Comparison of correlative data with HNO3 version 7 from the CLAES instrument deployed on the NASA Upper Atmosphere Research Satellite

Abstract

The cryogenic limb array etalon spectrometer (CLAES) aboard UARS made near‐global measurements of HNO3 and 388 days from January 9, 1992, to April 25, 1993, have been processed to data version 7 (V7). Results from UARS instruments, including CLAES, the improved stratospheric and mesospheric sounder, and the microwave limb sounder, provide the first near‐global documentation of the evolution of denitrification in the Antarctic 1992 winter and spring vortex. We provide a description of the CLAES HNO3 V7 quality that includes comparisons with correlative measurements to assess overall quality, accuracy, and precision. Correlative profiles of volume mixing ratio (vmr) included those obtained by the space shuttle deployed ATMOS in two missions, March–April 1992 and April 1993, data from a variety of balloon‐borne instruments at midlatitude (11 profiles), and in high‐latitude northern winter (six profiles), and LIMS data. In general, the CLAES V7 HNO3 maximum values of vmr were of the order of 6–15% less than correlative for CLAES values ≤8 parts per billion by volume (ppbv). However, when CLAES peak vmr values were 10 to 13 ppbv, then CLAES values exceeded correlative by 0–7%. The comparisons were within the combined instrumental error estimates, or observed measurement variability, for the large majority of comparisons. As discussed, the retrieval of future versions will utilize updated spectral parameters and will also correct for a small uncompensated drift in radiometric calibration that occurred in the latter part of the mission. This is expected to improve the comparisons in the ≤8 ppbv range, perhaps at the expense of those in the ≥8 ppbv range. The data obtained January 9 to April 15, 1992, in comparison with data obtained January 9 to April 15, 1993, reveal strikingly evident 1‐year period deseasonalized trends on a global basis. These trends agree quantitatively with available correlative data suitable for trend analysis. These include ATMOS in the southern midlatitudes and published long‐term time series of HNO3 column obtained at 45°S and 20°N. These trends reveal a large decrease in the southern hemisphere and small increases in the northern hemisphere, such that the global average is toward a decrease. The global average decrease we attribute to the diminishing influence of heterogeneous conversion of N2O5 to HNO3 as the Pinatubo aerosol settles out during this time period, and the HNO3 recovers toward pre‐Pinatubo conditions. We establish plausibility that the small increases in the north are due to hemispherically asymmetric QBO‐like effects that are strong in the northern hemisphere and weak in the southern hemisphere and are phased to produce an increase in HNO3 over the 1‐year time period of just the right magnitude to more than offset decrease due to settling out of the Pinatubo aerosol. Based on this study, our range of confidence in the CLAES HNO3 V7 product is from 70 to 3 mbar, in comparison with correlative data, and the precision on this range is of the order of 0.3–1.0 ppbv. This precision was derived from data repeatability and agrees within a factor of 2 or better with estimates based on instrument characterization and with error estimates embedded within the V7 data.