplemented rather than replaced existing monitoring programs. There is merit, nevertheless, in collecting and storing non-destructible samples, e.g., of suspended particulate matter, both from surface and upper-air stations. In the event that substantial improvements are made in chemical analysis techniques, or if substances not now monitored become important, the samples will be invaluable in studies of secular trends. A final topic that should be mentioned is sampling of the lower stratosphere. The suggestion has been made (CACR, 1970) that because diurnal and annual cycles are greatly damped in this region (see FIG. 8, for example), periodic stratospheric sampling might be very useful for establishing secular trends. Another justification for such a program is the need to study the effect of the emissions from the increasing numbers of

Global Air Pollution-Meteorological

P0:ific

OCeo:!

Los Angeles

Denver

Aspects

New Atlantic Cleveland Pittsburgh York

Kansas St. Chicago City Louis

395

Ocean

Distance FIG.

11. Schematic representation

of pollution concentrations (NEIBURGER,

in air crossingthe United States

1969).

aircraft flying above the tropopause. Water-vapour contrails, for example, can be considered as a form of stratospheric pollution. Although the total effect at the present time is probably small, we must maintain a prudent watch on this source of contamination. If the stratospheric water vapour content were to increase, the photochemistry of this region could be affected, changing for example, the mean ozone distribution. 7.4

International

uniformity in sampling procedures

The meteorological community has an enviable history of international cooperation that has resulted in uniformity of instrument siting criteria, sampling methods and data exchange procedures, and in the development of regular programs of intercomparisons of instruments. Atmospheric chemists are not yet in such a favourable position. However, a global sampling program can only be successful if international uniformity is achieved. Of particular importance are the periodic intercomparison of instruments and the development of long-lasting reference standards. 8. THE

8.1

EFFECT

OF POLLUTION

TRENDS

ON CLIMATE

CO2

Much has been written about the possible effects of pollution on climate. It is generally accepted that an increase in the amount of CO, in the atmosphere would lead to an increase in the average global temperature. A more precise assessment of the magnitude of such an increase is difficult, however, and quite different views have

R. E. Mcrz;~and B. BOUN

396

been propounded. %%QrE (1970a) estimates that a doubling of the carbon dioxide content of the atmosphere would lead to an increase of the global temperature by 2°C assuming unchanged absolute humidity, and by about 3’C assuming unchanged relative humidity. Clearly further numerical experimentation with general circulation models is required in order to clarify the interplay between the two basic radiative constituents of the atmosphere, i.e., water vapour and COz. Consideration should also be given to other factors that are influenced by a changing atmospheric temperature (clouds, sea surface temperature, and, for longer time periods, growth and extension of glaciers and sea ice, for example) (see also MANABE, 1970b). A critical consideration here is the existence of positive or negative feedback effects. For example, with an increase in temperature of surface waters, the CO2 solubility decreasesthus increasingtheatmospheric CO,content,a positive feedback mechanism. 5.2

Particli~~tes

Many attempts have also been made to try to assess the climatic effects of an increasing dust load of the atmosphere (BRYSON et al., 1970; MITCHELL,1970). Here the conclusions are even more uncertain, although increasing particulate matter probably produces temperature decreases. The first reason for uncertainty is that the radiative properties of particulate matter in the atmosphere are not suficientiy well known to permit a precise quantitative treatment. It is generally believed that scattering (causing an increase in the albedo of the earth) is the predominant effect but infra-red “greenhouse” absorption may also be important. Secondly, our knowledge of the possible increase in the dust load due to human activities is very limited. Large variations occur naturally due to volcanic activity @MITCHELL,1970). Thus the reversal in the upward trend of world temperature, that seems to have occurred around 1940, may be explained by the increasing volcanic activity during the last two of three decades. Additionally, however, man has probably modified the global dust loading, by changing his agricultural and recreational land-use policies. An analysis has not been undertaken, however, as to whether the net effect has been positive or negative: thus for example, we do not know whether the improvement in forest-fire protection services has counterbalanced the increase in fires in remote areas (due to the greater numbers of people able to visit those areas). 8.3

