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5.14 The study of wet removal mechanisms by means of atmospheric measurements

For the investigation of the importance of the different wet removal processes, it is usual to compare the chemical composition of cloud and precipitation waters sampled simultaneously. This would appear to separate the effects of rain-out and wash-out. However, as we shall see, the results of such a comparison have to be considered with some caution.

The composition of cloud and precipitation water was investigated by Petrenchuk and Drozdova (1966), among others; they developed a special cloud water collector that worked at positive as well as at negative temperatures. Their results, obtained over the European parts of the U.S.S.R., are given in Table 28. It can be seen that over clean northern regions the difference between the sum of ions in cloud and precipitation elements is not great. In these areas the concentration of sulfate and nitrate ions is relatively small while the chloride content is great. This situation can be explained by maritime influences. In comparison, sulfate is the predominant component over the south-western regions, and this is at least partly due to anthropogenic sources.

One of the most interesting (and disturbing) results of this investigation is the fact that in the case of some ions the concentrations are higher in cloud than in precipitation water, especially over the cleaner areas (north). Irt the case of chloride this unexpected difference was ubiquitous. Thus the Cl~/Na+ ratio in clouds is greater than the corresponding value in sea water (equal to 1.8) while in precipitation it is smaller. It is very difficult to explain this finding. A partial explanation may be that the precipitation elements were formed from the more dilute part of the cloud water (i.e., the larger drops). Further, Petrenchuk and Drozdova speculated that gaseous chlorine could be liberated below cloud base due to chemical reactions of the sea salt components (see, e.g. Subsection 3.S.4).

Finally, the measurements of Petrenchuk and Drozdova (1966) also showed that in clouds with smaller drops the water contains more dissolved materials than in clouds with greater average drop size, in agreement with the previous theoretical considerations.

Table 28

Chemical composition of cloud and precipitation waters over the European part of the U.S.S.R.

(Petrenchuk and Drozdova, 1966)

Table 28

Chemical composition of cloud and precipitation waters over the European part of the U.S.S.R.

(Petrenchuk and Drozdova, 1966)

Territory

Sample

cr

NO;

[mg 1

na']

k*

mg"

Ca"

pH

Conduct. OiScm'1]

Total [mgl']

CI/ Na

North

Precipitation 3.6

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