Nitric acid in the stratosphere based on Odin observations from 2001 to 2009 – Part 2: High-altitude polar enhancements

Abstract

Abstract. The wintertime abundance of nitric acid (HNO 3 ) 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 HNO 3 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 HNO 3 enhancements linked to EPP. In the first type, referred to as direct enhancements by analogy with the EPP/NO x direct effect, enhanced HNO 3 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/NO x indirect effect, the descent of mesospheric air triggers a stronger and longerlasting enhancement. Each of the three major SPEs that occurred during the Northern Hemisphere autumn or winter, in November 2001 and January 2005, are observed to lead to both direct and indirect HNO 3 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.

Variability of the nighttime OH layer and mesospheric ozone at high latitudes during northern winter: influence of meteorology

“…Moreover, during both the 2006 and 2009 major SSWs, the stratopause dropped dramatically, almost completely broke down, and then reformed at very high (∼75-80 km) altitude (Manney et al, 2009), affecting the temperature structure and trace gas distributions. These periods, together with part of the Northern polar winter 2004, have been extensively investigated because they were characterized by large descent of the constituents of the NO y family (including NO, NO 2 , HNO 3 and N 2 O 5 ) from the mesosphere to the stratosphere (see, for instance, Randall et al, 2005Randall et al, , 2006Randall et al, , 2009Natarajan et al, 2004;Clilverd et al, 2006;López-Puertas et al, 2007;Seppälä et al, 2007;Siskind et al, 2007;Hauchecorne et al, 2007;Funke et al, 2008;Orsolini et al, 2009). As pointed out by Randall et al (2009), it is atypical for such events to occur in the Northern Hemisphere (NH) three times in six years.…”

Middle atmosphere response to the solar cycle in irradiance and ionizing particle precipitation

“…In the mesosphere the wave energy spectrum has a −5/3 slope (e.g., Koshyk et al, 1999), which indicates a turbulent-like mixing regime. Observations of NO y transport (e.g., Urban et al, 2009;Orsolini et al, 2009) indicate that there is attenuation of mixing ratios in descending plumes of air at the poles. Mixing ratio conservation would require a volumetric collapse of descending tracer plumes but instead they disperse.…”

Analysis and parameterisation of ionic reactions affecting middle atmospheric HO<sub>x</sub> and NO<sub>y</sub> during solar proton events

“…In recent years, changes in other minor species have received substantial attention after a wealth of satellite observations of the winter polar regions became Published by Copernicus Publications on behalf of the European Geosciences Union. (López-Puertas et al, 2005b;von Clarmann et al, 2005;Orsolini et al, 2009;Verronen et al, 2008;Funke et al, 2008;Jackman et al, 2008;Winkler et al, 2009;Verronen et al, 2011;Winkler et al, 2011;Funke et al, 2011;Damiani et al, 2012).…”

“…Therefore, assuming instant HO x release from HNO 3 can lead to underestimation of ozone depletion. Satellite observations made during SPEs have shown significant enhancements of HNO 3 in the stratopause-lower mesosphere region (Orsolini et al, 2009). These so-called direct HNO 3 enhancements can be reasonably explained by ion chemical production (Verronen et al, 2008.…”