An intercomparison of nitrogen‐containing species in Nimbus 7 LIMS and SAMS data

Abstract

Nitrogen dioxide (NO2) and nitric acid (HNO3) distributions were derived with a two‐dimensional time‐dependent model using N2O, CH4, and temperature measurements from the stratospheric and mesospheric sounder (SAMS) along with H2O measurements from the limb infrared monitor of the stratosphere (LIMS) and O3 measurements from the solar backscatter ultraviolet (SBUV) instrument. All three instruments (SAMS, LIMS, and SBUV) were aboard the Nimbus 7 satellite. The computed NO2 and HNO3 were compared with LIMS NO2 and HNO3 measurements. Calculated NO2 is lower than the LIMS NO2 in much of the lower stratosphere by about a factor of 4 or more. Differences in the upper stratosphere between derived NO2 and LIMS NO2 are within the photochemical uncertainties of the computation. Derived NO2 is much larger than LIMS NO2 at high latitudes in the upper stratosphere in December, indicating that dynamics and/or photochemistry in the atmosphere are different from those used in the model. Derived HNO3 is in fairly good agreement with LIMS HNO3 in the upper stratosphere. Derived and LIMS HNO3 gradually become more different through the middle stratosphere and reach a fairly substantial disagreement in the lower stratosphere. The high‐latitude maxima observed in LIMS HNO3 are present at lower altitudes and at higher concentrations than those derived in the model computations. More significantly, the derived HNO3 at polar latitudes is highest in the summer while LIMS HNO3 is highest in the winter. Disagreements in the lower stratosphere between the computed and LIMS NO2 and HNO3 can only be reduced if two changes are made in the model computations: (1) additional lower stratospheric sources of odd nitrogen (other than N2O + O(1D)) are included and (2) a modified chemistry to allow the formation of HNO3 at the expense of N2O5 is used.