Page 186 - Fundamentals of Air Pollution 3E

152 10. Effects on the Atmosphere, Soil, and Water Bodies

during surface fog and conversion in nonprecipitating clouds. The RADM
also overestimated SO 2 and underestimated aerosol sulfate.
In the eastern United States, acid rain consists of —65% sulfuric acid,
—30% nitric acid, and ~5% other acids. In the West, windblown alkaline
dusts buffer the acidity in rains occurring over many rural areas, whereas
in urban areas 80% of the acidity is due to nitric acid (28). Average pH in
rainfall over the eastern United States for the period April 1979-March
1980 was less than 5.0, with some areas less than pH 4.2 (29). The lowest
annual pH recorded was 3.78 at De Bilt, The Netherlands, in 1967, and the
lowest in an individual rainfall was 2.4 at Pitlochry, Scotland, on April 10,
1974 (30).
One of the major effects of acidic deposition is felt by aquatic ecosystems
in mountainous terrain, where considerable precipitation occurs due to
orographic lifting. The maximum effect is felt where there is little buffering
of the acid by soil or rock structures and where steep lakeshore slopes
allow little time for precipitation to remain on the ground surface before
entering the lake. Maximum fish kills occur in the early spring due to the
"acid shock" of the first meltwater, which releases the pollution accumu-
lated in the winter snowpack. This first melt may be 5-10 times more acidic
than rainfall.
Although the same measurement techniques for rainfall acidity have not
been used over a long period of time and sampling has been carried out
at relatively few locations, the trend between 1955-1956 and 1975-1976
was for the area with a pH of less than 4.6 to expand greatly over the
eastern United States. The largest increases occurred over the southeastern
United States, where industrialization grew rapidly during the period. The
last several decades have also seen an increased area of lower pH over
northern Europe.

VI. EFFECTS OF ACIDIC DEPOSITION

Land, vegetation, and bodies of water are the surfaces on which acidic
deposition accumulates. Bodies of fresh water represent the smallest pro-
portion of the earth's surface area available for acidic deposition. Yet, the
best-known effect is acidification of freshwater aquatic systems.
Consider a lake with a small watershed in a forest ecosystem. The forest
and vegetation can be considered as an acid concentrator. SO 2 , NO 2, and
acid aerosol are deposited on vegetation surfaces during dry periods and
rainfalls; they are washed to the soil floor by low-pH rainwater. Much of
the acidity is neutralized by dissolving and mobilizing minerals in the soil.
Aluminum, calcium, magnesium, sodium, and potassium are leached from
the soil into surface waters. The ability of soils to tolerate acidic deposition
is very dependent on the alkalinity of the soil. The soil structure in the