Geoscience Reference
In-Depth Information
Table 3.12.
Sources and sinks of atmospheric nitric
oxide
Table 3.13.
Sources and sinks of atmospheric
nitrogen dioxide
Sources
Sinks
Sources
Sinks
Denitrification in soils
and plants
Lightning
Fossil fuel and biofuel
combustion
Biomass burning
Photolysis and kinetic
chemical reaction
Atmospheric chemical
reaction
Dissolution in surface water
Deposition to sea ice, snow,
soil, vegetation, and
structures
Atmospheric chemical
reaction
Atmospheric chemical
reaction
Fossil fuel and biofuel
combustion
Dissolution in surface
water
Biomass burning
Deposition to sea ice, snow,
soil, vegetation, and
structures
burning. During combustion or burning, NO
2
(g) emis-
sions are about 5 to 15 percent those of total NO
x
(g).
Indoor sources of NO
2
(g) include gas appliances,
kerosene heaters, wood-burning stoves, other biofuel
burning for heating and cooking, and cigarettes. Sinks
of NO
2
(g) include photolysis, chemical reaction, disso-
lution in surface water, and deposition to ground sur-
faces. NO
2
(g) is relatively insoluble in water.
percent of NO
x
(g) emissions were from transportation
sources, whereas much of the rest were from electric
power and industrial production. Between 1970 and
2008, NO
x
(g) emissions decreased 39.2 percent in the
United States.
3.6.7.2. Mixing Ratios
Atypical sea-level mixing ratio of NO(g) in the back-
ground troposphere is 5 pptv. In the upper troposphere,
NO(g) mixing ratios are 20 to 60 pptv. In urban regions,
NO(g) mixing ratios reach 0.1 ppmv in the early morn-
ing but may decrease to zero by midmorning due to
reaction with ozone.
3.6.8.2. Mixing Ratios
Mixing ratios of NO
2
(g) near sea level in the back-
ground troposphere range from 10 to 50 pptv. In the
upper troposphere, they range from 30 to 70 pptv,
and in urban regions, from 50 to 250 ppbv. Outdoors,
NO
2
(g) reaches its peak mixing ratio during midmorn-
ing because sunlight breaks down most NO
2
(g) past
midmorning. In homes with gas cooking stoves or
unvented gas space heaters, weekly average NO
2
(g)
mixing ratios range from 20 to 50 ppbv, although peak
mixing ratios may reach 400 to 1,000 ppbv (Spengler,
1993; Jones, 1999).
3.6.7.3. Health Effects
Nitric oxide has no harmful human health effects at
typical outdoor or indoor mixing ratios.
3.6.8. Nitrogen Dioxide
Nitrogen dioxide
[NO
2
(g)] is a brown gas with a strong
odor. It absorbs short (blue and green) wavelengths of
visible radiation, transmitting the remaining green and
all red wavelengths, causing NO
2
(g) to appear brown.
NO
2
(g) is an intermediary between NO(g) emission
and O
3
(g) formation. It is also an immediate precursor
to nitric acid, a component of acid deposition. Natural
NO
2
(g), like natural NO(g), reduces ozone in the upper
stratosphere. NO
2
(g) is one of the six criteria air pol-
lutants for which outdoor standards are set by the U.S.
EPA under CAAA70. It is regulated in many countries.
3.6.8.3. Health Effects
Although exposure to high mixing ratios of NO
2
(g)
harms the lungs and increases respiratory infections
(Frampton et al., 1991), epidemiologic evidence sug-
gests that exposure to typical mixing ratios has little
health impact. Children and asthmatics are more sus-
ceptible to illness associated with high NO
2
(g) mixing
ratios than are adults (Li et al., 1994). Pilotto et al.
(1997) found that levels of NO
2
(g) greater than 80 ppbv
resulted in more sore throats, colds, and absences from
school. Goldstein et al. (1988) found that exposure to
300 to 800 ppbv NO
2
(g) in kitchens reduced lung capac-
ity by about 10 percent. NO
2
(g) may trigger asthma by
damaging or irritating and sensitizing the lungs, making
3.6.8.1. Sources and Sinks
Table 3.13 summarizes sources and sinks of NO
2
(g).
Its major source is oxidation of NO(g). Minor sources
are fossil fuel and biofuel combustion and biomass
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