Validation of CLAES ClONO2 measurements

The cryogenic limb array etalon spectrometer (CLAES) aboard the Upper Atmosphere Research Satellite has made extensive measurements of thermal infrared radiation from the Earth's limb from which vertical concentration profiles of several stratospheric gases and multiwavelength aerosol absorption coefficients have been retrieved for the period from January 9, 1992, to May 5, 1993. This work examines stratospheric ClONO2 concentrations from the current calibration and retrieval software which are designated version 7 data. These data provide the first near-global view of this stratospheric species. This work evaluates data quality through (1) an analysis of estimated uncertainties and biases in the remote sensing process, (2) comparison with calculations using a two-dimensional chemical model, (3) comparison with correlative data, and (4) an examination of various known limitations. The precision of CLAES ClONO2 volume mixing ratio retrievals are within 15% in the range (10 < P < 50 mbar). The upper limit on estimated systematic error is 28% in the range (10 < P < 100 mbar) based on studies of error sources in midlatitude retrievals. The global distribution of ClONO2 computed with the Lawrence Livermore National Laboratory two-dimensional stratospheric chemistry model and the CLAES measurements agree qualitatively. However, above the profile peak the calculated concentration frequently exceeds the measurement. CLAES and ATMOS measurements show relatively good midlatitude agreement, suggesting that the major sourer of discrepancy is in the model. A possible explanation in terms of a missing reaction ClO + OH --> HCl + O-2 is suggested. Also, the ClONO2 diurnal cycle constructed from more than 30 days of CLAES data agrees well with the model. The CLAES ClONO2 data differ from correlative data acquired on flights of the shuttle-based ATMOS and balloon-borne instruments by less than 25% on the average in the 10 < P < 50 mbar range. At altitudes above 10 mbar the CLAES measurement is biased low with respect to correlative measurements. This discrepancy at high altitudes is consistent with the analysis showing a large increase of systematic errors above 10 mbar. Heavy tropical volcanic aerosol from the Mount Pinatubo eruption in June 1991 apparently interfered with ClONO2 retrievals in the period before July 1992, causing anomalous peaks in the 20 < P < 30 mbar region accompanied by very small concentrations below the peak (P > 30 mbar). A similar effect associated with thick polar stratospheric clouds is identified. Overall, this validation study indicates that the majority of these data are of good quality and should be very useful in quantitative and qualitative chemical studies of the stratosphere.