An enhanced HNO₃ second maximum in the Antarctic midwinter upper stratosphere 2003

Vertical profiles of stratospheric HNO₃ were retrieved from limb emission spectra recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) aboard the Envisat research satellite during the Antarctic winter 2003. A high second maximum of HNO₃ was found around 34 km altitude with abundances up to 14 ppbv HNO₃ during July. Similar high abundances have not been reported in the literature for previous winters, but for the subsequent Arctic winter 2003/2004, after severe perturbations due to solar proton events. The second HNO₃ maximum in the Antarctic stratosphere started to develop in early June 2003, reached peak values during July 2003, and decreased to about 7 ppbv at the end of August while being continuously transported downward before finally forming a single HNO₃ layer over all latitudes in the lower stratosphere together with the out-of-vortex primary HNO₃ maximum. The HNO₃ decrease in August 2003 was correlated with photochemical buildup of other NO_y species as ClONO₂ and NO_x. From the time scales observed, it can be ruled out that the 2003 long-term HNO₃ enhancements were caused by local gas phase reactions immediately after the solar proton event on 29 May 2003. Instead, HNO₃ was produced by ion cluster chemistry reactions and/or heterogeneous reactions on sulfate aerosols via N₂O₅ from high amounts of NO_x being continuously transported downward from the lower thermosphere during May to August.