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Fig. 47

Annual variation of sulfate concentration in precipitation at Chicago-Airport (Lodge et al., 1968). (By courtesy of the National Center for Atmospheric Research)

(Lodge et al. 1968) that the greater sulfate content of precipitation water in an arid environment is due to mineral dust particles and to the effect of evaporation below the cloud base. Soils in these areas contain calcium carbonate and calcium sulfate in significant proportions. Calcium carbonate particles in the air are transformed into calcium sulfate by reacting with sulfur dioxide. The process is an interesting interaction of natural and man-made trace constituents.

The high concentrations over the north-eastern part of the U.S.A. are explained by the effect of man's activities. This is further illustrated by annual variations in the sulfate concentration in precipitation water collected at Chicago airport, as shown in Fig. 47. We can see that at this station near Chicago the concentrations are highest in the winter, when fuel consumption is significant. The curve plotted also shows that during spring and early summer a secondary maximum can be observed in the concentration. This is in a good agreement with the annual change of the sulfate content of precipitation water in cleaner environments (see Subsection 5.4.5).

Finally, it is to be noted that the data analyses of Nisbet (see Hales, 1978) demonstrated that in North America the sulfate concentration in precipitation increased until 1965 by a rate of 60-65 % every decade. Although a levelling off occurred after 1966, the concentration has an upward trend, even at present.

5.4.3 The "Swedish" network

The first major precipitation chemistry program was initiated by Swedish scientists (see Egner and Eriksson, 1955). The network was extended to the northern and western parts of Europe and for some time chemical data were published in the scientific journal Tellus. At the beginning of the sixties more than one hundred stations were involved in the network. This network is still in operation, and it is now called the European Atmospheric Chemistry Network (Granat, 1978), since not only precipitation chemistry is measured in this program (see Egner et al., 1955). The number of stations at present is about 50. Monthly precipitations samples are collected by open rain gauges.

Figure 48 illustrates the spatial distribution of chloride ions measured by the Swedish network in 1957 and 1958 (Junge, 1963). In this figure the results of a Czechoslovakian program performed during the International Geophysical Year (Mackuet al., 1959)arealso included. One can see that theisolines practically follow the contour of the continent which makes evident that in locally unpolluted air the atmospheric chloride is of maritime origin. In Scandinavia the concentration initially decreases exponentially with distance from the ocean and reaches a rather constant value.

Concerning possible damage caused by atmospheric precipitation to other media of our environment (e.g. lakes, soil, vegetation), the acidity or pH of precipitation water is of crucial interest. It should be noted here that the neutral point of atmospheric waters is lowered by the presence of COz in the air. Thus, it can be calculated (Junge, 1963) that the neutral point is at pH = 5.6 at a temperature of

10 °C. This hydrogen ion concentration is then altered by different trace constituents.

The spatial and temporal distribution of pH in precipitation over Scandinavia was first studied by Barrett and Brodin (1955). According to their data, in South Sweden and in seaside areas, the pH is between 4 and 5 in the majority of cases. In a more continental environment the pH reaches the value of 6. The above authors

Fig. 48

Distribution of chloride concentration of precipitation water over North and North-West Europe (Junge. 1963). (By courtesy of Academic Press and the author)

attributed this spatial distribution to the advection of polluted air from West and Central Europe. The concept of anthropogenic effects was also confirmed by the annual variations of pH, which showed a clear winter minimum. The low values found in Norway near seaside were explained by the influence of H2S of maritime origin.

The pH distribution in precipitation over North-West Europe was studied by Oden. Figure 49 is taken from Oden's work, as published by Butcher and Charlson (1972). This distribution is believed to be caused by the authropogenic S02 emission, the maximum intensity of which approximately coincides with areas of lowest pH values.

Fig. 49

Spatial distribution of pH of precipitation according to Oden (see Butcher and Charlson, 1972). (By courtesy of Academic Press and the authors)

Fig. 49

Spatial distribution of pH of precipitation according to Oden (see Butcher and Charlson, 1972). (By courtesy of Academic Press and the authors)

Recently Granat (1978) has analyzed data on sulfur concentration in precipitation and sulfur wet deposition obtained in European Atmospheric Chemistry Network. According to his paper the deposition has been constant during the last ten years over North-West Europe. It is postulated that sulfur emitted in this area is transported and deposited over other areas, probably in an eastward direction (see Subsection 3.6.6). However, using the nitrate data of the same network, Soderlund (1977) reports an overall increase of 50-100 % in the nitrate wet deposition at many stations during the 20 years ending in 1973.

£4.4 Chemical composition of precipitation waters over the Soviet Union

The study of the chemical composition of precipitation in the U.S.S.R. by means of an extensive network was started in the International Geophysical Year under the auspices of the Main Geophysical Observatory, Leningrad. Stations were initially operated over the European part of the Soviet Union with collectors covered during dry periods. The network was later extended to the Asian part of the country. The results of analyses of daily and monthly precipitation samples collected at 13 European stations in 1958-1961 are summarized by Drozdova et al. (1964). Some of their data are reproduced in Table 28. The data tabulated clearly show that rather high concentrations of contaminants can be found in the southwest. In these areas the concentration of sulfate ions is particularly significant. Soviet workers attribute this finding to the effects of anthropogenic sources. However, dispersed soil components may also play a certain role in the control of the composition of precipitation (see Subsection 5.4.2).

