Global Stratospheric Ozone Reduction - Air Pollution and Global Warming: History, Science, and Solutions - page 251

O(g)

At any time, about 1 percent of the non-CFC chlorine
in the stratosphere is in the form of active chlorine,
whereas most of the rest is in the form of a chlorine
reservoir. Because CFCs release their chlorine by pho-
tolysis in the middle and upper stratosphere, HCl(g)
mixing ratios should also peak in the middle and upper
stratosphere. Indeed, observations confirm this suppo-
sition.
The HCl(g) reservoir leaks back to atomic chlorine
by photolysis, reaction with OH(g), and reaction with
O(g), all of which are slow processes. The e -folding
lifetime of HCl(g) against photolysis, for example, is
about 1.5 years at 25 km. HCl(g) also diffuses back to
the troposphere, where it can be absorbed by clouds.
The ClONO 2 (g) reservoir leaks back to atomic chlo-
rine by photolysis with an e -folding lifetime of about
4.5 hours at 25 km.

· + O 3 (g) → 2O 2 (g)

(11.31)

Atomic Ozone Molecular

oxygen oxygen

(net process)

The chain length of this cycle increases from about 100
at 20 km to about 10 4 at 40 to 50 km (Lary, 1997). The
chain length of the bromine catalytic cycle is longer
than is that of the chlorine catalytic cycle because Br(g)
is removed more slowly from the bromine cycle by reac-
tions with CH 4 (g) and H 2 (g) than Cl(g) is removed from
the chlorine cycle by reactions with the same chemicals.
When atomic bromine is removed from its catalytic
cycle, it forms hydrobromic acid [HBr(g)] by

H O 2 (g)
Hydroperoxy
radical

O 2 (g)
Molecular
oxygen

Br(g)

+ → +

Hydrobromic
acid

HBr(g) (11.32)

H O 2 (g)
Hydroperoxy
radical

Atomic
bromine

H 2 O 2 (g)
Hydrogen
peroxide

11.6. Effects of Bromine on Global
Ozone Reduction
Like chlorine, bromine reduces stratospheric ozone.
The primary source of stratospheric bromine is methyl
bromide [CH 3 Br(g)], which is produced biogenically
in the oceans and emitted as a soil fumigant. Other
sources of bromine are halons, defined in Section
11.5.1. The tropospheric mixing ratios of the most com-
mon halons, CF 2 ClBr(g) (H-1211) and CF 3 Br(g) (H-
1301), were about 4 and 3 pptv, respectively, in 2010,
less than 1 percent of the mixing ratios of CFC-12 (Table
11.2). Nevertheless, the efficiency of ozone destruction
by the bromine catalytic cycle is greater than is that by
the chlorine catalytic cycle.
Methyl bromide and halons are photolyzed in the
stratosphere above 20 km to produce atomic bromine.
Photolysis of methyl bromide occurs by

When BrO(g) is removed, it forms bromine nitrate
[BrONO 2 (g)] by
Br O(g)

M
→

NO 2 (g)

+ BrONO 2 (g) (11.33)

Bromine Nitrogen Bromine

monoxide dioxide nitrate

The HBr(g) reservoir leaks slowly back to atomic
bromine by reacting with OH(g). The BrONO 2 (g) reser-
voir quickly leaks back to atomic bromine by photoly-
sis. The e -folding lifetime of BrONO 2 (g) against pho-
tolysis is about 10 minutes at 25 km.

11.7. Regeneration Rates of
Stratospheric Ozone
The presence of chlorine and bromine has caused levels
of ozone in the stratosphere to decrease. If chlorine and
bromine could be removed easily from the stratosphere,
the stratospheric ozone layer could regenerate quickly.
However, natural removal of chlorine and bromine is
slow because the overall lifetimes of several chlorocar-
bons and bromocarbons against chemical removal are
on the order of 50 to 100 years.
Suppose, however, that all ozone in the stratosphere
were destroyed, all ozone-destroying compounds were
removed, but all oxygen remained. How long would
the ozone layer take to regenerate? An estimate can be
obtained from Figure 11.14, which shows results from
two computer simulations of the global atmosphere in
which all ozone in the present-day atmosphere was ini-
tially removed, but oxygen was not. In the first simula-
tion, ozone regeneration was simulated in the absence of

CH 3 (g)

+ → +

Br(g)

<

CH 3 Br(g) h 260 nm

Methyl Methyl Atomic

(11.28)

bromide radical bromine

The e -folding lifetime of CH 3 Br(g) against loss by this
reaction is about 10 days at 25 km.
Once atomic bromine forms in the stratosphere, it
reacts in the bromine catalytic ozone destruction
cycle ,

Br(g)

Br O(g) + O 3 (g) → + O 2 (g)

(11.29)

Atomic Ozone Bromine Molecular

bromine monoxide oxygen

Br O(g)

O(g) Br(g)

+· → + O 2 (g)

(11.30)

Bromine Atomic Atomic Molecular

monoxide oxygen bromine oxygen