An intercomparison of nitrogen-containing species in Nimbus 7 LIMS and SAMS Data

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.