Geoscience Reference
In-Depth Information
Nitric oxide naturally reduces ozone in the upper strato-
sphere by
60
50
UV < 250 nm
NO(g)
NO 2 (g)
+
O 3 (g)
+
O 2 (g)
(11.10)
40
Nitric
Ozone
Nitrogen
Molecular
oxide
dioxide
oxygen
O 3 (g)
30
(11.11)
NO 2 (g)
O(g)
NO(g)
+
O 2 (g)
20
Nitrogen
Atomic
Nitric
Molecular
dioxide
oxygen
oxide
10
O(g)
·
+
O 3 (g)
2O 2 (g)
O 2 (g)
(11.12)
Atomic
Ozone
Molecular
oxygen
oxygen
0
(net process)
0
1
2
3
4
5
The result of this sequence is that one molecule
of ozone is destroyed, but neither NO(g) nor NO 2 (g)
is lost. This sequence is called a catalytic ozone
destruction cycle because the species causing the
O 3 (g) loss, NO(g), is recycled. This particular cycle
is the NO x (g) catalytic ozone destruction cycle, where
NO x (g)
Figure 11.7. Variation with altitude of ultraviolet
radiation
<
250 nm in wavelength (photons/cm 2 /s
×
10 -14 ), O 2 (g) (molec./cm 3
×
10 -18 ), and O 3 (g)
(molec./cm 3
10 -12 )onaclearJanuary 1 day
at 10 Sand20 E.
×
and 11.2 to produce high ozone densities, whereas, at
lower altitudes, UV radiation is not intense enough for
oxygen photolysis to produce high ozone densities.
NO 2 (g) and NO(g) is the catalyst.
The number of times the cycle is executed before
NO x (g) is removed from the cycle by reaction with
another gas is the chain length. In the upper stratosphere,
the chain length of this cycle is about 10 5 (Lary, 1997).
Thus, 10 5 molecules of O 3 (g) are destroyed before one
NO x (g) molecule is removed from the cycle. In the
lower stratosphere, the chain length decreases to near
10. When NO x (g) is removed from this cycle, its major
loss processes are the formation of nitric acid and per-
oxynitric acid by the reactions
=
NO(g)
+
11.3.2. Effects of Nitrogen on the Natural
Ozone Layer
Oxides of nitrogen [NO(g) and NO 2 (g)] naturally
destroy ozone, primarily in the upper stratosphere, help-
ing shape the vertical profile of the ozone layer. In
the troposphere, the major sources of nitric oxide are
surface anthropogenic and natural emissions and light-
ning. The major source of NO(g) in the stratosphere is
transport from the troposphere and the breakdown of
nitrous oxide [N 2 O(g)] (laughing gas), a colorless gas
emitted during denitrification by anaerobic bacteria in
soils (Section 2.3.6). It is also emitted by bacteria in
fertilizers, sewage, and the oceans, as well as during
biofuel and biomass burning, automobile and aircraft
combustion, nylon manufacturing, and aerosol spray
cans. In the troposphere, N 2 O(g) is lost by transport to
the stratosphere, deposition to the surface, and chemi-
cal reaction. Because its loss rate from the troposphere
is slow, nitrous oxide is long lived and well diluted in
the troposphere, with an average mixing ratio of about
0.33 ppmv in 2011. The mixing ratio of N 2 O(g) is rel-
atively constant up to about 15 to 20 km, but decreases
above that as a result of photolysis. Throughout the
atmosphere, N 2 O(g) produces nitric oxide by
N 2 O(g)
M
NO 2 (g)
OH(g)
+
HNO 3 (g)
(11.13)
Nitrogen
Hydroxyl
Nitric
dioxide
radical
acid
M
H O 2 (g)
NO 2 (g)
+
HO 2 NO 2 (g)
(11.14)
Hydroperoxy
Nitrogen
Peroxynitric
radical
dioxide
acid
Nitric acid and peroxynitric acid photolyze back to the
reactants that formed them, but such processes are slow.
Peroxynitric acid also decomposes thermally, but ther-
mal decomposition is slow in the stratosphere because
temperatures are low there.
The natural NO x (g) catalytic cycle erodes the ozone
layer above ozone's peak altitude shown in Figure 11.4.
Most of this cycle is natural because most NO x (g) in
the stratosphere is from natural sources. An increas-
ing anthropogenic source of stratospheric NO x (g) is
aircraft emission. Worldwide, about 24 percent of air-
craft emissions occur in the lower stratosphere , and
almost all stratospheric emissions occur between 30 N
O( 1 D )(g)
2 NO(g)
(11.9)
Nitrous
Excited
Nitric oxide
oxide
atomic
oxygen
 
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