Formation of stratospheric nitric acid by a hydrated ion cluster reaction: Implications for the effect of energetic particle precipitation on the middle atmosphere

Abstract

In order to improve our understanding of the effects of energetic particle precipitation on the middle atmosphere and in particular upon the nitrogen family and ozone, we have modeled the chemical and dynamical middle atmosphere response to the introduction of a chemical pathway that produces HNO 3 by conversion of N 2 O 5 upon hydrated water clusters H + ̇(H 2 O) n. We have used an ensemble of simulations with the National Center for Atmospheric Research (NCAR) Whole-Atmosphere Community Climate Model (WACCM) chemistry-climate model. The chemical pathway alters the internal partitioning of the NO y family during winter months in both hemispheres, and ultimately triggers statistically significant changes in the climatological distributions of constituents including: i) a cold season production and loss of HNO 3 and N 2 O 5, respectively, and ii) a cold season decrease and increase in NO x /NO y -ratio and O 3, respectively, in the polar regions of both hemispheres. We see an improved seasonal evolution of modeled HNO 3 compared to satellite observations from Microwave Limb Sounder (MLS), albeit not enough HNO 3 is produced at high altitudes. Through O 3 changes, both temperature and dynamics are affected, allowing for complex chemical-dynamical feedbacks beyond the cold season when the pathway is active. Hence, we also find a NO x polar increase in spring-to-summer in the southern hemisphere, and in spring in the northern hemisphere. The springtime NO x increase arises from anomalously strong poleward transport associated with a weaker polar vortex. We argue that the weakening of zonal-mean polar winds down to the lower stratosphere, which is statistically significant at the 0.90 level in spring months in the southern hemisphere, is caused by strengthened planetary waves induced by the middle and sub-polar latitude zonal asymmetries in O 3 and short-wave heating. © 2012. American Geophysical Union. All Rights Reserved.