Acid Deposition - Air Pollution and Global Warming: History, Science, and Solutions - page 234

water. The dissolved gas then reacts with the hydrogen
ion to form the ammonium ion by

and the European Community, was the first agreement
to deal with an international air pollution problem. Since
then, fifty-one countries have signed on to the treaty, and
it has been amended eight times. As part of a 1985
amendment (the Sulfur Protocol), member countries
were required to reduce their emissions or transbound-
ary fluxes of sulfur by 30 percent below 1980 levels
by 1993. A 1988 amendment (the Nitrogen Oxide Pro-
tocol) required countries to reduce their emissions or
transboundary fluxes of nitrogen oxide to their 1987
levels by December 1994. Because the first Sulfur Pro-
tocol did not address forest loss in central Europe suf-
ficiently, a second Sulfur Protocol was signed in 1994
that was intended to result in a 60 percent reduction
in sulfur emissions compared with 1980 values by
2010.

H +
Hydrogen
ion

NH 4
Ammonium
ion

NH 3 (aq)
Dissolved
ammonia

+ (10.20)

The loss of H + that results from this reaction increases
pH, reducing acidity. In many cases, the pH of rainwa-
ter containing the ammonium ion exceeds 6. Aerosol
particles and raindrops containing the ammonium ion
deposit to soils and lakes, providing these surfaces with
a neutralizing agent. Soils downwind of high ammonia
gasemissions tend to have a better neutralizing capacity
against acid deposition than do soils far from ammonia
sources, if all other conditions are the same.

10.7. Recent Regulatory Control
of Acid Deposition
The first major effort to control acid deposition was
the British Alkali Act of 1863, which mandated large
reductions in hydrochloric acid gas emissions by soda
ash manufacturers. In more recent years, the U.S. Clean
Air Act Amendments of 1970 led to lower emissions of
acid deposition precursors, namely SO 2 (g) and NO 2 (g).
In 1977, the United States initiated the National Atmo-
spheric Deposition Program (NADP), whose purpose
wastomonitor trends of acidity in precipitation. In
1980, the U.S. Congress passed the Acid Precipita-
tion Act ,which funded a program, the National Acid
Precipitation Assessment Program. Under the program,
the network of monitoring stations under NADP was
enlarged to produce a National Trends Network.
The Clean Air Act Amendments of 1990 mandated
a 10-million-ton reduction in sulfur dioxide [SO 2 (g)]
emissions from 1980 levels and a 2-million-ton reduc-
tion in nitrogen oxide [NO x (g)] emissions from 1980
levels by 2010. To implement these reductions, the
U.S. EPA established an emission trading system,
whereby emitters could trade among themselves for
limited rights to release SO 2 (g). Power plants were also
required to install emission monitoring systems. In Jan-
uary 2000, the U.S. EPA issued a new rule requiring
U.S. refiners to cut the sulfur content of gasoline to
one-tenth its value by 2006.
Meanwhile, several studies in the 1970s concluded
that winds were transporting acid deposition precur-
sors over long distances and political boundaries. Such
studies culminated in the 1979 Geneva Convention on
Long-Range Transboundary Air Pollution .The con-
vention, originally signed by thirty-four governments

10.7.1. Methods of Controlling Emissions
Several mechanisms are available to control emissions
of acid deposition precursors. These include the manda-
tory use of low-sulfur coal instead of high-sulfur coal
and the use of emission control technologies. The
quantity of SO 2 (g) emitted during coal combustion
depends on the sulfur content of the coal .Inthe
United States, about 40 percent of coal in 2009 was
mined in Wyoming, and 32 percent was mined in the
Appalachian Mountains (Energy Information Admin-
istration, 2011c). Coal from the Appalachian Moun-
tains has a high sulfur content. The cost of transporting
Appalachian coal to power plants, most of which are
in the midwestern and eastern United States, is lower
than is the cost of transporting low-sulfur coal from
Wyoming or other western states to these plants. As
such, coal burners prefer to use high-sulfur coal. How-
ever, CAAA90 requirements to reduce SO 2 (g) emis-
sions caused an increase in Wyoming's share of U.S.
coal from 31 to 40 percent between 2000 and 2009 and
a decrease in Appalachia's share from 39 percent to
32 percent.
The use of low-sulfur coal is one mechanism to
reduce emission of SO 2 (g) during coal burning. Another
is to remove a certain fraction of sulfur from high-
sulfur coal before burning it. A technology available for
reducing SO 2 (g) emission from a stack is the scrubber ,
first developed by William Gossage to reduce HCl(g)
emission. A modern-day scrubber technique is called
flue gas desulfurization .With this technique, a fan
first extracts a hot, gaseous exhaust stream containing
SO 2 (g) into a reaction tower. A mixture of aerosolized
water and dissolved limestone [CaCO 3 (aq)] or slaked