Page 331 - Fundamentals of Air Pollution 3E

V. Removal Mechanisms 287

source with distance from the source to account for the particles removed
by deposition.
For 20-100 jjirn particles, the deposition w on the ground is

where the air concentration x is evaluated for a height above ground z of
about 1 m.
Particles larger than 100 ptm fall through the atmosphere so rapidly that
turbulence has less chance to act upon and disperse them. The trajectories
of such particles are treated by a ballistic approach.

B. Reaction (Transformation)
Transformations due to chemical reactions throughout the plume are
frequently treated as exponential losses with time. The concentration x(0
at travel time t when pollutant loss is considered compared to the concentra-
a
tion x t the same position with no loss is

where L is the half-life of the pollutant in seconds. The half-life is the time
required to lose 50% of the pollutant.

C. Dry Deposition
Although it does not physically explain the nature of the removal process,
deposition velocity has been used to account for removal due to impaction
with vegetation near the surface or for chemical reactions with the surface.
McMahon and Denison (12) gave many deposition velocities in their review
1
paper. Examples (in cm s" ) are sulfur dioxide, 0.5-1.2; ozone, 0.1-2.0;
iodine, 0.7-2.8; and carbon dioxide, negligible.

D. Wet Deposition
Scavenging of particles or gases may take place in clouds (rainout) by
cloud droplets or below clouds(washout) by precipitation. A scavenging
ratio or washout ratio W can be defined as

l
where k is concentration of the contaminant in precipitation in Mg g '> P
3
is the density of air, approximately 1200 g m~ ; and x is the concentration,
3
/u,g m" , of the pollutant in the air prior to scavenging. McMahon and
Denison (12) gave a table of field observations of washout ratios. The values
for various pollutants range from less than 100 to more than 4000. These
values are a function of particle size and rainfall intensity, generally decreas-
ing with the latter and increasing with the former.