Important new observations have been obtained during the springtime recovery
phases of the antarctic and arctic vortices. With simultaneous measurements of
NO and ClO and a trajectory model, Schoeberl et al. [1993b]
show that deactivation of reactive chlorine over the arctic occurs by fast
photochemical formation of ClONO
, at rates consistent with decomposition
of HNO
into NO
by photolysis and by reaction with OH.
This suggests that model treatments of OH abundances, which were not measured,
are generally correct. Solomon and Keys [1992] and Keys et al.
[1993] have shown that NO
abundances increase in regions that
still contain HNO
. Within the ozone hole over Antarctica, however, ClO remains
perturbed well into springtime. UARS observations of HNO
, ClO, ClONO
,
and O
complete the picture for Antarctica. Roche et al.
[1994] and Santee et al. [1995] show a broad region
where nitric acid has been completely removed, presumably by sedimentation of
PSC particles earlier in the winter, as shown by lidar observations of Gobbi
et al. [1991] and Deshler et al. [ 1991],
and where there is little formation of ClONO
. Thus, reactive chlorine is
maintained at large abundances well into springtime and nearly all of the ozone
is destroyed in about a month [ Anderson et al., 1991].
In this case, recovery of HCl via the Cl + CH
reaction
is more rapid than recovery of ClONO
[ Liu et al., 1993],
a situation that is opposite to that of the Arctic [ Webster
et al., 1993a]. Outside of this denitrified region,
nitric acid forms a ``collar'' around the ozone hole in mid- to late-winter in
the same region that later exhibits a similar feature for ClONO
, presumably
as a result of photochemistry that is similar to that which occurs over most
of the Arctic in spring [ Santee et al, 1995].