Nitric acid in the stratosphere based on Odin observations from 2001 to 2009 – Part 2: High-altitude polar enhancements Y. J. Orsolini1, J. Urban2, and D. P. Murtagh2 1Norwegian Institute for Air Research, Kjeller, Norway 2Chalmers University of Technology, Department of Radio and Space Science, Göteborg, Sweden
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 (EPP), primarily by
protons during solar proton events (SPEs), but also by precipitating auroral
or relativistic electrons. We analyse a multi-year record (August 2001 to
April 2009) of middle atmospheric HNO3 measurements by the
Sub-Millimeter Radiometer instrument aboard the Odin satellite, with a focus
on the polar upper stratosphere. SMR observations show clear evidence of two
different types of polar high-altitude HNO3 enhancements linked to EPP.
In the first type, referred to as direct enhancements by analogy with the
EPP/NOx direct effect, enhanced HNO3 mixing ratios are observed
for a short period (1 week) after a SPE, upwards of a level typically in the
mid-stratosphere. In a second type, referred to as indirect enhancements by
analogy with the EPP/NOx indirect effect, the descent of mesospheric
air triggers a stronger and longer-lasting enhancement. Each of the three
major SPEs that occurred during the Northern Hemisphere autumn or winter, in
November 2001, October–November 2003 and January 2005, are observed to lead
to both direct and indirect HNO3 enhancements. On the other hand,
indirect enhancements occur recurrently in winter, are stronger in the
Southern Hemisphere, and are influenced by EPP at higher altitudes.
Citation: Orsolini, Y. J., Urban, J., and Murtagh, D. P.: Nitric acid in the stratosphere based on Odin observations from 2001 to 2009 – Part 2: High-altitude polar enhancements, Atmos. Chem. Phys., 9, 7045-7052, doi:10.5194/acp-9-7045-2009, 2009.