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respiratory tract and lungs. Above 100 ppbv, ozone
affects animals by increasing their susceptibility to bac-
terial infection. It also interferes with the growth of
plants and trees and deteriorates organic materials, such
as rubber, textile dyes and fibers, and some paints and
coatings (U.S. EPA, 1978). Ozone increases plant and
tree stress and their susceptibility to disease, infestation,
and death.
from 1 to 30 ppbv. SO 2 (g) levels are usually lower
indoors than outdoors. The indoor-to-outdoor ratio of
SO 2 (g) is typically 0.1:1 to 0.6:1 in buildings without
indoor sources (Jones, 1999). Indoor mixing ratios can
range from 30 to 57 ppbv in homes equipped with a
kerosene heater or a gas stove (Leaderer et al., 1993).
3.6.6.3. Health Effects
Because SO 2 (g) is soluble, it dissolves and oxidizes
in the mucous membranes of the nose and respira-
tory tract to sulfurous acid [H 2 SO 3 (aq)], burning the
respiratory tract upon inhalation. Particles containing
sulfuric acid [H 2 SO 4 (aq)] are also soluble, but their
deposition rates into the respiratory tract depend on
particle size (Maroni et al., 1995). High concentrations
of SO 2 (g) and H 2 SO 4 (aq) can harm the lungs (Islam
and Ulmer, 1979). Bronchiolar constrictions and respi-
ratory infections can occur at mixing ratios greater than
1.5 ppmv. Long-term exposure to SO 2 (g) is associated
with impaired lung function and other respiratory ail-
ments (Qin et al., 1993). People exposed to open coal
fires emitting SO 2 (g) are likely to suffer from breath-
lessness and wheezing more than are those not exposed
to such fires (Burr et al., 1981). SO 2 (g) can cause death
at mixing ratios of 400 to 500 ppmv and above.
3.6.6. Sulfur Dioxide
Sulfur dioxide [SO 2 (g)] is a colorless gas that exhibits a
taste above 0.3 ppmv and a strong odor above 0.5 ppmv.
SO 2 (g) is a precursor to sulfuric acid [H 2 SO 4 (aq)], an
aerosol particle component that affects acid deposition,
global climate, and the global ozone layer. SO 2 (g) is one
of the six air pollutants for which NAAQS standards are
set by the U.S. EPA under CAAA70. SO 2 (g) is regulated
in most countries.
3.6.6.1. Sources and Sinks
Table 3.11 summarizes the major sources and sinks of
SO 2 (g). Some sources include coal-fired power plants,
vehicles, and volcanos. SO 2 (g) is also produced chemi-
cally in the air from biologically produced dimethylsul-
fide [DMS(g)] and hydrogen sulfide [H 2 S(g)]. SO 2 (g) is
removed by chemical reaction; dissolution in water (as
it is relatively soluble); and deposition to sea ice, snow,
soil, vegetation, and structures. Anthropogenic SO 2 (g)
emissions decreased in the United States between 1970
and 2008 by about 63 percent (Figure 3.14). In 2008,
its total emissions were
3.6.7. Nitric Oxide
Nitric oxide [NO(g)] is a colorless gas and a free rad-
ical. It is important because it is a precursor to tropo-
spheric ozone, nitric acid [HNO 3 (g)], and particulate
nitrate [NO 3 ]. Whereas NO(g) does not directly affect
acid deposition, nitric acid does. NO(g) does not affect
climate directly either, but ozone and nitrate particu-
late matter produced from it do. Natural NO(g) reduces
ozone in the upper stratosphere. Emissions of NO(g)
from jets that fly in the stratosphere also reduce strato-
spheric ozone. Outdoor levels of NO(g) are not regu-
lated in any country.
10.4 million tonnes.
3.6.6.2. Mixing Ratios
In the background troposphere, SO 2 (g) mixing ratios
range from 10 pptv to 1 ppbv. In polluted air, they range
Table 3.11. Sources and sinks of atmospheric sulfur
dioxide
3.6.7.1. Sources and Sinks
Table 3.12 summarizes the sources and sinks of NO(g).
NO(g) is emitted by microorganisms in soils and plants
during denitrification, and it is produced by light-
ning, combustion, and chemical reaction. Combustion
sources include fossil fuel, biofuel, and biomass burn-
ing. The primary sink of NO(g) is atmospheric chemical
reaction. Figure 3.14 shows that, in 2008, 14.8 million
tonnes of anthropogenic NO x (g) [
Sources
Sinks
Oxidation of
dimethylsulfide
Vo l canic emission
Fossil fuel combustion
Mineral ore processing
Chemical manufacturing
Atmospheric chemical
reaction to produce sulfuric
acid
Dissolution in cloud drops and
surface water
Deposition to sea ice, snow,
soil, vegetation, and
structures
NO 2 (g)]
were emitted in the United States. NO(g) emissions rep-
resent about 5 to 15 percent those of NO x (g). About 58
=
NO(g)
+
 
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