Three-dimensional structure and evolution of stratospheric HNO3 based on UARS Microwave Limb Sounder measurements

The Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) measured the global distribution of stratospheric HNO3 through more than seven complete annual cycles in both hemispheres. Here we present an overview of the seasonal, interhemispheric, and interannual variations in the distribution of HNO3 throughout the lower and middle stratosphere from 420 to 960 K potential temperature based on the UARS MLS version 6 HNO3 measurements. The version 6 MLS data have much better precision and a larger vertical range than previous MLS HNO3 data sets and have also been corrected to account for an oversight in the retrieval algorithms that led earlier versions to overestimate HNO3 abundances by as much as 35% at some levels in the stratosphere. HNO3 exhibits little vertical, seasonal, or interannual variability in the tropics. For the first ~1.5 years of the mission, however, a persistent enhancement is seen at low and middle latitudes that we attribute to perturbations in reactive nitrogen chemistry under conditions of high aerosol loading from the eruption of Mount Pinatubo. The signature of Pinatubo-induced HNO3 enhancement is considerably weaker at 420 and 465 K than it is at higher altitudes, and it is also considerably weaker at northern middle and high latitudes than it is in the Southern Hemisphere. HNO3 abundances increase toward the pole in both hemispheres at all levels and in all seasons, with the exception of the severely denitrified region inside the Antarctic vortex. A pronounced seasonal cycle is present at middle and high latitudes up to at least 960 K (~34 km), with a winter maximum and a summer minimum. Large interannual variability in the timing, magnitude, and duration of enhanced wintertime HNO3 abundances is seen in both hemispheres. Even in the coldest Arctic winters, HNO3 depletion is modest and limited in both horizontal and vertical extent. In contrast, virtually complete removal of gas-phase HNO3 occurs at the highest southern latitudes by July in every year throughout the lower stratosphere. Indications of denitrification are present up to at least 740 K, well above the highest altitude at which dehydration is observed, providing further evidence that denitrification can proceed in the absence of dehydration.