An unprecedented ozone loss occurred in the Arctic in spring 2011. The
details of the event are revisited from the twice-daily total ozone and
NO2 column measurements of the eight SAOZ/NDACC (Systeme d'Analyse par
Observation Zenithale/Network for Detection of Atmospheric Composition
Changes) stations in the Arctic. It is shown that the total ozone
depletion in the polar vortex reached 38% (approx. 170 DU) by the end
of March, which is larger than the 30% of the previous record in 1996.
Aside from the long extension of the cold stratospheric NAT PSC period,
the amplitude of the event is shown to be resulting from a record daily
total ozone loss rate of 0.7% d(-1) after mid-February, never seen
before in the Arctic but similar to that observed in the Antarctic over
the last 20 yr. This high loss rate is attributed to the absence of NOx
in the vortex until the final warming, in contrast to all previous
winters where, as shown by the early increase of NO2 diurnal increase,
partial renoxification occurs by import of NOx or HNO3 from the outside
after minor warming episodes, leading to partial chlorine deactivation.
The cause of the absence of renoxification and thus of high loss rate,
is attributed to a vortex strength similar to that of the Antarctic but
never seen before in the Arctic. The total ozone reduction on 20 March
was identical to that of the 2002 Antarctic winter, which ended around
20 September, and a 15-day extension of the cold period would have been
enough to reach the mean yearly amplitude of the Antarctic ozone hole.
However there is no sign of trend since 1994, either in PSC (polar
stratospheric cloud) volume (volume of air cold enough to allow
formation of PSCs), early winter denitrification, late vortex
renoxification, and vortex strength or in total ozone loss. The
unprecedented large Arctic ozone loss in 2011 appears to result from an
extreme meteorological event and there is no indication of possible
strengthening related to climate change.
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