Despite the considerable progress toward understanding polar chemistry, significant uncertainties remain about the effects of dynamics and particle microphysics on chemistry. Although the vortices are relatively isolated from mid-latitudes, there is still a question of the extent of isolation [ Schoeberl and Hartmann, 1991 and Randel, 1993]. If there is little exchange of air between the vortex and mid-latitudes, then ozone losses resulting from PSC chemistry are confined to the polar regions and will only affect middle latitudes when the vortex breaks up. Since less than 10% of the area of the hemisphere is represented by the vortex, the perturbation to mid-latitude ozone would be small. On the other hand, if air were to flow through the vortex rapidly, more air could be processed by polar chemistry, and the resulting dilution might significantly impact mid-latitude chemical constituent abundances. From tracer measurements it is known that air within the vortex has descended a great deal over the course of the winter. However, less clear is the rate at which this air is ejected back to mid-latitudes before final breakup.
Using UARS water and methane data, Tuck et al. [1993] and
Pierce et al. [1994b] have argued that detection of
dehydration at southern hemisphere mid-latitudes is a signature of significant
outflow. Another analysis by Tuck et al. [1994] of NO
deficits based on N
O as a conservative tracer yields apparently similar
results. Together, these studies imply a flushing time of a few months for air
in the vortex. (It is important to note that the data upon which one of these
studies is based, that of Tuck et al. [1993], are being
revised as UARS validation efforts continue.) On the other hand, in order to
replenish the air in the vortex, there most be correspondingly large inward
flow, either by strong vertical descent or by compensating horizontal
transport. Other data from UARS reported by Russell et al.
[1993] and Lahoz et al. [1993] show
strong, unmixed vertical descent in the vortex.
Three-dimensional model simulations of Rood et al. [1992], trajectory calculations of Bowman [1993a,b] and Bowman and Mangus [1993], and contour advection models of Waugh et al. [1994a] suggest that small amounts of air are irreversibly ejected out of the vortex, and that this occurs in a few events at the expense of vortex size (i.e. the flow is mainly out of the vortex). Plumb et al. [1994] have identified horizontal transport into the vortex, but this appears to be a relatively rare phenomenon. It is hoped that tracer data from UARS, which has a more global perspective than focussed aircraft studies, will shed additional light on this issue.
Beyond the questions of PSC phase and composition discussed above, is
the question of whether changing climate will lead to an Arctic ozone
hole [ Austin et al., 1992]. Based on radiative arguments,
increases in CO
should lead to colder stratospheric temperatures
which, in turn, could increase the frequency of PSCs. However, Mahlman
[1992] argues that reliable predictions will require a number
of years of intensive research into the pertinent chemistry and dynamics.
This research should include studies of aerosol microphysics [
Tabazadeh and Turco, 1993b] and chemistry in both the laboratory and
directly within PSCs. The latter studies should also focus on
leewave-induced clouds [ Deshler et al., 1994b],
which are often difficult to observe by satellite but could process
a significant amount of air and, thus, may have a disproportionate
impact on chemical partitioning. Laboratory experiments, such as
those that describe the surface structure of ice films [
Keyser and Leu, 1993], will also provide valuable insight into
PSC formation and growth.