Geoscience Reference
In-Depth Information
Figure 4.5.
Mean column ozone
totals (Dobson units) by month
versus latitude for 1964-1980
and 1984-1993 (from Bojkov
and Fiolotov,
1995
, by permission
of AGU).
altitude when temperatures fall below 195K. These clouds may be spectacularly
observed when the sun is near or just below the horizon. The odd nitrogen absorbed
by the cloud particles (nitric acid hydrates and water ice) allows reactions that lead
to catalytic destruction of ozone. A bromine compound, BrO, can trigger similar cat-
alytic reactions. Although the Montreal Protocol, which entered into force in 1989,
phased out the production of CFCs, it will be decades until the ozone hole heals. In
the Arctic stratosphere, the low temperatures associated with ozone destruction are
less frequent than in the Antarctic but may still occur during about two months in a
normal winter (Levi,
1992
), and “mini” ozone holes have been observed (Solomon,
1999
). One can see a reduction in total column ozone between the earlier and later
periods plotted in
Figure 4.6
. The largest Arctic ozone loss ever observed occurred
in 2011. For the first time in the record, the ozone destruction was comparable to
that in the Antarctic ozone hole (Manney et al.,
2011
). The event was a result of
unusually low stratopsheric temperatures.
