Encyclopedia of Environmental Science and Engineering, Volume I and II

ATMOSPHERIC CHEMISTRY 135

with ozone to form ClO. This ClO may react with itself to

form the dimer, (ClO) 2 :

ClO  ClO  M → (ClO) 2  M (59)

Under high-ClO-concentration conditions, the following

catalytic cycle could be responsible for the destruction of

ozone:

2 × (Cl  O 3 → ClO  O 2 ) (51)

ClO  ClO  M → (ClO) 2  M (59)

(ClO) 2  h → Cl  ClOO (60)

ClOO  M → Cl  O 2 (61)

2O 3 → 3O 2 (Net)

This ClO-driven catalytic cycle can effectively destroy O 3 ,

but it requires the presence of sunlight to photolyze Cl 2 and

(ClO) 2. The presence of sunlight will lead to an increase in

temperature that releases HNO 3 back to the gas phase. The

photolysis of HNO 3 can release NO 2 , which can react with

ClO by reaction (58) to re-form ClONO 2. This can termi-

nate the unusual chlorine-catalyzed destruction of ozone that

occurs in polar regions.

Anderson (1995) suggests that the same processes occur

in both the Arctic and Antarctic polar regions. The main

distinction is that it does not get as cold in the Arctic, and

the polar stratospheric clouds do not persist as long after the

polar sunrise. As the temperature rises above 195 K, nitric

acid is released back into the gas phase only shortly after

Cl 2 photolysis begins. As nitric acid is photolyzed, forming

NO 2 , the ClO reacts with NO 2 to form ClONO 2 and termi-

nate the chlorine-catalyzed destruction of ozone. Anderson

(1995) suggests that the temperatures warmed in late January

1992, and ozone loss was only 20 to 30% at the altitudes of

peak ClO. The temperatures remained below 195 K until late

February 1993, and significantly more ozone will be lost.

The delay between the arrival of sunlight and the rise of

temperatures above 195 K are crucial to the degree of ozone

loss in the Arctic.

Summary

The observations made in the polar regions provided the key

link between chlorine-containing compounds in the strato-

sphere and destruction of stratospheric ozone. These experi-

mental results led to the Montreal Protocol agreements and

their subsequent revisions to accelerate the phase-out of

the use of CFCs. A tremendous amount of scientific effort

over many years has led to our current understanding of the

effects of Cl-containing species on the stratosphere.

CLOSING REMARKS

Our knowledge and understanding has improved consider-

ably in recent years. Much of the reason for this improved

knowledge is the result of trying to understand how we are

affecting our environment. From the foregoing discussion, it

POLAR NIGHT SUNLIGHT

• COOLING


• DESCENT

PSC

CHEMISTRY

Cl 2 + 2Cl ClO.Cl 3 O 2

HNO 2 NO 2

ClO + NO 2 ClO + NO 2

CH 4 + Cl HCl + CH 3

ClO + 2Cl NO + ClO Cl + NO^2

2 O 3

ClONO 2

ClONO 2

O 3 LOSS

TIME

0

1

2

3

MIXING RATIO (ppbv)

RECOVERY

HCl

HCl

FIGURE 11 Schematic of the time evolution of the chlorine chemistry, illustrating the

importance of the initial HCl/ClONO 2 ratio, the sudden formation of ClO with returning

sunlight, the way in which ClONO 2 levels can build up to mixing ratios in excess of its initial

values, and the slow recovery of HCl levels. From Webster et al. (1993). With permission.

C001_008_r03.indd 135C001_008_r03.indd 135 11/18/2005 10:15:24 AM11/18/2005 10:15:24 AM