Stratospheric effects

We turn next to a brief consideration of the effects of stratospheric pollution on ciimate. Much of what has been written in the popular press is science fiction, and there is a great need for realistic physical models and carefuhy designed experiments. By way of introduction, it should be noted that extremely minute trends in the average global values of the meteorological elements can be cumulative. For example, a radiative temperature change of lO’4” C day-’ is equivalent to 0.9”C per 25 yr. In general, the radiative balance of the earth’s surface depends on bulk factors (total ozone, total C02, tota water vapour, total clouds, etc.) and on properties of the lower troposphere (local pollution, local emissions of heat and water vapour, etc.). In other words, in most spectral regions the surface either “sees” only the lower troposphere or else essentially the entire atmosphere. There is, however, one very important exception. In the 8-13 pm Hz0 and CO2 window (in which the surface “sees” the entire atmosphere, and outer space as well, if skies are clear), there is a

Global Air Pollution-Meteorological Aspects

397

narrow ozone band centred at 9.6 pm. Since the ozone concentration increases very rapidly with height above the tropopause, there is a direct radiative linkage between the surface and the lower stratosphere in the 9-10 pm spectral band. Since the lower stratosphere is in very delicate radiative balance (O,, CO* and Hz0 contributing in quite different ways but with comparable magnitudes), any long-term changes in this balance, due for example to changes in stratospheric water vapour, can influence directly the temperature of the earth’s surface. Having admitted, however, that stratospheric pollution can affect climate, we must emphasize that at the present time, predictions of trends are almost pure speculation. 8.4

Research

requirements

Returning now to a consideration of the troposphere, it should be noted that the sources and sinks of pollution are not distributed uniformly around the world. Thus lack of knowledge about global dispersion patterns is an obstacle in attempts to assess possible climatic changes caused by increasing emissions. The pollution, heat and water vapour from an individual chimney usually have little influence on local climate. Multiple urban sources, however, can change the climate significantly, influencing the frequencies of fog and precipitation and contributing to urban “heat islands”. In the megalopolis from Boston to Washington, for example, the total effect is substantial. It is clear from the previous treatment that the theory for dispersion from a point source under the assumption of a steady synoptic wind pattern is inadequate for an assessment of regional effects. Indeed there is an urgent need for knowledge of, (1) the dispersion of individual smoke plumes over distances of several hundred kilometres, (2) statistics on the fluctuations in space and time of the synoptic wind field, including the vertical component, (3) reliable estimates of the residence times of the pollutants. So far, limited progress has been made in the construction of a coherent procedure for dealing with this complex problem. We know reasonably well the characteristic variations of the horizontal component of the synoptic flow pattern but we do not understand precisely how the large-scale vertical motion fields and the vertical stratification in the free atmosphere influence the dispersion of pollution. Here numerical models for weather forecasting should be used much more extensively than has been the case so far. There exist a few cases in the literature in which dispersion of ozone and radioactive debris in the atmosphere has been dealt with rather successfully, e.g., HUNT and MANABE (1968). To generate adequate statistics with the aid of such models requires, however, a considerable effort. As long as the removal and large-scale diffusion processes for pollution are poorly known, we are not able to devise sufficiently realistic models of regional or global three-dimensional dispersion to justify extensive numerical integrations. The lack of knowledge about dispersion in the free atmosphere and on the physics and chemistry of the removal processes calls for extensive studies of the behaviour of pollutants above the friction layer. The design of such experiments must go beyond what has so far been attempted. Since there is no way of deciding in advance the relative importance of dispersion, chemical reactions, and removal processes over time

R. E. MUNN and B. BOLIN

398

scales of days, simultaneous observations of several tracers are desirable, some of which react chemically with each other, some of which do not. Not until we know the source strengths (natural and/or man-made) more adequately, shall we be able to forecast the effects of increasing emissions from extensive sources like those in Western Europe or in the Eastern United States. One further complication in our endeavour to try to make such forecasts is the natural variability of the weather and the difficulty in distinguishing between natural processes and those induced by man. To make such a separation possible requires detailed analyses of pollution observations in the way outlined ‘by MUNN and RODHE (1971) and an extension of such studies to include series of observations from the free atmosphere. It is quite clear that observational programs extending over several years are required in order to obtain meaningful results. It is also clear that because of the importance of variations of the synoptic flow patterns in such studies, and because of the requirements for observations from the free atmosphere, the incorporation of a limited and carefully planned observational experiment within the Global Atmospheric Research Program (GARP) seems justified indeed. It is generally accepted that the residence time of carbon dioxide in the atmosphere is several years, while the residence times for some other gaseous constituents (for example, sulfur dioxide) and for particulate matter are of the order of a month or weeks (or even less, since emissions take place in the surface boundary layer). It is also generally accepted that the characteristic horizontal mixing time when considering a hemisphere is of the order of a month or more and that it may amount to half a year or a year if the whole globe is considered (BOLIN and KEELISG, 1963; JUNGE and CZEPLAK, 1968). It therefore follows that we may deal with the globe as a whole when studying the possible climatic effects of changes in CO, concentrations; this simplification is probably not permissible for many other pollutants, especially particulate matter. The dust load of the southern hemisphere will certainly be much less affected by an increase in human activities than it is in the northern hemisphere. The distribution of continents and oceans and the uneven distributions of human sources of pollution imply that the effects of increasing man-made emissions may appear as regional sources or sinks of radiative energy. The prime effects may thus not be a change in the global or hemispheric temperature distribution but rather a modification of the geographical distribution. When such permanent features become of sufficient magnitude, they may influence average flow patterns not only in the polluted regions but also in other parts of the world; this realignment of the global flow pattern could occur without any significant trend in mean hemispheric or global conditions. Also the fact that a small change in weather patterns (and thus in wind distributions) has a feedback effect on the dispersion of pollution (and also on the radiative patterns) indicates that time series of pollution observations at a single point must be treated with great care. S.5

