Geoscience Reference
In-Depth Information
and 90
◦
N(Whitt et al., 2011). However, such emissions
occur simultaneously with emissions of organic gases,
carbon monoxide, and NO
x
(g), which together slightly
increase ozone in the lower stratosphere via reaction
mechanisms such as those in Sections 4.2.4 and 4.2.5.
In the 1970s and 1980s, a concern arose about a
proposal to introduce a fleet of
supersonic transport
(SST) aircraft into the middle stratosphere. In the mid-
dle and upper stratosphere, the NO
x
(g) catalytic destruc-
tion cycle of ozone is strong; thus, NO
x
(g) has more
potential to destroy ozone than it does in the lower
stratosphere, even in the presence of organic gases and
carbon monoxide. However, the plan to introduce a fleet
of SSTs never materialized.
HO
x
(g) species can be removed temporarily from the
catalytic cycle by Reactions 11.13, 11.14, and 11.19:
H O
2
(g)
OH(g)
+
→
H
2
O(g)
+
O
2
(g)
(11.19)
Hydroperoxy
Hydroxyl
Water
Molecular
radical
radical
vapor
oxygen
This mechanism is particularly efficient at removing
HO
x
(g) from the cycle because it removes two HO
x
(g)
molecules at a time.
11.3.4. Effects of Carbon on
the Natural Ozone Layer
Carbon monoxide and methane produce ozone by the
chemical reaction mechanisms shown in Sections 4.2.4
and 4.2.5, respectively. The contributions of CO(g) and
CH
4
(g) to ozone production in the stratosphere are rela-
tively small, but they increase when NO
x
(g) simultane-
ously increases, such as in the case of aircraft emissions
in the lower stratosphere (Section 11.3.2).
An important
by-product of methane oxidation in the
stratosphere is water vapor
,produced by
11.3.3. Effects of Hydrogen on the Natural
Ozone Layer
Hydrogen-containing compounds, particularly the
hydroxyl radical [OH(g)] and the hydroperoxy radical
[HO
2
(g)], are responsible for shaping the ozone profile
in the lower stratosphere. The hydroxyl radical is pro-
duced in the stratosphere by one of several reactions:
OH(g)
CH
3
(g)
CH
4
(g)
+
→
+
H
2
O(g)
(11.20)
Methane
Hydroxyl
Methyl
Water
radical
radical
vapor
H
2
O(g)
Water
vapor
OH(g)
Hydroxyl
radical
Because water vapor mixing ratios in the stratosphere
are low and transport of water vapor from the tropo-
sphere to stratosphere is slow, this reaction is a relatively
important source of water vapor in the stratosphere. An
anthropogenic source of water vapor into the strato-
sphere is aircraft exhaust.
CH
3
(g)
Methyl
radical
O(
1
D
)(g)
OH(g)
·
+
CH
4
(g)
→
+
(11.15)
Excited
atomic
oxygen
Methane
Hydroxyl
radical
H
Atomic
Hydrogen
H
2
(g)
Molecular
hydrogen
11.4. Recent Changes to the Ozone Layer
Changes in stratospheric ozone since the 1970s can
be categorized as global stratospheric changes, Antarc-
tic
The hydroxyl radical participates in an
HO
x
(g) cat-
alytic ozone destruction cycle
,where HO
x
(g)
=
OH(g)
stratospheric
changes,
and
Arctic
stratospheric
+
HO
2
(g). HO
x
(g) catalytic cycles are important in the
lower stratosphere. The most effective HO
x
(g) cycle,
which has a chain length in the lower stratosphere of 1
to 40 (Lary, 1997), is
OH(g)
changes.
11.4.1. Global Stratospheric Changes
Between 1979 and 2011, the global stratospheric ozone
column abundance decreased by approximately 5 per-
cent (Figure 11.8). Unusual decreases in global ozone
were detected following the El Chichon (Mexico) vol-
canic eruption in April 1982, and the Mount Pinatubo
(Philippines) eruption in June 1991 (Figure 11.9). These
eruptions injected aerosol particles into the strato-
sphere. On the surfaces of these particles, chemical
reactions involving chlorine occurred that contributed
to ozone loss. Over time, the concentration of these
particles decreased, and the global ozone layer partially
H O
2
(g)
+
→
+
O
3
(g)
O
2
(g)
(11.16)
Hydroxyl
Ozone
Hydroperoxy
Molecular
radical
radical
oxygen
H O
2
(g)
OH(g)
+
O
3
(g)
→
+
2O
2
(g)
(11.17)
Hydroperoxy
Ozone
Hydroxyl
Molecular
radical
radical
oxygen
2O
3
(g)
→
3O
2
(g)
(11.18)
Ozone
Molecular
oxygen
(net process)
Search WWH ::
Custom Search