Temporal evolution of chlorine and minor species related to ozone
depletion observed with ground-based FTIR at Syowa Station, Antarctica and satellites during austral fall to spring in 2007 and 2011
Authors: Nakajima Hideaki , Murata Isao , Nagahama Yoshihiro , Akiyoshi Hideharu , Saeki Kosuke , Takeda Masanori , Tomikawa Yoshihiro , Jones Nicholas B. 
Published: June 2018

Abstract. To understand and project future ozone recovery, understanding of mechanisms related to polar ozone destruction is crucial. For polar stratospheric ozone destruction, chlorine species play an important role, but detailed temporal evolution of chlorine species in the Antarctic winter is not well understood. 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 (69.0º S, 39.6º E) from March to December 2007 and September to November 2011. 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. When the stratospheric temperature over Syowa Station fell below polar stratospheric cloud (PSC) saturation temperature in early winter, PSCs started to form and heterogeneous reaction on PSCs convert chlorine reservoirs into reactive chemical species. HCl and ClONO2 decrease occurred at both 18 and 22 km, and soon ClONO2 was almost depleted in early winter. When the sun returned to Antarctica in spring, enhancement of ClO and gradual O3 destruction were observed. During the ClO enhanced period, 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 distance between Syowa Station and the inner edge of the polar vortex. Temporal variation of chlorine species over Syowa Station was affected by both heterogeneous chemistry related to PSC occurrence deep inside the polar vortex, and transport of an NONOx-rich airmass from lower latitudinal polar vortex boundary region which can produce additional ClONO2 by reaction between ClO and NO2. We used MIROC3.2 Chemistry-Climate Model (CCM) results to see the comprehensive behavior of chlorine and related species inside the polar vortex and the edge region in more detail. Rapid conversion of chlorine reservoir species (HCl and ClONO2) into Cl2, gradual conversion of Cl2 into Cl2O2, increase of ClO when sunlight became available, and conversion of ClO into HCl, was successfully reproduced by the CCM. HCl decrease in the winter polar vortex core continued to occur due to the transport of ClONO2 from the subpolar region (55–65º S) to higher latitudes (65–75º S), providing a flux of ClONO2 from more sunlit latitudes into the polar vortex. The deactivation pathways from active ClO into reservoir species (HCl and/or ClONO2) were found to be highly dependent on the availability of ambient O3 and NOx. At an altitude where most ozone was depleted in Antarctica, most ClO was converted to HCl. However, when there were some O3 and NOx available, super-recovery of ClONO2 can occur, similar to the case in the Arctic.