Although this is a review of chemistry, it is difficult to ignore the role of
transport. In the lower stratosphere, photochemical lifetimes of reservoir
species like HNO
and HCl can be relatively long, approaching months or
longer at high latitudes in wintertime. Thus, rapid quasi-horizontal advection
and mixing, such as that described by Plumb and Ko [1992], can alter
the steady-state abundances of important species, which in turn can influence
ozone loss rates. Transport of processed material out of the arctic polar
vortex may be the most celebrated issue of the past few years in this regard.
Douglass et al. [1991] argued that depletion of HCl can be
used as a tracer of processed vortex air, and showed that some chemical
features appearing in February high-latitude ClO data could be simulated with a
three-dimensional chemical transport model. However, structure near the edge was
difficult to
reproduce. Tuck et al. [1992] showed that the small-scale
variability corresponded well with features in potential vorticity (a
semi-conserved tracer of air motion) maps from
the European Centre for Medium-range Weather Forecasting (ECMWF) analyses, and
argued that such transport could leave a chemical signature throughout
mid-latitudes.
Using contour advection, Waugh et al. [1994a] showed that
air pulled out from the vortex is stretched into thin filaments across
mid-latitudes that maintain their identities for many days to weeks. While
such episodic events may occur frequently near the edge of the vortex
the
total mass transported by such events on average is likely less than about
one-third of the mass of the arctic vortex in a month
[ Rood et al., 1992,1993, Fisher et
al., 1993, Pierce et al., 1993, and Manney et al., 1994b].
Similar filamentation has been observed between the tropics and middle
latitudes [
Waugh et al., 1994b]. As pressures mount to assess the impact of NO
exhaust from supersonic transports on stratospheric ozone [
Albritton et al., 1993], there will be vigorous investigations of
tropics-to-midlatitude transport and planetary-wave mixing [
Randel et al., 1993].
Finally, there is a growing research emphasis on the linkage between ozone depletion and temperature in the lower stratosphere [ Miller et al., 1992, Mahlman et al., 1994, and McCormack and Hood, 1994]. Two important questions are, (1) How have ozone losses contributed to the radiative balance of the lower stratosphere? and (2) How have temperature trends contributed to ozone losses? There will be significant new contributions to this issue in the next review.