Chlorine partitioning near the polar vortex edge observed with ground-based FTIR and satellites at Syowa Station, Antarctica in 2007 and 2011
Authors: Nakajima Hideaki , Murata Isao , Nagahama Yoshihiro , Akiyoshi Hideharu , Kinase Takeshi , Tomikawa Yoshihiro , Jones Nicholas 
Published: March 2020

&lt;p&gt;We retrieved lower stratospheric vertical profiles of O&lt;sub&gt;3&lt;/sub&gt;, HNO&lt;sub&gt;3&lt;/sub&gt;, and HCl from solar spectra taken with a ground-based Fourier-Transform infrared spectrometer (FTIR) installed at Syowa Station, Antarctica (69.0&amp;#176;S, 39.6&amp;#176;E) from March to December 2007 and September to November 2011. &amp;#160;This was the first continuous measurements of chlorine species throughout the ozone hole period from the ground in Antarctica. &amp;#160;We analyzed temporal variation of these species combined with ClO, HCl, and HNO&lt;sub&gt;3&lt;/sub&gt; data taken with the Aura/MLS (Microwave Limb Sounder) satellite sensor, and ClONO&lt;sub&gt;2&lt;/sub&gt; data taken with the Envisat/MIPAS (The Michelson Interferometer for Passive Atmospheric Sounding) satellite sensor at 18 and 22 km over Syowa Station. &amp;#160;HCl and ClONO&lt;sub&gt;2&lt;/sub&gt; decrease occurred from the end of May at both 18 and 22 km, and eventually in early winter, both HCl and ClONO&lt;sub&gt;2&lt;/sub&gt; were almost depleted. &amp;#160;When the sun returned to Antarctica in spring, enhancement of ClO and gradual O&lt;sub&gt;3&lt;/sub&gt; destruction were observed. &amp;#160;During the ClO enhanced period, negative correlation between ClO and ClONO&lt;sub&gt;2&lt;/sub&gt; was observed in the time-series of the data at Syowa Station. &amp;#160;This negative correlation was associated with the relative distance between Syowa Station and the edge of the polar vortex. &amp;#160;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. &amp;#160;From CCM model results, rapid conversion of chlorine reservoir species (HCl and ClONO&lt;sub&gt;2&lt;/sub&gt;) into Cl&lt;sub&gt;2&lt;/sub&gt;, gradual conversion of Cl&lt;sub&gt;2&lt;/sub&gt; into Cl&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;, increase of HOCl in winter period, increase of ClO when sunlight became available, and conversion of ClO into HCl, was successfully reproduced. &amp;#160;HCl decrease in the winter polar vortex core continued to occur due to both transport of ClONO&lt;sub&gt;2&lt;/sub&gt; from the subpolar region to higher latitudes, providing a flux of ClONO&lt;sub&gt;2&lt;/sub&gt; from more sunlit latitudes into the polar vortex, and the heterogeneous reaction of HCl with HOCl. &amp;#160;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 airmass from the polar vortex boundary region which can produce additional ClONO&lt;sub&gt;2&lt;/sub&gt; by reaction of ClO with NO&lt;sub&gt;2&lt;/sub&gt;. &amp;#160;The deactivation pathways from active chlorine into reservoir species (HCl and/or ClONO&lt;sub&gt;2&lt;/sub&gt;) were confirmed to be highly dependent on the availability of ambient O&lt;sub&gt;3&lt;/sub&gt;. &amp;#160;At 18 km where most ozone was depleted, most ClO was converted to HCl. &amp;#160;At 22km where some O&lt;sub&gt;3&lt;/sub&gt; was available, additional increase of ClONO&lt;sub&gt;2&lt;/sub&gt; from pre-winter value occurred, similar as in the Arctic.&lt;/p&gt;