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6.2 Sources of Soil Pollutants 171

depositions may occur everywhere, while cloud deposition mainly occurs in coastal
and mountainous due to their immergence in cloud and fog. Wet deposition occurs
due to incorporation of particles and gases into cloud droplets and their subsequent
precipitation as rain or snow. Wet deposition may also result from scavenging of

particles and gases by raindrops and snowflakes as they fall. These processes are
controlled by concentration and size distribution of particles and the solubility and
reactivity of gases and by meteorological factors. There are such gases as oxides
of nitrogen and sulfur dissolved in rainwater. In dry deposition, particles >5 μm
diameter are deposited mainly by gravitational sedimentation and inertial impaction.
Brownian diffusion is the process of dry deposition of particles ~2 μm (Fowler
1980 ). Particles between 0.2 and 2 μm are not readily dry deposited. They are
deposited by wet deposition. The characteristics of the chemical species of interest

in air pollutant dry deposition on soil and ecosystems have sufficiently been studied.
The particle-associated substances derived primarily from erosion of soil containing
K, Ca, Mg, Al, and Si tend to reside on larger airborne particles which can be deposited
by gravitational sedimentation (Lindberg and McLaughlin 1986 ). In contrast,
+
+
2−
the majority of airborne mass of particulate SO 4 , NH 4 , H , and Pb reside with
sub-micrometer aerosol, which can be deposited by wet deposition. Nitrates reside
2−
on particles of wide ranges of sizes. Nitrate and SO 4 may occur on larger particles
through adsorption of reactive gases like SO 2 and nitric acid vapor (HNO 3 ) on large
alkaline or sea salt particles in the atmosphere (Butler 1988 ). Deposition in the form
of dry particulate matter and wet precipitation can be acidic due to air pollution
from burning fossil fuels. Sulfur dioxide (SO 2 ) and nitrogen oxides (NO, NO 2 ) react
in the atmosphere to form sulfuric and nitric acids, respectively. These acids can
alter the environment where they fall from the atmosphere, which can be long
distances from the pollution source. The impacts of wet acidic deposition or acid
rain are usually noticed in the higher elevations or ridgetops that receive more
deposition due to the orographic effect and also in areas that have geological formations
with limited buffering capacity (Kimmel 1999 ). Impacts to the environment include
damage to trees, depletion of nutrients in the soil, and acidic stream water. The
combination of low pH and toxic aluminum in water causes osmoregulatory failure

and can cause the depletion of fish populations in headwater streams.
Heavy metals are emitted into the atmosphere through industrial, motor transport
and volcanic activities, soil erosion, forest fires, evaporation, etc. (Salomons and

Forstner 1984 ). Most of the metals in the air are in particulate form with the exception
of mercury (Hg). Usually, toxic metals such as Pb, Cd, and As are found in fi ne particles
(<2.5 μm). Fine particles are respirable and tend to persist in the atmosphere where
they can undergo chemical reactions and be transported from their sources over
long distances to pristine areas of the environment (Ross 1987 ). Most of heavy
metals are soluble in water; thus, they can migrate in ecosystems by water pathways.
Even small amounts and low concentrations of heavy metals can damage ecosystems
and are dangerous for human health (Ovadnevaite et al. 2006 ). Luo ( 2009 ) reported
atmospheric deposition of As, Cr, Hg, Ni, and Pd to soils.

The deposition fluxes of inorganic chemicals New Jersey precipitation is given
in Table 6.3 for an understanding of the magnitude of atmospheric deposition.