Abstract:
Record-low ozone column densities (with a minimum
of 212 DU) persisted over three weeks at the Río
Gallegos NDACC (Network for the Detection of Atmospheric
Composition Change) station (51.5 S, 69.3 W) in
November 2009. Total ozone remained two standard deviations
below the climatological mean for five consecutive
days during this period. The statistical analysis of 30 years
of satellite data from the Multi Sensor Reanalysis (MSR)
database for Río Gallegos revealed that such a long-lasting
low-ozone episode is a rare occurrence. The event is examined
using height-resolved ozone lidar measurements at
Río Gallegos, and observations from satellite and groundbased
instruments. The computed relative difference between
the measured total ozone and the climatological monthly
mean shows reductions varying between 10 and 30% with
an average decrease of 25 %. The mean absolute difference
of total ozone column with respect to climatological
monthly mean ozone column is around 75 DU. Extreme values
of the UV index (UVI) were measured at the ground
for this period, with the daily maximum UVI of around 13
on 15 and 28 November. The high-resolution MIMOSACHIM
(Modelisation Isentrope du transport Méso-échelle de
l’Ozone Stratosphérique par Advection) model was used to
interpret the ozone depletion event. An ozone decrease of
about 2 ppmv was observed in mid-November at the 550K
isentropic level ( ~22 km). The position of Río Gallegos relative
to the polar vortex was classified using equivalent latitude
maps. During the second week of November, the vortex
was over the station at all isentropic levels, but after 20
November and until the end of the month, only the 10 lower
levels in the stratosphere were affected by vortex overpasses
with ozone poor air masses. A rapid recovery of the ozone
column density was observed later, due to an ozone rich filament
moving over Río Gallegos between 18 and 24 km in the
first two weeks of December 2009.
