Ozone losses over both polar regions have been quantified by satellites, aircraft, balloons, and ground-based instruments. Over Antarctica, the column losses have increased annually, reaching a record low in October of 1993. During this same period, Hofmann et al. [1994] reported that ozone was undetectable between 14 to 19 km. They inferred that these depletions were more severe than in previous years because of chemistry enhanced by aerosol loading from Mount Pinatubo. Further, Hofmann et al. [1993] showed that ozone losses appeared before the winter in 1992, consistent with enhanced heterogeneous chlorine chemistry implicit from the early detection of OClO that year [ Solomon et al., 1993]. Waters et al. [1993] showed that the main ozone losses developed in springtime and in regions colocated with high abundances of ClO, a global perspective that complements the high-resolution aircraft results [ Anderson et al., 1991]. Waters et al. [1995a] now have three years of simultaneous observations of ClO and ozone showing the same behavior. Recent analysis of 1970s Nimbus 4 data [ Labow et al. 1992], and examination of ground-based data from the Antarctic [ Newman, 1994] have found no evidence for ozone holes in 1958 or the early 1970s.
As reported in the last review [ Brune, 1991], Anderson et
al. [1991] have shown that ozone loss within the antarctic vortex
can be explained well by known reactions of ClO and BrO at measured abundances.
However, since this report, there have been relatively few attempts to further
constrain ozone loss rates over Antarctica. Laboratory studies of Dlugokencky
and Ravishankara [1992] have shown that direct ozone loss on ice
(and presumably PSCs) can be neglected. Murphy [1991] has shown that
ozone loss rates due to gas-phase reactions are greatest below 20 km, raising
the possibility that air transported from the bottom of the vortex could be a
source of ozone decreases observed at mid-latitudes. In addition, Sanders
et al. [1993] and Solomon et al. [1993] report
inconsistencies between observations of OClO, NO
and NO
, and models that
assume air over the pole remains in darkness for prolonged periods (more
than about 7 days) during the winter. They conclude that vortex air must encounter
sunlight during the winter, and significant ozone loss could be occurring
throughout the winter. In light of these results, more work is required to
quantify ozone loss over Antarctica, especially during the early winter.
There are more such studies for the Arctic. As shown by Proffitt and coworkers
[1992,1993] and Waters et al. [1993],
accounting for ozone loss over the Arctic is difficult because there is
significant poleward transport of ozone-rich air during the winter and spring
seasons. Proffitt and coworkers [1992,1993] have extracted
chemical destruction when the ozone changes are referenced to a long-lived
tracer, such as N
O, and approximately 15% to 40% depletion is deduced
in some regions, especially below 20 km. Lidar measurements from the NASA DC-8
research aircraft over the Arctic from
January through March of 1992 [ Browell et al.,
1993] and by UARS [
Manney et al., 1994a] have also detected ozone losses
of up to about 25% in some regions and extending as high as 25 km. Brune
et al. [1991] and Salawitch et al. [1993], using
data from the ER-2 aircraft, and Waters et al. [1993] and
Manney et al. [1994a], using data from UARS, have shown that
these ozone losses can be explained by the same reactions of chlorine and
bromine compounds that occur over Antarctica, although less ozone is destroyed
over the Arctic due to less extensive denitrification there.