Geoscience Reference
In-Depth Information
Table 3.3. Volume mixing ratios of some spatially and temporally varying gases in three atmospheric regions
Volume mixing ratio (ppbv)
Gas name
Chemical formula
Clean troposphere
Polluted troposphere
Stratosphere
Inorganic
Water vapor
3,000-4.0( + 7) a
5.0( + 6)-4.0( + 7) a
H 2 O(g)
3,000-6,000
Carbon dioxide
CO 2 (g)
390,000
390,000
390,000
Carbon monoxide
CO(g)
40-200
2,000-10,000
10-60
Ozone
O 3 (g)
10-100
10-350
1,000-12,000
Sulfur dioxide
SO 2 (g)
0.02-1
1-30
0.01-1
Nitric oxide
NO(g)
0.005-0.1
0.05-300
0.005-10
Nitrogen dioxide
NO 2 (g)
0.01-0.3
0.2-200
0.005-10
CFC-12
CF 2 Cl 2 (g)
0.55
0.55
0.22
Organic
Methane
CH 4 (g)
1,850
1,850-2,500
150-1,700
Ethane
C 2 H 6 (g)
0-2.5
1-50
- b
Ethene
C 2 H 4 (g)
0-1
1-30
-
Formaldehyde
HCHO(g)
0.1-1
1-200
-
Toluene
C 6 H 5 CH 3 (g)
-
1-30
-
Xylene
C 6 H 4 (CH 3 ) 2 (g)
-
1-30
-
Methyl chloride
CH 3 Cl(g)
0.61
0.61
0.36
CFC, chlorofluorocarbon.
a
10 7 .
4.0(
+
7) means 4.0
×
b
A(-) indicates that the volume mixing ratio is negligible, on average.
3.6.1. Water Vapor
Water vapor [H 2 O(g)] is the most important spatially
and temporally varying gas in the air. It is a natural
greenhouse gas in that it readily absorbs thermal-IR
radiation, but it also plays a vital role in the hydro-
logic cycle on Earth. As a natural greenhouse gas, it
is much more important than carbon dioxide for main-
taining a climate suitable for life on Earth. However,
as a global warming agent, its anthropogenic emis-
sions due to combustion of fossil fuels and evapora-
tion during irrigation and power plant cooling have
amuch smaller impact than do emissions of other
global warming agents. Nevertheless, anthropogenic
water vapor emissions do affect local temperatures, sta-
bility, the relative humidity, and cloudiness, which feed
back to air pollution (e.g., Jacobson, 2008a). Water
vapor is not considered a health-affecting air pollu-
tant; thus, no regulations control its mixing ratio or
emission.
Other well-mixed but inert gases present in trace con-
centrations include neon, helium, krypton, and xenon.
3.5.2. Spatially and Temporally
Varying Gases
Gases whose volume mixing ratios change in time and
space are variable gases. Table 3.3 summarizes the vol-
ume mixing ratios of some such gases in the clean tropo-
sphere, the polluted troposphere (e.g., urban areas), and
the stratosphere. Many organic gases degrade chemi-
cally before they reach the stratosphere, so their mixing
ratios are low in the stratosphere.
3.6. Characteristics of Selected Gases
and Aerosol Particle Components
Table 3.4 lists gases and aerosol particle components
relevant to each of the five major air pollution problems
discussed in this topic. The table indicates that each
problem involves a different set of pollutants, although
some pollutants are common to two or more problems.
Next, a few gases and aerosol particle components
listed in Table 3.4 are discussed in terms of their rele-
vance, abundance, sources, sinks, and health effects.
3.6.1.1. Sources and Sinks
Table 3.5 summarizes the sources and sinks of water
vapor. The main source, evaporation from the oceans,
 
 
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