This assumption is also supported by the spatial distribution of the sums of the concentrations of all ions measured over the whole of Soviet Union as shown in Fig. 50 (Petrenchuk and Selezneva, 1970). This figure indicates that very high concentrations are measured in the south over arid areas. One can also see from Fig. 50 that minimal concentrations are detected over the northern territories and over

Fig. 50

Distribution of the sum of the masses of ions in precipitation (mg I ~ '(over the U.S.S.R. (Petrenchuk and Selezneva, 1970). (By courtesy of the American Geophysical Union and the authors)

Fig. 50

Distribution of the sum of the masses of ions in precipitation (mg I ~ '(over the U.S.S.R. (Petrenchuk and Selezneva, 1970). (By courtesy of the American Geophysical Union and the authors)

Siberia. It is possible that the Siberian value of 15 mg 1 ~1 represents the continental background of Europe and Asia.

Petrenchuk and Selezneva (1970) made model calculations to evaluate the removal both in the cloud and beneath the cloud. They found that, in a clean tropospheric environment, 55 % of the trace constituents found in precipitation collected at the ground level is due the rain-out processes, in agreement with Georgii's results discussed in Subsection 5.3.4.

Soviet data also demonstrate that the sum of the concentrations of the different ions is independent of sampling location in the case of frontal precipitation systems, and is equal to about 6 mg 1"1 over the whole of the Soviet Union. The pattern shown in Fig. 50 is thus mostly produced by local precipitation systems, which are much more sensitive to local pollution. Petrenchuk and Selezneva (1970) argue that in frontal systems the rain-out of aerosol particles by condensation is the dominant mechanism in the control of precipitation water composition.

On the basis of precipitation chemistry data, Selezneva (1972) was also able to estimate that, over rural areas of the Soviet Union 30-40 % of the contamination found in precipitation is due to the background level of trace constituents, while 6070 % of the concentration reflects the effects of local sources. About half of this latter fraction is provided by pollutants. In this important work she also showed that about 20-30 % of the quantity of ions is anthropogenic even over such clean areas as Siberia.

&45 Annual variations of the chemical composition

In the discussion of Fig. 45 it was stated that no differences are detected in the precipitation composition in the summer (April-September) as compared with the winter (October-March) half-years. However, if the more detailed annual variation of the concentration of different ions is studied by means of monthly averages, an interesting spring maximum can be identified. Figure 51 shows the average annual variation of the concentration of several parameters of precipitation water collected at four rural stations in Hungary (E. Meszaros, 1974a). In the figure the average precipitation amount is also plotted. One can see that the concentrations of all components except pH have a maximum in March or April. The presence of a late summer or early fall minimum is also obvious. In the winter months, in the case of all the ions studied, a secondary maximum emerges, while ammonium and calcium distributions also show a small summer peak. It is evident that the secondary winter maximum is caused by anthropogenic effects. Furthermore, the small summer peaks are obviously explained by the influence of natural sources. The exception is the pH, the monthly values of which vary as a function of the difference between the sum of calcium and ammonium and the sum of nitrate and sulfate ions. Its annual variation is similar to that found in Scandinavia by Barrett and Brodin (1955).

In Subsection 5.4.1 it was shown that there is an inverse relation between the concentration of chemical components and precipitation amount. However, the spring maximum cannot be attributed to this effect since in the spring months the quantity of monthly precipitation increases. Only the October peak in the distribution of chemical parameters can be interpreted by this relationship.

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Fig. 51

Annual variation ofchemicalcomposition and precipitation quantity (Prec.) over Hungary (E. Mészâros,

1974a). (By courtesy of Tellus)

We can thus conclude that the spring maximum cannot be explained either by the annual variation of source intensity at the Earth's surface or by the variation of the quantity of precipitation. It has been postulated (E. Meszaros, 1974a) that this maximum is due to the oxidation effects of tropospheric ozone, the concentration of which also has a maximum during the spring (see Fig. 13). Ozone oxidizes S02 and N02 in atmospheric liquid water (see Subsection 5.3.2) which leads to the lowering of the pH. The increase in the concentration of hydrogen ions promotes the absorption of ammonia gas from the air, as well as the transformation of insoluble mineral components (e.g. calcium carbonate) into water-soluble materials. If this speculation is correct, this process provides a non-negligible ozone sink in the troposphere (Subsection 3.4.S) and plays an important role in the cleansing of the lower atmospheric layers from anthropogenic pollutants.

The existence of the spring maximum was also illustrated by Soviet and Swedish precipitation chemistry data (E. Mészáros, 1974a). Furthermore, Granat (1978) has demonstrated more recently, on the basis of a large amount of data gained in the European Atmospheric Chemistry Network, that the sulfur concentration in precipitation has a maximum in March. Further evidence is needed, however, before the acceptance of the above hypothesis

546 The future of precipitation chemistry measurements

It becomes more and more obvious that anthropogenic effects can modify (and will modify considerably in the future) the chemical composition of the atmosphere, even on a global scale (see Chapter 3). Since these modifications may lead to inadvertent climatic modification (see Chapter 6), there is an increasing need to monitor the trends in large scale atmospheric composition. Since, on the one hand, sampling of precipitation water is rather easy and, on the other hand, the precipitation composition is a good indirect indicator of the pollution of a large part of the troposphere, the Workl Meteorological Organization decided to include the measurement of the chemical composition of precipitation in its background air pollution monitoring program. Considering that the density of this network is adequate at present only in Europe and in North America, further efforts have to be made so that this program becomes really world-wide. The standardization of sampling and chemical analysis procedure is also an obvious need.

Furthermore, in all international programs intended to determine the long-range transport of sulfur and nitrogen containing pollutants (e.g. across national boundaries) one has to organize a dense precipitation chemistry network to follow the wet deposition of these components. Such studies (e.g. the present program of the Economic Commission for Europe) seem to be indispensable in order to understand the atmospheric pathways of trace constituents on a continental scale in a more reliable way and to make international air quality management possible.

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