Chlorine partitioning near the polar vortex edge observed with ground-based FTIR and satellites at Syowa Station, Antarctica, in 2007 and 2011

Abstract

We retrieved lower stratospheric vertical profiles of O3, HNO3, and HCl from solar spectra taken with a ground-based Fourier transform infrared spectrometer (FTIR) installed at Syowa Station, Antarctica (69.0∘ S, 39.6∘ E), from March to December 2007 and September to November 2011. This was the first continuous measurement of chlorine species throughout the ozone hole period from the ground in Antarctica. We analyzed temporal variation of these species combined with ClO, HCl, and HNO3 data taken with the Aura MLS (Microwave Limb Sounder) satellite sensor and ClONO2 data taken with the Envisat MIPAS (the Michelson Interferometer for Passive Atmospheric Sounding) satellite sensor at 18 and 22 km over Syowa Station. An HCl and ClONO2 decrease occurred from the end of May at both 18 and 22 km, and eventually, in early winter, both HCl and ClONO2 were almost depleted. When the sun returned to Antarctica in spring, enhancement of ClO and gradual O3 destruction were observed. During the ClO-enhanced period, a negative correlation between ClO and ClONO2 was observed in the time series of the data at Syowa Station. This negative correlation was associated with the relative distance between Syowa Station and the edge of the polar vortex. We used MIROC3.2 chemistry–climate model (CCM) results to investigate the behavior of whole chlorine and related species inside the polar vortex and the boundary region in more detail. From CCM model results, the rapid conversion of chlorine reservoir species (HCl and ClONO2) into Cl2, gradual conversion of Cl2 into Cl2O2, increase in HOCl in the winter period, increase in ClO when sunlight became available, and conversion of ClO into HCl were successfully reproduced. The HCl decrease in the winter polar vortex core continued to occur due to both transport of ClONO2 from the subpolar region to higher latitudes, providing a flux of ClONO2 from more sunlit latitudes into the polar vortex, and the heterogeneous reaction of HCl with HOCl. The temporal variation of chlorine species over Syowa Station was affected by both heterogeneous chemistries related to polar stratospheric cloud (PSC) occurrence inside the polar vortex and transport of a NOx-rich air mass from the polar vortex boundary region, which can produce additional ClONO2 by reaction of ClO with NO2. The deactivation pathways from active chlorine into reservoir species (HCl and/or ClONO2) were confirmed to be highly dependent on the availability of ambient O3. At 18 km, where most ozone was depleted, most ClO was converted to HCl. At 22 km where some O3 was available, an additional increase in ClONO2 from the prewinter value occurred, similar to the Arctic.