Page 164 - Basic physical chemistry for the atmospheric sciences

1 5 0 Basic physical chemistry
(7 .35)
Therefore, it was argued, the chlorine atom freed by these reactions
could serve as the catalyst X in Reactions (7 . 2 8) and destroy ozone in
the cycle
Cl + 03___,. CIO + 02 (7.36a)
CIO + O___,. Cl + 02 (7.36b)

.
3
Net: (7 7)
The first evidence for depletions in stratospheric ozone produced by
anthropogenic chemicals in the stratosphere came, surprising y , from
l
measurements over the Antarctic. In 1 9 85 British scientists who had
been making ground-based , remote-sensing measurements of ozone at
Halley Bay (76°S) in the Antarctic for many years reported that there
had been about a 30% decrease in total column ozone each October
(i.e . , in the austral spring) since 1 9 77. These observations were subse­
quently confirmed by remote-sensing measurements from satellite and
by airborne measurements. Satellite measurements show that the re­
gion of depleted ozone over the Antarctic in spring has grown progres­
sively deeper since 1 9 79, and in 1 9 87 through 1 9 9 1 it occupied an area
larger than the A n tarctic continent.
Detection of the so-called "ozone hole" over the Antarctic raised
several intriguing questions. Why over the Antarctic? Why during
spring? Also, the magnitudes of the measured decreases in ozone over
the Antarctic were much greater than any predictions based solely on
gas-phase chemistry, of the type outlined above - why? The answers
to these questions provide an excellent demonstration of the maxim
that in the environment processes rarely, if ever, act in isolation .
During the austral winter ( J une-September) stratospheric air over
the Antarctic continent is restricted from interacting with air from
lower latitudes by a large-scale vortex circulation, which is bound at
its perimeter by strongly circulating winds, through which very cold
air slowly sinks. High-level clouds , called polar stratospheric clouds
x
(PSCs), form in the cold core of this vorte , where temperatures can
fall below -80°C. In the austral spring, as temperatures rise, the winds
around the vortex weaken , and by November the vortex has disap­
peared. However, during winter the vortex serves as a giant chemical
reactor in which anomalous chemistry can go on. For example , al­
though the concentrations of ozone in the vortex are normal in August,
the concentrations of CIO are ten times greater than just outside the
"wall" of the vortex and, by September, ozone concentrations within