Acid Deposition - Air Pollution and Global Warming: History, Science, and Solutions - page 226

hydrogen ions chemically, acidifying the water. This
process is discussed in Section 10.3.2.

of primarily anthropogenic rather than natural acids.
In the eastern United States, about 60 to 70 percent
of excess acidity of rainwater has been due to sulfuric
acid, whereas 30 to 40 percent has been due to nitric
acid (Glass et al., 1979). Thus, sulfuric acid has been
the predominant acid of concern.
In polluted cities where fog is present, such as in
Los Angeles or London, nitric acid fog is a prob-
lem. In locations where HCl(g) is emitted anthro-
pogenically, such as near wood burning or industrial
processing, HCl(aq) affects the acidity of rainwater.
Today, however, HCl(aq) contributes to less than 5 per-
cent of total rainwater acidity by mass. Other acids
that are occasionally important in rainwater include
formic acid [HCOOH(aq), produced from formalde-
hyde] and acetic acid [CH 3 COOH(aq), produced from
acetaldehyde].

10.2.3. Nitric Acid
When nitric acid gas dissolves in raindrops, it forms
aqueous nitric acid [HNO 3 (aq)], a strong acid that dis-
sociates almost completely by

H +
Hydrogen
ion

NO 3
Nitrate
ion

HNO 3 (g)
Nitric
acid gas

HNO 3 (aq)
Dissolved
nitric acid

+ (10.7)

adding one proton to solution. As with sulfuric acid,
nitric acid decreases the pH of rainwater below that of
rainwater affected by only carbonic acid.

10.2.4. Hydrochloric Acid
When gas-phase hydrochloric acid dissolves in rain-
drops, it forms aqueous hydrochloric acid [HCl(aq)],
astrong acid that dissociates almost completely by

10.2.6. Acidity of Rainwater and Fog Water
Rainwater with a pH less than that of natural rainwater
is acid rain. The pH of acid rain varies between 2 and 5 ,
although typical values are near 4 and extreme values of
less than 2 have been observed (Likens, 1976; Marsh,
1978; Graves, 1980; Graedel and Weschler, 1981). A
pH of 4 corresponds to an H + molarity 40 times that of
natural rainwater, whereas a pH of 2 corresponds to an
H + molarity 4,000 times that of natural rainwater.
In Los Angeles, where fogs are common and nitric
acid mixing ratios are high, fog water pHs are typically
2.2 to 4.0 (Waldman et al., 1982; Munger et al., 1983),
butlevels as low as 1.7 have been recorded (Jacob,
1985). Nitrate ion molarities in those studies were about
2.5 times those of sulfate ions. A fog with a pH below
5isan acid fog .

H +
Hydrogen
ion

Cl −
Chloride
ion

HCl(g)
Hydrochloric
acid gas

HCl(aq)
Dissolved
hydrochloric acid

+ (10.8)

adding one proton to solution. Hydrochloric acid also
decreases the pH of rainwater below that of rainwater
affected by only carbonic acid.

10.2.5. Natural and Anthropogenic
Sources of Acids
Some of the enhanced acidity of rainwater from sul-
furic, nitric, and hydrochloric acids is natural. Vol-
canos, for example, emit SO 2 (g), a source of sulfuric
acid, and HCl(g). Phytoplankton over the oceans emit
dimethylsulfide [DMS(g)], which oxidizes to SO 2 (g).
The main natural source of HNO 3 (g) is gas-phase oxi-
dation of NO 2 (g) originating from lightning-produced
NO(g). The addition of natural acids to rainwater
already containing carbonic acid results in typical nat-
ural rainwater pHs of between 5.0 and 5.6, as shown in
Figure 10.3.
Acid deposition occurs when anthropogenically pro-
duced acids are deposited to soils, lakes, plants, tree
leaves, or buildings as gases or within aerosol particles,
fog drops, or raindrops. The two most important anthro-
pogenically produced acids today are sulfuric and nitric
acids, although hydrochloric acid can be important in
some areas. In the United States, about 70 percent of
SO 2 (g) and more than 85 percent of NO x (g) emissions
are anthropogenic in origin. Thus, the excess acidifica-
tion of rain due to sulfuric and nitric acids is a result

10.3. Sulfuric Acid Deposition
The most abundant acid in the air is usually sulfuric
acid, whose source is sulfur dioxide gas, emitted anthro-
pogenically from coal-fired power plants, metal smelter
operations, and other sources (Section 3.6.6).
Power plants usually emit SO 2 (g) from high stacks
so that the pollutant is not easily deposited to the ground
nearby. The higher the stack, the further the wind carries
the gas before it is removed from the air. The wind trans-
ports SO 2 (g) long distances, sometimes hundreds to
thousands of kilometers. Thus, acid deposition is often
aregional and long-range transport problem. When
acids or acid precursors are transported across politi-
cal boundaries, they create transboundary air pollu-
tion problems, prevalent between the United States and