Health Assessment Document for Diesel Emissions - NSCEP | US ...

1 2.4.3. Physical Removal Processes
2 2.4.3.1. Dry Deposition
3 Dry deposition is the removal of particles and gases from the atmosphere through the
'4 delivery of mass to the surface by nonprecipitation atmospheric processes and the subsequent
5 physical attachment to, or chemical reaction with, surfaces such as vegetation, soil, water, or the
6 built'environment (Dolske and Gatz, 1985). It should be noted that the surface itself may be wet
7 or dry; the term "dry deposition" refers to the mechanism of transport to the surface, not to the
8 nature of the surface itself. Dry deposition plays an important role as a removal mechanism of
9 pollutant in the absence of precipitation. Even in remote locations such as Siskiwit Lake, located
10 on a wilderness island in northern Lake Superior, the dry deposition of aerosol was found to
11 exceed the wet removal mechanism by an average ratio of9:1 (McVeety and Hites, 1988).
12 For particles, the deposition velocities depend on the particle size, exhibiting a minimum
13 for particles of mean diameter of "'0 .1 to "' 1 IJ.m.
14
1"5 2.4.3.2. Wet Deposition
16 Wet deposition encompasses all processes by which airborne pollutants are transported to
17 the Earth's surface in aqueous form (i.e., in rain, snow, or fog). The mechanisms ofwet removal
18 from the atmosphere may be very different for particle-associated compounds and for gas-phase
19 compounds. However, because many organic compounds are partitioned between the aerosol
20 . and vapor phase, processes of both gas and particle scavenging may be important for a given
21 compound (Ligocki et al., 1985a,b; Bidleman, 1988). When there is no exchange ofmaterial
22 between the particulate and dissolved phases in the rain, the total scavenging of a given
23 compound can be expressed as (Pankow et al., 1984):
24
25 w = w g (1-) + w p