Page 167 - Basic physical chemistry for the atmospheric sciences

Photochemistry l 'H

At this point, the reader might well ask if an ozone hole develops in
the Arctic stratosphere in winter and , if not, why not? I n fact, dra­
matic depletions of ozone have not been measured over the Arctic,
although there is evidence for anomalous chlorine chemistry similar to
i
that in the Antarctic. In a field study carried out in the Arctic n 1 9 88-
89, sharp increases were measured in the concentrations of ClO in the
s
stratosphere, and these appeared to be associated with PSC . Also,
increases in OClO were measured, which provides support for the
reaction cycle (7. 4 2). 7 On the other hand, although some denitrifica­
tion was measured at altitudes around 20 km, it was not as great as in
e
the Antarctic stratospher , perhaps because the PSCs evaporated in
the lower stratosphere. Also, dehydration was much less in the Arctic.
In any case, the decreases in total ozone column in the Arctic in 1 9 88-
89 were only a f e w percent, much less than observed in the Antarctic.
It is not known whether this was due to insufficient anomalous chemis­
try or to less than optimal meteorological conditions for ozone deple­
tion. For example, stratospheric temperatures remained very low until
the middle of February in 1 9 89, when there was a sudden warming and
the PSCs disappeared. Thus, air that was sufficiently cold for Reaction
(7.42a) to proceed rapidly may not have received sufficient solar radia­
tion for Reaction (7.42b) to proceed effectively . In the Antarcti ,
c
stratospheric ozone is depleted primarily in September (which corres­
ponds to March in the Arctic) when temperatures are still very low,
but solar radiation is increasing rapidly. It would appear that while
concentrations of Freons remain high in the atmosphere, the Arctic
stratosphere has the potential to cause the same dramatic losses in
ozone as the Antarctic stratosphere, but that the combination of chem­
ical and meteorological conditions that lead to such reductions may
not be as common in the Arctic as in the Antarctic.
On a global scale, ground-based and satellite observations show
significant decreases of total column ozone at middle latitudes in the
northern hemisphere of 2 . 7 % per decade in winter, 1 . 3% decade in
summer, and 1 . 2% per decade in the fall . Similar decreases are appar­
ent at middle latitudes in the southern hemisphere ; and at high lati­
tudes, beneath the region of the Antarctic ozone hole, the decreases
are 14% per year. The decreases have occurred primarily in the lower
stratosphere. No trends in ozone concentrations have been observed
in the tropics .
Concerns about the health and environmental hazards of increased