Geoscience Reference
In-Depth Information
ClO O(g)
Cl(g)
The net result of the heterogeneous reactions is to
convert relatively inactive forms of chlorine in the
stratosphere, such as HCl(g) and ClONO
2
(g), to photo-
chemically active forms, such as Cl
2
(g), HOCl(g), and
ClNO
2
(g). This conversion process is called
chlorine
activation
.The most important heterogeneous reaction
is Reaction 11.35 (McElroy et al., 1986; Solomon et al.,
1986), which generates Cl
2
(g).
Reaction 11.36 does not activate chlorine. Its only
effect is to remove nitric acid from the gas phase.
When nitric acid adsorbs to a Type II PSC particle,
which is larger than a Type I PSC particle, the nitric
acid can sediment out along with the PSC particle to
lower regions of the stratosphere. This removal process
is called
stratospheric denitrification
.Denitrification
is important because it removes nitrogen that might oth-
erwise reform Type I PSCs or tie up active chlorine as
ClONO
2
(g).
Cl
2
O
2
(g)
+
h
→
+
<
360 nm
Dichlorine
Chlorine
Atomic
(11.44)
dioxide
peroxy
chlorine
radical
M
→
ClO O(g)
Cl(g)
+
+
O
2
(g)
(11.45)
Chlorine
Atomic
Molecular
peroxy
chlorine
oxygen
radical
2O
3
(g)
→
3O
2
(g)
(11.46)
Ozone
Molecular
oxygen
This mechanism is called the
dimer mechanism
(Molina and Molina, 1986). It is important in the spring-
time polar stratosphere because, at that time and loca-
tion, the ClO(g) required for Reaction 11.43 has a
high enough concentration for the reaction to proceed
rapidly. More specifically, the rate of Reaction 11.43
depends on the square of the concentration of ClO(g).
So, the rate increases superlinearly with increasing
ClO(g). Once the ClO(g) concentration passes a thresh-
old, the rate of Reaction 11.43 becomes high enough for
the entire dimer mechanism to proceed rapidly. If the
concentration falls below the threshold, the mechanism
is less important than is the normal chlorine catalytic
cycle in Reactions 11.23 to 11.25, which is not suffi-
cient for creating the Antarctic ozone hole. A second
polar catalytic ozone destruction cycle is
Cl(g)
11.8.3. Springtime Polar Chemistry
Chlorine activation occurs in the stratosphere during the
polar winter, a time during which the sun does not rise
over the polar region and temperatures are extremely
low. When the sun does rise during early spring, Cl-
containing gases created by PSC reactions during the
winter photolyze by
2 Cl(g)
Cl O(g)
Cl
2
(g)
+
h
→
<
450 nm
(11.39)
+
O
3
(g)
→
+
O
2
(g)
(11.47)
Molecular
Atomic
Atomic
Ozone
Chlorine
Molecular
chlorine
chlorine
chlorine
monoxide
oxygen
Br(g)
Br O(g)
Cl(g)
OH(g)
+
O
3
(g)
→
+
O
2
(g)
(11.48)
HOCl(g)
+
h
→
+
<
375 nm
Atomic
Ozone
Bromine
Molecular
Hypochlorous
Atomic
Hydroxyl
(11.40)
bromine
monoxide
oxygen
acid
chlorine
radical
Br O(g)
Cl O(g)
Br(g)
Cl(g)
+
→
+
+
O
2
(g)
(11.49)
Cl(g)
NO
2
(g)
ClNO
2
(g)
+
h
→
+
<
370 nm
Bromine
Chlorine
Atomic
Atomic
Molecular
Chlorine
Atomic
Nitrogen
monoxide
monoxide
bromine
chlorine
oxygen
(11.41)
nitrite
chlorine
dioxide
2O
3
(g)
→
3O
2
(g)
(11.50)
Once Cl is released, it immediately attacks ozone.
The catalytic cycle that destroys ozone in the spring-
time polar stratosphere differs from that in Reactions
11.23 to 11.25, which reduces ozone on a global scale.
A polar stratosphere catalytic ozone destruction cycle
is
Ozone
Molecular
oxygen
(McElroy et al., 1986), which is important in the polar
lower stratosphere.
In sum, chlorine activation and springtime pho-
tochemical reactions convert chlorine from reservoir
forms, such as HCl(g) and ClONO
2
(g), to active forms,
such as Cl(g) and ClO(g), as shown in Figure 11.18.
The active forms of chlorine destroy ozone in catalytic
cycles.
Every November, the Antarctic warms up sufficiently
for the polar vortex to break down and for PSCs to melt
and sublimate. Ozone from outside the polar region then
2x (Cl(g)
Cl O(g)
+
O
3
(g)
→
+
O
2
(g))
(11.42)
Atomic
Ozone
Chlorine
Molecular
chlorine
monoxide
oxygen
M
→
Cl O(g)
Cl O(g)
(11.43)
+
Cl
2
O
2
(g)
Chlorine
Dichlorine
monoxide
dioxide
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