Inferring the photolysis rate of NO₂ in the stratosphere based on satellite observations

Abstract

NO and NO2 (NOx) play major roles in both tropospheric and stratospheric chemistry. This paper provides a novel method to obtain a global and accurate photolysis rate for NO2 based on satellite data. The photolysis rate J(NO2) dominates the daytime diurnal variation of NOx photochemistry. Here the spatial variation of J(NO2) at 50–90∘ S in December from 20–40 km is obtained using data from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) experiment. Because NO and NO2 rapidly exchange with one another in the daytime, J(NO2) can be attained assuming steady state, and the results are shown to be consistent with model results. The J(NO2) value decreases as the solar zenith angle increases and has a weak altitude dependence. A key finding is that satellite-derived J(NO2) increases in the polar regions, in good agreement with model predictions, due to the effects of ice and snow on surface albedo. Thus, the method presented here provides an observation-based check on the role of albedo in driving polar photochemistry.