Nitrogen species in the post‐Pinatubo stratosphere: Model analysis utilizing UARS measurements

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We present an analysis of the impact of heterogeneous chemistry on the partitioning of nitrogen species measured by the Upper Atmosphere Research Satellite (UARS) instruments. The UARS measurements utilized include N2O, HNO3, and ClONO2 from the cryogenic limb array etalon spectrometer (CLAES), version 7 (v.7), and temperature, methane, ozone, H2O, HCl, NO and NO2 from the halogen occultation experiment (HALOE), version 18. The analysis is carried out for the UARS data obtained between January 1992 and September 1994 in the 100‐ to 1‐mbar (∼17–47 km) altitude range and over 10° latitude bins from 70°S to 70°N. The spatiotemporal evolution of aerosol surface area density (SAD) is adopted from analysis of the Stratospheric Aerosol and Gas Experiment (SAGE) II data. A diurnal steady state photochemical box model, constrained by the temperature, ozone, H2O, CH4, aerosol SAD, and columns of O2 and O3 above the point of interest, has been used as the main tool to analyze these data. Total inorganic nitrogen (NOy) is obtained by three different methods: (1) as a sum of the UARS‐measured NO, NO2, HNO3, and ClONO2; (2) from the N2O‐NOy correlation, and (3) from the CH4‐NOy correlation. To validate our current understanding of stratospheric heterogeneous chemistry for post‐Pinatubo conditions, the model‐calculated monthly averaged NOx/NOy ratios and the NO, NO2, and HNO3 profiles are compared with the UARS‐derived data. In general, the UARS‐constrained box model captures the main features of nitrogen species partitioning in the post‐Pinatubo years, such as recovery of NOx after the eruption, their seasonal variability and vertical profiles. However, the model underestimates the NO2 content, particularly in the 30‐ to 7‐mbar (∼23–32 km) range. Comparisons of the calculated temporal behavior of the partial columns of NO2 and HNO3 and ground‐based measurements at 45°S and 45°N are also presented. Our analysis indicates that ground‐based and HALOE v. 18 measurements of the NO2 vertical columns are consistent within the range of their uncertainties and are systematically higher (up to 50%) than the model results at midlatitudes in both hemispheres. Reasonable agreement is obtained for HNO3 columns at 45°S, suggesting some problems with nitrogen species partitioning in the model. Outstanding uncertainties are discussed.