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The raindrop will wash out a fraction eMt of this mass, where e is the impaction efficiency as defined in Subsection 4.1.2 (for its numerical value see Mason, 1957). If the drop radius lies between 50-2000 /xm the impaction efficiency is 1 for 10 /im g r < R and decreases with decreasing particle size. It ranges from 0.5 to 0.1 if the particle radius varies between 5 fim and 2 /im. This indicates that only giant particles in the coarse mode (Subsection 4.3.2) are significantly washed out below the clouds. Let us multiply the right-hand side of equation [5.16] by e and divide the result by the volume of the raindrop. This yields the trace concentration in the drop (C2(l) due to the capture of aerosol particles:

C2a = —\er3N(r)dr which is inversely proportional to drop radius.

The change of the mass concentration of aerosol particles (M) caused by washout can also be calculated easily. Let us designate by v(R) the falling speed of the drops with number concentration N(R). Suppose that this speed is much higher than the deposition velocity of the particles. Under these conditions the particle mass loss in the air per unit time is dAf

- — -eMR nv(R)N(R) [5.18] di or after integration:

where M(t) and M(0) are the mass concentrations at time t and at the beginning of the removal process, respectively. By the following substitution,

equation [5.19] yields:

Physically, relation [5.20] means that the mass concentration of aerosol particles below the cloud base decreases exponentially due to wet removal.

Equation [5.20] was used by Greenfield (1957) among others. During his model investigation he utilized the results obtained by Best (1950) according to which the rainfall intensity unambiguously determines the drop characteristics. In this way he found that at a rainfall rate of 2.5 mm hr "1 75-80 % of particles with a radius of 10 fjaa are removed during one hour. More recently the wash-out of sulfate particles below cloud base was modelled theoretically by Scott (1978). His calculations show that the wash-out of sulfate particles is of secondary importance compared to sulfate rain-out caused mainly by condensation. This result is not surprising when one considers the size distribution of sulfate particles (Subsection 4.4.2).

The absorption of trace gases below the cloud is controlled by the same laws as for the rain-out process. The wash-out of trace gases may be particularly significant if the concentration (partial pressure) of the gas considered increases with decreasing height, which is generally the case, especially in more polluted air.

Because of different rain-out and wash-out processes, the concentration of trace constituents, C, that can be measured in rainwater samples collected at the surface will be as follows (Junge, 1963):

where / represents the effect of evaporation (/ is equal to or greater than unity),

10 Mazaros while indexes 1,2, a and g refer to rain-out, wash-out, aerosol particles and gases, respectively.

The significance of the terms in equation [5.21 ] can be estimated by modelling in an appropriate way the wet removal of gaseous and particulate compounds. Table 27 gives the relative importance of the different terms, expressed in percentage of the surface concentration in rainwater for each sulfur species. Ail authors listed made model calculations using results of laboratory experiment and atmospheric observations. It can be seen that the relative influence of the wet removal of sulfate particles lies between 25 and 85 %. In a more recent paper, Scott (1978) speculates on the basis of his removal modelling that the sulfate mass measured at the surface in rainwater can be attributed totally to the rain-out and wash-out of airborne sulfate particles. This discrepancy in the models is obviously caused by the different assumptions used. It cannot be excluded, however, that this fraction really varies as a function of the degree of pollution of the atmosphere, or of other parameters.

Table 27

Contribution of different scavenging processes to the total sulfate concentration in rain water (%)

Table 27

Contribution of different scavenging processes to the total sulfate concentration in rain water (%)

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