First spaceborne observations of Antarctic stratospheric ClONO₂ recovery: Austral spring 2002

We report on the first spaceborne observations of ClONO 2 recovery in the springtime Antarctic polar vortex, performed by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) from 8 September to 13 October 2002. ClONO 2 was retrieved by constrained nonlinear least squares fit from MIPAS measurements with a vertical resolution of 3.3–3.8 km and an estimated accuracy of 6–11% in the lower stratosphere (18–27 km). During the end of September 2002 an unusual major stratospheric warming affected the polar vortex. At the 475 K potential temperature level, ClONO 2 showed a pronounced collar structure with high mixing ratios at the vortex edge and values of 0.5–0.6 ppbv in the inner region from 8 to 17 September. During the following 8 days, mean inner vortex ClONO 2 increased by 0.8 ppbv accompanied by a decrease of ClO. Thus from 18 to 25 September the conversion into ClONO 2 was a major pathway for chlorine deactivation at 475 K. After 25 September, ClONO 2 decreased because of conversion into HCl. The onset of chlorine deactivation at 475 K is consistent with the latest polar stratospheric clouds detected by MIPAS on 18 September. At 625 K, mean inner vortex ClONO 2 mixing ratios decreased from 2 to 1.35 ppbv from 8 September to 13 October. This evolution was interrupted by temporary chlorine activation from 12 to 16 September when values of 1.55 ppbv were reached. Comparison with previous measurements and model results of 1992 show similar chlorine partitioning at 475 K before the major warming in 2002. However, there is indication that in 2002 the evolution of chlorine deactivation at 475 K was characterized by a faster and more pronounced increase of ClONO 2 compared with 1992.