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More recently, Fricke et al. (1978) reported the results of a similar investigation. In this study, carried out over Bavaria, F.R.G., the composition of cloud water collected at the cloud base was compared with the composition of rainwater sampled at the surface. It was found that the concentration of heavy metals at cloud base was about twice the value at ground level in rainwater, in agreement with some of the results of Petrenchuk and Drozdova (1966). Fricke's study also indicated that the total sulfur content (sulfate -I- S02 + sulfite; about 30 % of the total was S02 and sulfite) was doubled between the cloud base and the surface, probably due to S02 wash-out.

The understanding of wet removal of trace constituents is further facilitated by sampling at several altitudes above and below the cloud base. Such an investigation was done by Georgii (196S) in the Alps. He collected rainwater samples at four levels. The altitude difference between the highest and lowest station was 1800 m (the horizontal distance was 6.5 km). At the two higher sampling points, which were generally in the cloud, the cloud water was also sampled. The mean results for electrolytic conductivity (measure of the total ion content) obtained in 1963-1964 are plotted in Fig. 43. The abscissa gives values expressed in percentage of the conductivity observed at the lowest level (Innsbruck), while for the cloud and rainwater collected inside the clouds the absolute values are also given. One can see from this figure that the electical conductivity of cloud water was found to be practically the same as that of rainwater in Innsbruck. On the basis of this finding, in the absence of other information, we should say that the wash-out was negligible during this study. However, the quantity of ions in precipitation collected above the cloud base was found to be significantly smaller than the same parameter in cloud water. This is consistent with the hypothesis that precipitation forms from more diluted part of cloud water, as mentioned above.

Fig. 43

Variation of the electrical conductivity of cloud and precipitation as a function of altitude according to Georgii (1965). The solid line corresponds to rain water and is expressed in percent of conductivity measured in Innsbruck. (By courtesy of Deutscher Wetterdienst)

Fig. 43

Variation of the electrical conductivity of cloud and precipitation as a function of altitude according to Georgii (1965). The solid line corresponds to rain water and is expressed in percent of conductivity measured in Innsbruck. (By courtesy of Deutscher Wetterdienst)

It also follows from Fig. 43 that the composition of cloud and precipitation water, at least in the lower part of the cloud, does not depend very much upon the altitude. On the other hand, below the cloud base, which was generally between Hungerburg and Seegrube, the electrical conductivity increases by a factor of two, which means that rain-out and wash-out processes contribute about equally to the final concentration of salts in precipitation water measured at the surface.

Wet removal processes can also be studied by measuring the variation of the chemical composition during individual rainfalls. Figure 44 gives the results of such a study (unpublished work by the present author; samples taken near Budapest). The part (a) of this figure refers to calcium ions, which come only from particulate matter, while part (b) represents the concentration variation in case of ammonium ions, which are due both to ammonium containing particles and to gaseous ammonia in the air. In Fig. 44 the change of rainfall rate during precipitation is also given (dotted lines). It is to be noted that concentrations in rainwater (ordinate) are expressed in units of the average for total rainfall, while the abscissa gives the time in percentage of the duration of precipitation. One can see from Fig. 44 that calcium and ammonium concentrations in rainwater decreased to a constant value in the middle of the rainfall period, when the precipitation intensity is at a maximum. At the end of the rainy interval the calcium concentration begins to rise, but it does not reach the starting values. This phenomenon is not observed in the case of ammonium ions. It can also be seen that at the beginning of the rainfall calcium concentration increases slightly.

Fig. 44

Average variation of calcium and ammonium concentration in precipitation samples (solid lines) as a function of the duration (expressed in %) of rainfall. Dashed line is the precipitation intensity

Fig. 44

Average variation of calcium and ammonium concentration in precipitation samples (solid lines) as a function of the duration (expressed in %) of rainfall. Dashed line is the precipitation intensity

On the basis of our foregoing discussion, the pattern represented by Fig. 44 may be explained as follows. Let us consider a cloud with precipitation which moves over our sampling site. At the beginning, precipitation elements fall from the frontal part of the cloud. In this part, near the edge of the cloud, the liquid water content is low and consequently the water is more concentrated. Since there is a direct relation between liquid water content in the cloud and precipitation intensity at the surface the rainfall rate is also low at this time. The light rain falls in an "unwashed" polluted atmosphere which is frequently unsaturated with water vapour. Both circumstances promote the formation of higher concentrations in rainwater. Some larger, more dilute drops reach the surface faster than smaller ones (see the curve for calcium) because of their higher falling speed. In the middle of the rain period precipitation elements arrive from the centre of the cloud where the liquid water content is higher and t he water is less contaminated. Also, in this period the air below the cloud base is cleaner and already saturated. At the end of the precipitation the drops fall from the rear part of the clouds where liquid water is again small. However, the air is much cleaner then before the rainfall due to the wet removal. Hence the concentration increase is not large enough to reach the initial values (calcium); it may not occur at all (ammonium).

It follows implicitly from this discussion that smaller precipitation elements are generally more concentrated than larger ones since rain of low intensity is composed of smaller drops (Best, 1950). This relation was experimentally proved by Georgii and Wotzel (1967) who constructed a special rainwater collector that classified rain drops according to their size. In this way they demonstrated that the concentration increased with decreasing drop size.

We can expect that the level of trace constituents in the air decreases during rainfall. However, variations of surface air concentration of atmospheric substances are not representative of the wet removal taking place at higher tropospheric levels. It is very difficult to make a separation between effects caused by wash-out and those which are due to air mass exchanges. With these problems in mind the data obtained by Georgii (1960) in Frankfurt am Main are presented in Table 29. In this table the concentration of different gaseous and particulate constituents are given, measured before and after precipitation. It can be seen that the decrease may be as great as 73 % (nitrate particles).

Table 29

Concentrations of trace constituents in the air before and after rainfall in Frankfurt am Main between June 1956 and May 1957 (Georgii, 1960)

Table 29

Concentrations of trace constituents in the air before and after rainfall in Frankfurt am Main between June 1956 and May 1957 (Georgii, 1960)

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