Page 332 - Fundamentals of Air Pollution 3E

288 18. The Meteorological Bases of Atmospheric Pollution

Fig. 18-6. Typical values of the washout coefficient as a function of rainfall rate and
particle diameter. Source: After Engelmann (13).

Scavenging may also be considered as an exponential decay process:

3
where x(0 is the concentration in /ug m at time t in seconds, x(0) is the
concentration at time 0, and A is the scavenging or washout coefficient,
1
s" . Figure 18-6, after Engelmann (13), gives the washout coefficient as a
function of particle diameter and rainfall rate. McMahon and Denison (12)
give a table of field measurements of scavenging coefficients. This same
concept can be applied to gaseous pollutants. Fewer data are available for
5
5
gases. Values ranging from 0.4 x 10~ to 6 x 10 ~ for SO 2 are given by
McMahon and Denison (12) and compare reasonably well with an equation
for SO 2 by Chamberlain (14):

where / is rainfall intensity in mm h *.

REFERENCES

1. Firket, ]., Trans. Faraday Soc. 32, 1192-1197 (1936),
2. Roholm, K., /. Ind. Hyg. 19, 126-137 (1937).
3. Schrenk, H. H., Heimann, H., Clayton, G. D., Gafafer, W. M., and Wexler, H., "Air
Pollution in Donora, Pa," Public Health Bulletin 306. U.S. Public Health Service, Washing-
ton, DC, 1949.