Contaminant Fate and Transport - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 298

Particulate
pollutants in
atmosphere

Gaseous
pollutants in
atmosphere

Sources

Pollutants in
cloud water
and
precipitation

VOC NO x

Wet
deposition

Hg

NO x

VOC

SO 2

Natural

Receptors

Anthropogenic

FIGURE 8.17
Process of wet and dry deposition of air contaminants. (From United States Environmental Protection Agency
(USEPA), What is acid rain? http://www.epa.gov/acidrain/what/index.html (accessed June 29, 2010), 2010.)

low-to-medium solubility, and low molecular weight. As a result of these chemical char-
acteristics, VOCs are common air, soil, and water contaminants (USGS 2006a; USEPA
2008a). Automobile exhaust contains VOCs. When combined with other common air pol-
lutants and sunlight, urban smog will form if atmospheric conditions are favorable—that
is, there is an ample supply of the combined sources of VOCs and other smog-forming
contaminants. This type of smog formation produces photochemical smog , and is shown
in Figure 8.18.

Step 1 Step 2

Accumulation of
air pollution

Smog
formation

Sunlight

Inversion (trapping pollutants)

NO NO 2

O 3
SO 2

Smog
layer

Haze

CO
VOC

Industrial
facility

Vehicle exhaust
(cars, trucks, buses)

Electric power
generating plant

FIGURE 8.18
Formation of photochemical smog.