MIPAS-TRANSALL OBSERVATIONS OF THE VARIABILITY OF CLONO2 DURING THE ARCTIC WINTER OF 1994 95/

In the winter of 1994/95 the German Transall research aircraft perform ed 5 campaigns in the European Arctic with 22 flights altogether. An e xtensive dataset of HNO3, ClONO2 and O-3 column amounts was obtained b y MIPAS-FT (Michelson Interferometer for Passive Atmospheric Sounding - Flugzeug Transall) onboard the aircraft. In this paper we present th e variability of the ClONO2 reservoir gas in the course of the winter. We include groundbased FTIR measurements of HF, HCl and ClONO2 to dis cuss the airborne observations with regard to the partitioning of inor ganic chlorine. From mid-December until the end of January, MIPAS meas ured a stable ClONO2 collar with constantly low column amounts inside the polar vortex and maxima at the edge. This observation reflected wi despread conversion of ClONO2 to reactive chlorine inside the vortex f or at least six weeks. In good accordance, the ground stations measure d low in-vortex HCl and ClONO2 column amounts and conversion of HCl in to ClONO2 in the region of the ClONO2 maxima. In the first week of Feb ruary the CLONO2 amounts started to increase in the edge region as wel l as inside the vortex. Between March 21 and 27, just one week after t he last cold period, MIPAS observed exclusively high ClONO2 column amo unts inside the vortex, indicating fast deactivation of active chlorin e. In the same period the ground stations measured an excess of ClONO2 over HCl. Further, the high ClONO2 implies that the polar vortex was renoxified in March. Lower ClONO2 values, observed inside the vortex o n the flights of April 5 and 8, and an increased HCl/ClONO2 ratio, mea sured from ground, marked the starting redistribution within the chlor ine reservoir species to the photochemically more stable HCl. In Febru ary, March and April, MIPAS observed mixing of ClONO2-rich air masses with midlatitude air at the vortex edge. A very clear event happened o n March 27. On this flight a distinct ClONO2 minimum was measured at t he vortex edge, which was closely correlated with a filament of midlat itude air observed by OLEX (Ozone Lidar EXperiment) onboard the Transa ll.