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HCl(g) against reaction by OH(g) is about 15 to 30
days. HCl(g) is also soluble in water and is absorbed
by clouds. Volcanos, which emit water vapor and
hydrochloric acid, produce clouds and rain that remove
HCl(g), preventing most of it from reaching the strato-
sphere (Lazrus et al., 1979; Pinto et al., 1989; Tabazadeh
and Turco, 1993). The fact that the two major natural
sources of chlorine, CH 3 Cl(g) and HCl(g), have short
chemical lifetimes against destruction by OH(g), and
the fact that HCl(g) is soluble in water, whereas CFCs
have long chemical lifetimes and are insoluble, sup-
port the contention that CFCs and not naturally emit-
ted chlorine compounds are responsible for most ozone
destruction in the stratosphere.
contention that stratospheric ozone reductions result
primarily from anthropogenic chlorine, not natural
chlorine.
11.5.3. Catalytic Ozone Destruction
by Chlorine
Once released from their parent compounds in the
stratosphere, chlorine atoms from CFCs and non-CFCs
react along one of several pathways. Chlorine reacts in
the chlorine catalytic ozone destruction cycle ,
Cl(g)
Cl O(g)
+
+
O 3 (g)
O 2 (g)
(11.23)
Atomic
Ozone
Chlorine
Molecular
chlorine
monoxide
oxygen
(11.24)
Cl O(g)
O(g)
Cl(g)
+
O 2 (g)
11.5.2.3. Emissions of Chlorine Compounds
to the Stratosphere
Table 11.3 summarizes the relative emissions of anthro-
pogenic and natural chlorine-containing compounds
into the stratosphere from one analysis. About 82 per-
cent of chlorine entering the stratosphere originated
from anthropogenic sources. Of the remainder, about
15 percent was methyl chloride, emitted almost exclu-
sively by biogenic sources in the oceans, and 3 percent
washydrochloric acid, emitted by volcanos, evapo-
rated from sea spray, and otherwise produced natu-
rally. The relatively large anthropogenic versus natural
source of chlorine into the stratosphere supports the
Chlorine
Atomic
Atomic
Molecular
monoxide
oxygen
chlorine
oxygen
O(g)
·
+
O 3 (g)
2O 2 (g)
(11.25)
Atomic
Ozone
Molecular
oxygen
oxygen
(net process)
At midlatitudes, the chain length of this cycle increases
from about 10 in the lower stratosphere to about 1,000
in the middle and upper stratosphere (Lary, 1997).
The primary removal mechanisms of active chlo-
rine [Cl(g)
ClO(g)] from the catalytic cycle are reac-
tions that produce the chlorine reservoirs ,HCl(g)and
chlorine nitrate [ClONO 2 (g)]. Chlorine reservoirs are
called such because they temporarily store active chlo-
rine, preventing it from destroying ozone. Conversion
of Cl(g) to HCl(g) occurs by
+
Table 11.3. Relative emissions of selected chlorine
compounds into stratosphere
Contribution to
stratospheric
CH 3 (g)
Methyl
radical
CH 4 (g)
Methane
Trade name or
Chemical
emissions
chemical name
formula
(percent)
H O 2 (g)
Hydroperoxy
radical
O 2 (g)
Molecular
oxygen
Anthropogenic Sources
CFC-12
CF 2 Cl 2 (g)
28
CFC-11
CFCl 3 (g)
23
Cl(g)
+
+
HCl(g)
(11.26)
Carbon tetrachloride
CCl 4 (g)
12
H 2 (g)
Molecular
hydrogen
Atomic
chlorine
Hydrochloric
acid
H(g)
Atomic
hydrogen
Methyl chloroform
CH 3 CCl 3 (g)
10
CFC-113
6
CFCl 2 CF 2 Cl(g)
H 2 O 2 (g)
Hydrogen
peroxide
H O 2 (g)
Hydroperoxy
radical
HCFC-22
CF 2 ClH(g)
3
Natural Sources
Methyl chloride
CH 3 Cl(g)
15
Hydrochloric acid
HCl(g)
3
Conversion of ClO(g) to ClONO 2 (g) occurs by
TOTAL
100
M
CFC, chlorofluorocarbon; HCFC, hydrochlorofluorocarbon.
Source :World Meteorological Organization (1995).
Cl O(g)
NO 2 (g)
+
ClONO 2 (g)
(11.27)
Chlorine
Nitrogen
Chlorine
monoxide
dioxide
nitrate
 
 
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