Geoscience Reference
In-Depth Information
Reaction 4.5 requires the presence of the
hydroxyl
radical
[OH(g)], an oxidizing agent that decomposes
(scavenges) many gases. Given enough time, OH(g)
breaks down every organic gas and most inorganic gases
in the air.
The daytime average OH(g) concentration in the
clean background troposphere usually ranges from 2
4.2.4. Ozone Production from Carbon
Monoxide
Daytime ozone production in the background tro-
posphere is enhanced by carbon monoxide [CO(g)],
methane [CH
4
(g)], and certain nonmethane organic
gases. CO(g) produces ozone by
×
10
5
10
6
molec cm
−
3
.Inurban air, OH(g) con-
centrations typically range from 1
×
to 3
OH(g)
Hydroxyl
radical
H(g)
Atomic
hydrogen
+
→
+
CO(g)
Carbon
monoxide
CO
2
(g)
Carbon
dioxide
(4.11)
10
7
molec cm
−
3
.The primary free-tropospheric source of
OH(g) is the pathway
×
10
6
×
to 1
M
→
H(g)
Atomic
hydrogen
H O
2
(g)
Hydroperoxy
radical
+
O
2
(g)
Molecular
oxygen
(4.12)
O(
1
D
)(g)
Excited
atomic
oxygen
O
3
(g)
Ozone
+
h
→
O
2
(g)
Molecular
Oxygen
+·
<
310 nm
(4.6)
NO(g)
Nitric
oxide
H O
2
(g)
Hydroperoxy
radical
NO
2
(g)
Nitrogen
dioxide
OH(g)
Hydroxyl
radical
+
→
+
(4.13)
O(
1
D
)(g)
Excited
atomic
oxygen
2 OH(g)
Hydroxyl
radical
·
+
H
2
O(g)
Water
vapor
→
(4.7)
NO
2
(g)
Nitrogen
dioxide
NO(g)
Nitric
oxide
O(g)
Atomic
oxygen
+
h
→
+·
<
420 nm
(4.14)
Thus, OH(g) concentrations in the background tropo-
sphere depend on ozone and water vapor contents.
M
→
O(g)
Atomic
oxygen
·
+
O
2
(g)
Molecular
oxygen
O
3
(g)
Ozone
(4.15)
4.2.3. Nighttime Nitrogen Chemistry
During the night, Reaction 4.2 (nitrogen dioxide photol-
ysis) shuts off, eliminating the major chemical sources
of O(g) and NO(g). Because O(g) is necessary for the
formation of ozone, ozone production also shuts down
at night. Thus, at night, neither O(g), NO(g), nor O
3
(g)
is produced chemically. However, NO(g) can be emit-
ted at night. When this occurs, it destroys ozone by
Reaction 4.1, causing a local loss in ozone.
Because NO
2
(g) photolysis shuts off at night, NO
2
(g)
becomes available to produce NO
3
(g), N
2
O
5
(g), and
HNO
3
(aq) at night by the sequence
NO
2
(g)
Nitrogen
dioxide
Because the
e
-folding lifetime of CO(g) against break-
down by OH(g) in the background troposphere is
28 to 110 days, the sequence does not affect the
photostationary-state relationship among O
3
(g), NO(g),
and NO
2
(g) much, except to increase the NO
2
(g):NO(g)
ratio slightly, thus increasing ozone. Reaction 4.11 also
produces carbon dioxide, and Reaction 4.12 is almost
instantaneous.
4.2.5. Ozone Production from Methane
Methane
[CH
4
(g)] ,with a mixing ratio of 1.85 ppmv,
is the most abundant organic gas in the Earth's atmo-
sphere. Its free-tropospheric
e
-folding lifetime against
chemical destruction is eight to twelve years. This
long lifetime has enabled it to mix uniformly up to
the tropopause. Above the tropopause, its mixing ratio
decreases due to chemical reaction and photolysis.
Methane's most important tropospheric loss mecha-
nism is its reaction with the hydroxyl radical. The
methane loss pathway produces ozone by increasing
the NO
2
(g):NO(g) ratio, but the incremental quantity
of ozone produced is small compared with the pure
photostationary quantity of ozone. The methane oxida-
tion sequence producing ozone is
N O
3
(g)
Nitrate
radical
+
Ozone
→
+
O
3
(g)
O
2
(g)
Molecular
oxygen
(4.8)
M
NO
2
(g)
Nitrogen
dioxide
N O
3
(g)
Nitrate
radical
+
N
2
O
5
(g)
Dinitrogen
pentoxide
(4.9)
N
2
O
5
(g)
Dinitrogen
pentoxide
+
H
2
O(aq)
Liquid
water
→
2HNO
3
Dissolved
nitric acid
(aq)
(4.10)
Reaction 4.10 occurs on aerosol or hydrometeor parti-
cle surfaces. During the morning, sunlight breaks down
NO
3
(g) within seconds, so NO
3
(g) is not a source
of pollution during the day. Because N
2
O
5
(g) forms
from NO
3
(g) and decomposes thermally within sec-
onds at high temperatures by the reverse of Reaction
4.9, N
2
O
5
(g) is also unimportant during the day.
OH(g)
Hydroxyl
radical
CH
3
(g)
Methyl
radical
CH
4
(g)
Methane
+
→
+
H
2
O(g)
Water
vapor
(4.16)
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