Downward transport of upper atmospheric NO_x into the polar stratosphere and lower mesosphere during the Antarctic 2003 and Arctic 2002/2003 winters

Pronounced upper stratospheric and mesospheric NO_x enhancements were measured by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) in the Southern Hemisphere (SH) polar vortex from May to August 2003, reaching average abundances of 60 ppbv at 50-60 km in July. Peak mixing ratios of around 200 ppbv were measured in the polar night, representing the highest values ever recorded in the SH. The observed NO_x enhancements are attributed to production by electron precipitation in the upper mesosphere and lower thermosphere and subsequent descent with the meridional circulation. Using measured CH₄ and CO distributions as dynamic tracers, the downward transport of NO_x-rich air masses into the lower and middle stratosphere has been investigated. Upper atmospheric air with average NO_x abundances of 15 ppbv reached the 800-1000 K potential temperature region (around 30 km) by the end of July, where it remained until the final warming in late October. The NO_x descent was confined to the polar vortex, although significant mixing of tropical and NO_x rich vortex air masses began already in August above 40 km. The amount of upper atmospheric NO_y measured inside of the SH vortex in late spring was 1.1 Gigamoles (GM) which is in good agreement with previous estimates from HALOE data. The global coverage of MIPAS data further allows to quantify the upper atmospheric NO_x dispersed into the stratosphere during August-September, estimated in 1.3 GM. The net deposition of NO_x into the stratosphere during the 2003 Antarctic winter (2.4 GM) makes up 9% of the N₂O oxidation source in the SH, twice as much as estimated in previous studies. NO_x and tracer distributions observed on several days during the NH winter 2002 /2003 have been analyzed for comparison. We found that high planetary wave activity, resulting in the major midwinter warming had led to a rather inefficient NO_x downward transport with negligible deposition of NO_x into the lower and middle stratosphere.