Geoscience Reference
In-Depth Information
4.2. Gas-Phase Chemistry of the
Background Troposphere
Today, no region of the global atmosphere is unaf-
fected by anthropogenic pollution. Nevertheless, the
background troposphere is cleaner than are urban areas,
and to understand photochemical smog, it is useful to
examine the gas-phase chemistry of the background
troposphere. The background troposphere is affected
by inorganic, light organic, and a few heavy organic
gases. The sources of many gases in the background
troposphere are natural and include microorganisms,
vegetation, volcanos, lightning, and fires (which may be
natural or anthropogenic). The major heavy organic gas
in the background troposphere is isoprene, a hemiter-
pene emitted by vegetation. Because rural areas can
have a lot of vegetation, isoprene emissions occur over
many land areas of the world. The background tropo-
sphere also receives inorganic and light organic gases
emitted anthropogenically from urban regions. How-
ever, most heavy organic gases emitted in urban air,
such as toluene and xylene, break down chemically over
hours to a few days; thus, these gases do not reach the
background troposphere, even though their breakdown
products do. In the following subsections, inorganic and
light organic chemical pathways important in the back-
ground troposphere are described.
can deplete local ozone at night because NO(g) mixing
ratios at night may exceed those of O
3
(g).
If
k
1
(cm
3
molec
−
1
s
−
1
)isthe rate coefficient of
Reaction 4.1 and
J
(s
−
1
)isthe photolysis rate coefficient
of Reaction 4.2, the volume mixing ratio of ozone can
be calculated from these two reactions as
N
d
k
1
NO
2
(g)
J
O
3
(g)
=
(4.4)
NO(g)
where
is volume mixing ratio (molecule of gas per
molecule of dry air) and
N
d
is the concentration of dry
air (molecules of dry air per cubic centimeter). This
equation is called the
photostationary-state relation-
ship
.The equation does not state that ozone is affected
by only NO(g) and NO
2
(g). Indeed, other reactions
affect ozone, including ozone photolysis. Instead, Equa-
tion 4.4 says that the ozone mixing ratio is a function
of the ratio of the NO
2
(g) to NO(g) ratio. Many other
reactions can affect the NO
2
(g):NO(g) ratio, thereby
affecting ozone.
Example 4.1
Find the photostationary-state mixing ratio of
O
3
(g) at midday when
p
d
=
1,013 hPa,
T
=
298 K,
0.01 s
−1
,
k
1
=
10
−14
cm
3
molec
−1
s
−1
,
J
≈
1.8
×
NO(g)
=
5 pptv, and
NO
2
(g)
=
10 pptv (typical
free-tropospheric mixing ratios).
Solution
From Equation 3.12,
N
d
4.2.1. Photostationary-State Ozone
Concentration
In the background troposphere, the ozone [O
3
(g)] mix-
ing ratio is controlled primarily by a set of three reac-
tions involving itself, nitric oxide [NO(g)], and nitrogen
dioxide [NO
2
(g)]. These reactions are
NO(g)
Nitric
oxide
10
19
molec
cm
−3
. Substituting this into Equation 4.4 gives
O
3
(g)
=
2.46
×
=
45.2 ppbv, which is a typical
free-
tropospheric ozone mixing ratio.
NO
2
(g)
Nitrogen
dioxide
+
O
3
(g)
Ozone
→
+
O
2
(g)
Molecular
oxygen
(4.1)
4.2.2. Daytime Removal of Nitrogen Oxides
During the day, NO
2
(g) isremoved slowly from the
photostationary-state cycle by the reaction
NO
2
(g)
Nitrogen
dioxide
NO(g)
Nitric
oxide
O(g)
Atomic
oxygen
+
h
→
+·
<
420 nm
(4.2)
M
→
NO
2
(g)
Nitrogen
dioxide
OH(g)
Hydroxyl
radical
+
HNO
3
(g)
Nitric
acid
(4.5)
M
→
O(g)
Atomic
oxygen
·
+
O
2
(g)
Molecular
Oxygen
O
3
(g)
Ozone
(4.3)
Although
+
OH(g), its
e
-folding lifetime against photolysis is 15
to 80 days, depending on the day of the year and the
latitude. Because this lifetime is fairly long, HNO
3
(g)
serves as a sink for
nitrogen oxides
[NO
x
(g)
HNO
3
(g)
photolyzes
back
to NO
2
(g)
Background tropospheric mixing ratios of O
3
(g) (20-60
ppbv) are much higher than are those of NO(g) (1-60
pptv) or NO
2
(g) (5-70 pptv). Because the mixing ratio
of NO(g) is much lower than is that of O
3
(g), Reaction
4.1 does not deplete ozone during the day or night in
background tropospheric air. In urban air, Reaction 4.1
+
NO
2
(g)] in the short term. In addition, because HNO
3
(g)
is soluble, much of it dissolves in cloud drops or aerosol
particles before it photolyzes back to NO
2
(g).
=
NO(g)
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