Atmospheric

oxygen

Finally, a comment not directly related to Some ecologists have burning of fossil fuels diatoms, the principal

should be included about the world oxygen supply, a topic climate but nevertheless one of the “environmental anxieties”. predicted that the oxygen balance is being disrupted by the and by the release of pesticides into the oceans, killing marine source of photosynthetically produced oxygen. MACKTA and

Global Air Pollution-Xleteorological

Aspects

399

HUGHES (1970) however, have reported that there have been no significant changes in the atmospheric oxygen concentrations since at least 1910. Furthermore, RYTHER (1970) has reasoned that if all photosynthesis in the oceans were to stop suddenly, at least a million years would elapse before the atmospheric concentration of oxygen decreased by 10 per cent. 9. CONCLUSION

Because the atmosphere has a very great dilution capacity, it has been exploited as a giant sewer. Chimneys have been built to solve local problems in villages and towns while a combination of very tall chimneys and intelligent land-use practices has been employed to alleviate regional problems. It is thus interesting to note that the average height of a power plant stack in the United States completed in 1960 was 73 m as compared with a 1969 value of 183 m (TVA 1970). Such engineering solutions provide acceptibte air quality by diluting the pollution through a large votume of air. Occasionally, of course, there is a break-down in the efficiency of the system when the air space becomes limited, locally in a valley, for example, or regionally in a stagnant warm anticyclone. Special controls, particularly of low-level sources, may then be necessary for a few hours or days. For toxic substances that are practically nondegradable (e.g., DDT), great care must naturally be exercised in the release of even minute quantities, because of the cumulative effect. As emissions increase, the heights of smoke stacks also increase and thus the area of influence expands. In addition, the expanding multiplicity of ground-level sources contributes increasingly to background pollution. The engineering problems gradually change in character, therefore, and the political aspects become much more critical. Public attitudes are changing, resulting in concern about possible degradation of rural and wilderness areas as well as about possible ciimatic deterioration. The discussion in previous sections has indicated that our present knowledge of diffusion and sink mechanisms on the regional and global scales is still inadequate. To answer the questions posed by engineers and politicians, therefore, additional large-scale studies of the behaviour of atmospheric pollutants are urgently required. We need experimental data as well as realistic models, not only to assess present conditions but also to predict future trends in degredation of the “global village”. If this review serves to stimulate such activity, we will indeed have fulfilled our objective. Acknowledgement-Helpful discussions with Professor GODSON(Section 8) are gratefully acknowledged.

E. ERIKSSON,H. RODHE and Dr. W. L.

REFERENCES (1969) Small-scale variations of the contamination of rain caused by washout from the low layers of the atmosphere. Tellus 21, 685-692. ASGELL J. K. (1961) Use of constant level balloons in meteorology. Adr. Geophys. 8, 137-X9. BALL R. J. (1969) fnsterstate transport of air pollution in southwestern Connecticut. &clanuscript. Air Pollution Controf Section. Connecticut State Health Dpt., Hartford, Conn. BARRY P. J. and iMu,ux R. E. (1967) Use of radioactive tracers in studying mass transfer in the atmospheric boundary layer. Phys. Fluids Suppl., 263-266. BATCHELORG. K. (1950) The application of the similarity theory of turbulence to atmospheric diffusion. Q. J. R. Meteorol, Sot. 76, 133-146. BEILKES. and GEORGII H. W. (1968) Investigation on the incorporation of sulphur-dioxide into fogand rain-droplets. Tellus 20,435-442. ANDERSSONT.

R. E. MM

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