Acid Deposition - Air Pollution and Global Warming: History, Science, and Solutions - page 235

lime [Ca(OH) 2 (aq)] is then sprayed into the stream. Sul-
fur dioxide can dissolve in the spray drops, producing
calcium sulfate [CaSO 4 (s)], which is readily removed
as a solid. The remaining exhaust and pollutants are
vented to the air.

deposition. Acid deposition problems in the sulfur tri-
angle (Section 8.2.4) are still severe, although reduc-
tions in emissions of acid gases and other pollutants
resulted in visibility improvements during the 2000s
(Figure 8.6), suggesting that acid deposition problems
have also become less severe over time.
Despite success in some parts of the world, acid rain
became more pronounced in China during the 2000s
than in prior decades, primarily due to the substan-
tial growth in coal combustion there. For example, in
2010, 258 cities experienced acid rain, primarily from
sulfuric and nitric acid (Press Trust of India, 2011).
In many of these cities, every raindrop measured was
acidic. Despite having some of the highest concen-
trations of sulfate and nitrate ions in rainwater in the
world, the pH of rainwater in Beijing is less acidic
than in many other cities ,oftenbetween 5 and 6.8
(Xu and Han, 2009). The reason is that frequent dust
storms from the Gobi Desert pass through Beijing.
Dust contains calcium carbonate, which is a neutral-
izing agent (Section 10.6.3). Thus, although rainwater
in Beijing contains sulfate and nitrate, it also contains
calcium, which moderates the pH of rainwater. The pH
of rainwater in many other cities in China is frequently
between 4 and 5, as is the pH of rainwater in Tokyo,
Istanbul, Mexico City, and Itatiaia, Brazil (Xu and
Han, 2009).

10.7.2. Effects of Regulation
Between the 1970s and 2011, reductions in sulfur diox-
ide emissions reduced the extent of acid deposition in
most of North America and Europe, but increases in
such emissions exacerbated acid deposition in parts of
Asia.
In the United States, rainwater acidity decreased
and pH increased between the 1990s and late 2000s,
as illustrated in Figure 10.4. For example, between
1994 and 1996, more than 40 percent of the United
States experienced acid rain in at least 50 percent of
all rainwater samples. One percent of all samples had
apH 5. Most acidity occurred in the northeastern

United States, particularly Ohio, Pennsylvania, and
nearby areas. Between 2002 and 2004, the area over
which at least 50 percent of rainwater samples were
acidic dropped to 35 percent (Lehmann et al., 2007).
Although the pH increase occurred simultaneously with
a decrease in the sulfate ion in rainwater, it also coin-
cided with an increase in the ammonium ion. For exam-
ple, between 1985 and 2004, the sulfate ion content of
U.S. rainwater decreased by 46 percent and the ammo-
nium ion content increased by 29 percent (Lehmann
et al., 2007). Ammonia is a neutralizing agent that helps
increase the pH (Section 10.6.5).
Reductions in sulfur dioxide emissions in Canada
have similarly reduced the acidity of some Canadian
lakes and forests. For example, in the late 1960s, the
Sudbury, Ontario nickel smelting plant was the largest
individual source of SO 2 (g) in North America, emit-
ting 4,500 tonnes of SO 2 (g) per day, devastating nearby
lakes and forests. In 1972, a large stack was built to dis-
perse the pollutants further downwind (Section 6.6.2.4),
and in the 1990s, pollution scrubbing equipment was
added to reduce emissions to less than 450 tonnes of
SO 2 (g) per day. As a result, nearby lakes and forests
partially regenerated, although significant pollution still
occurred. Reductions in the acidity of lakes in Quebec,
Atlantic Canada, and other areas of Ontario were less
dramatic.
In Europe, reductions in sulfur dioxide emissions
have similarly reduced the acidity of rainwater. Dur-
ing the last three decades, many lakes in Sweden have
been restored, but many more are still damaged by acid

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10.8. Summary
In this chapter, the history, science, effects, and control
of acid deposition are discussed. Acidity is determined
by pH, which ranges from less than 0 (very acidic) to
more than 14 (very basic). The pH of distilled water is
7, of natural rainwater is 5 to 5.6, and of acid rain or
fog is less than 5. Acid deposition occurs when sulfu-
ric, nitric, or hydrochloric acid is emitted into or forms
chemically in the air and is subsequently deposited as a
gasorliquid to the ground, where it harms microorgan-
isms, fish, forests, agriculture, and structures. In high
concentrations in the air, acids can also harm humans.
Severe acid deposition problems arose from increased
coal combustion in the UK during the Industrial Revolu-
tion and from the growth of the alkali industry in France
and the UK during the 1800s. Today, sulfuric acid is
usually the most abundant acid in rainwater. Sulfuric
acid is produced by gas- and aqueous-phase oxidation
of sulfur dioxide. The latter process is most efficient
when cloud drops are present. In polluted coastal air,
nitric acid fog is often a problem. A method of amelio-
rating the effects of acid deposition on lakes is to add a