Nitric acid in the stratosphere based on Odin observations from 2001 to 2007 – Part 2: High-altitude polar enhancements

Abstract

Abstract. The wintertime abundance of nitric acid (HNO3) in the polar upper stratosphere displays a strong inter-annual variability, and is known to be strongly influenced by energetic particle precipitation, primarily during solar proton events, but also by precipitating electrons in the auroral zone. While wintertime HNO3 enhancements in the polar upper stratosphere had been occasionally observed before, from the ground or from satellite, we present here measurements by the Sub-Millimeter Radiometer instrument aboard the Odin satellite through 6 full annual cycles (2001 to 2007). Major solar proton events, e.g. during November 2001 or the Halloween solar storms of autumn 2003, lead to a two-stage HNO3 enhancement, likely involving different chemical reactions: a fast (about 1 week) in-situ enhancement from the mid to the upper stratosphere is followed by a slower, longer-lasting one, whereby anomalies originating in the upper stratosphere can descend within the polar vortex into the lower stratosphere. We highlight the fact that the actual chemical coupling between the upper and lower atmosphere involves a complex interplay of chemistry, dynamics and energetic particle precipitation.