Geoscience Reference
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nitrogen dioxide and an organic radical with increasing
temperature. For example, at high temperature, PAN
thermally dissociates by the reverse of
of HO
2
(g). Because HO
2
(g) decreases with increasing
H
2
O(g) through Reaction 12.11 when NO
x
(g) is low,
the NO(g) that is available under such conditions pro-
duces little NO
2
(g) by Reaction 12.10 with increasing
H
2
O(g). As such, ozone decreases slightly with increas-
ing H
2
O(g). In addition, because OH(g) increases with
increasing H
2
O(g) by Reaction 12.9, OH(g) speeds the
conversion of existing NO
2
(g) to organic nitrates under
low NO
x
(g) conditions, decreasing the NO
2
(g) : NO(g)
ratio, thereby decreasing ozone.
Temperature and water vapor affect surface ozone
through other processes aside from chemistry. For
example, higher temperatures increase the emission
rates of biogenic organic gases, such as isoprene and
monoterpenes, both of which increase ozone. Higher
temperatures also increase electric power demand for
air conditioning and, thus, summer daytime emissions
from the energy sector. Conversely, warmer nighttime
and winter temperatures reduce natural gas and electric-
ity usage for heating, offsetting some of the increased
daytime and summer increases. Third, higher tempera-
tures increase carbon monoxide and hydrocarbon emis-
sions from vehicles (e.g., Rubin et al., 2006; Motallebi
et al., 2008;). In contrast, warmer winter and nighttime
temperatures tend to offset some of the higher temper-
ature increases (U.S. EPA, 2006).
High temperatures can increase not only ozone,
butalso particulate matter. First, higher temperatures
increase the rates of wildfire occurrence due to dryer
conditions in many locations (Westerling et al., 2006).
Wildfires are a major source of particulate matter (Jaffe
et al., 2008).
Second, the increase in biogenic organic gas emis-
sions that occurs when temperatures rise increases the
rate of secondary organic matter particle formation.
Third, higher water vapor due to higher temperatures
can increase the relative humidity in some locations,
whereas higher temperatures can decrease the relative
humidity in others. An increase in the relative humidity
increases the uptake of liquid water onto aerosol parti-
cles, increasing the surface area on which condensation
of gases occurs and the volume in which dissolution
of other gases occurs. Thus, an increase in the relative
humidity due to global warming can increase particulate
matter, whereas a decrease can cause the reverse. Simi-
larly, higher temperatures can reduce particulate matter
by evaporating some organic and inorganic aerosol con-
stituents to the gas phase (Aw and Kleeman, 2003).
Fourth, global warming is usually characterized by
an increase in surface air temperatures and a slightly
lesser increase in ground temperatures. This increase in
CH
3
C(
=
O)O
2
(g)
+
NO
2
(g)
⇔
CH
3
C(
=
O)O
2
NO
2
(g)
Peroxyacetyl
Nitrogen
Peroxyacetyl nitrate
radical
dioxide
(PAN)
(12.8)
whereas, at low temperature, the formation of PAN (for-
ward reaction) is favored. When PAN thermally dissoci-
ates, it produces an organic radical and nitrogen dioxide,
both of which produce ozone. The thermal dissociation
at high temperature is more important in polluted air
than it is in clean air because, in clean air, PAN mixing
ratios are low; thus, less NO
2
(g) and organics are avail-
able for release upon a temperature increase. The gen-
eral increase in surface ozone with higher temperatures
is supported not only by data analysis (e.g., Olszyna
et al., 1997), but also by many computer model studies
(e.g., Sillman and Samson, 1995; Mickley et al., 2004;
Stevenson et al., 2005; Steiner et al., 2006; Brasseur
et al., 2006; Kleeman, 2008; Jacobson, 2008a, 2010a).
Higher temperatures due to increases in carbon diox-
ide and other global warming agents also increase evap-
oration of ocean, lake, and soil water to produce water
vapor. An increase in water vapor increases surface
ozone in polluted air but can slightly reduce ozone in
clean air, as illustrated in Figure 12.30a. In both clean
and polluted air, an increase in water vapor increases
the hydroxyl radical, OH(g), by
O(
1
D
)(
g
)
Excited atomic
oxygen
H
2
O(g)
Water
vapor
+
⇔
2OH(g)
Hydroxyl
radical
(12.9)
Much of the OH(g) converts to HO
2
(g) by reaction of
OH(g) with aldehydes and other organic gases.
In polluted surface air, NO
x
(g) levels are high, so the
resulting HO
2
(g) reacts with NO(g) by
NO(g)
Nitric
oxide
+
HO
2
(g)
Hydroperoxy
radical
⇔
NO
2
(g)
Nitrogen
dioxide
+
OH(g)
Hydroxyl
radical
(12.10)
This reaction increases the NO
2
(g) : NO(g) ratio,
increasing ozone. As such, increases in water vapor
increase ozone in polluted surface air (Figure 12.30a).
However, in clean surface air, NO
x
(g) levels are low,
so HO
2
(g) reacts primarily with itself by
HO
2
(g)
Hydroperoxy
radical
+
HO
2
(g)
Hydroperoxy
radical
+
M
⇔
H
2
O
2
(g)
Hydrogen
peroxide
+
O
2
(g)
Molecular
oxygen
+
M
(12.11)
The rate coefficient of this reaction is proportional to
the concentration of water vapor; thus, the more water
vapor, the faster the reaction and the faster the loss
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