Applications of Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 140

partitioning process. Because of their small sizes and long atmospheric residence
times, fog droplets can also be considered to attain equilibrium with the atmosphere
rather quickly.
Using the above equation for washout ratio, the overall flux of pollutants to the
surface (in mol/m 2 /s) by wet deposition can be obtained as

J wet ( G + P ) = R I · W T · C i a , (4.46)

where R I is the rainfall intensity (cm/h) and C i a is the measured concentration of i
in air.
If K aw is large, then the vapor uptake into droplets is negligible and only the
aerosol particulate fraction is removed by wet deposition. A number of environ-
mentally significant hydrophobic organic compounds fall in this category. Examples
are n -alkanes, polychlorinated biphenyls, chlorinated pesticides, and polyaromatic
hydrocarbons. Compounds with small K aw values are removed mostly via gas scav-
enging by droplets. Examples of this category include phenols, low-molecular-weight
chlorobenzenes, and phthalate esters.A number of investigators have reported exper-
imentally determined values of both W g and W p for a variety of metals and organics.
The range of values observed for typical pollutants in some regions of the world is
summarized in Table 4.7. Aerosols have overall washout ratios approximately 10 6 .
Aerosols with washout ratios less than 10 5 are generally considered to be insoluble
and relatively young in that their dimensions are of the order of 0.1 μ m. It has been

TABLE 4.7
Range of Washout Ratios for Typical Pollutants
Compound W g W p

1.3-2.2 × 10 4 n -Alkanes Portland, OR

3.5-5.8 × 10 5

College Station, TX

4.5-1.6 × 10 6 Norfolk, VA

10 2 -1.8 10 4 10 3 -1.1 10 4 PAHs 1.9 × × 2 × × Portland, OR

10 5

1.4-2.5 × Isle Royale

10 6

Phenanthrene 64 ± 46 1.1 × Chespeake Bay, MD

10 5

Pyrene 390 ± 280 8.5 × A

2.0 × 10 5 -1 × 10 6

Metals Ontario, Canada

1 × 10 5 -5 × 10 5 Northeast U.S.

Mercury, Hg 0

10-100

2 × 10 5 -2 × 10 6

Radionuclides

1 × 10 4 -1 × 10 5

PCDDs/PSDFs

1 × 10 2 -2 × 10 6 (rain)
5 × 10 4 -4 × 10 6 (snow)

PCBs

Sulfate, SO 2 −
4

10 4 -5 10 6

6 × ×

Source: From Bidleman, T.F. 1998. Environmental Science and Technology 22, 361-367; Ligocki,
M.P., Leuenberger, C., and Pankow, J.F. 1985. Atmospheric Environment 19, 1609-1617;
Baker, J.T. 1997. Atmospheric Deposition of Contaminate to the Great Lakes and Coasted
Waters . Pensacola, FL: SETAC Press.