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 NOy species as ClONO2 and NOx. 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 NOx being continuously transported downward from the lower thermosphere during May to August. Copyright 2005 by the American Geophysical Union.