Validation of CH4 and N2O measurements by the cryogenic limb array etalon spectrometer instrument on the upper atmosphere research satellite

CH4 and N2O are useful as dynamical tracers of stratospheric air transport because of their long photochemical lifetimes over a wide range of altitudes. The cryogenic limb array etalon spectrometer (CLAES) instrument on the NASA UARS provided simultaneous global measurements of the altitude profiles of CH4 and N2O mixing ratios in the stratosphere between October 1, 1991, and May 5, 1993. Data between January 9, 1995 and May 5, 1993 (388 days), have been processed using version 7 data processing software, and this paper is concerned with the assessment of the quality of this data set. CLAES is a limb-viewing emission instrument, and approximately 1200 profiles were obtained each 24-hour period for each constituent over a nominal altitude range of 100 to 0.1 mbar (16 to 64 km). Each latitude was sampled 30 times per day between latitudes 34 degrees S and 80 degrees N, or 34 degrees N and 80 degrees S depending on the yaw direction of the UARS, and nearly all local times were sampled in about 36 days. This data set extends the altitude, latitude, and seasonal coverage of previous experiments, particularly in relation to measurements at high winter latitudes. To arrive at estimates of experiment error, we compared CLAES profiles for both gases with a wide variety of correlative data from ground-based, rocket, aircraft, balloon, and space-borne sensors, looked at the repeatability of multiple profiles in the same location, and carried out empirical estimates of experiment error based on knowledge of instrument characteristics. These analyses indicate an average single-profile CH4 systematic error of about 15% between 46 and 0.46 mbar, with CLAES biased high. The CH4 random error over this range is 0.08 to 0.05 parts per million, which translates to about 7% in the midstratosphere. For N2O the indicated systematic error is less than 15% at all altitudes between 68 and 2 mbar, with CLAES tending to be high below 6.8 mbar and low above. The N2O random error is 20 to 5 ppb between 46 and 2 mbar, which also translates to 7% in the low to midstratosphere. Both tracers have useful profile information to as low as 68 mbar, excluding the tropics, and as high as 0.2 mbar (CH4) and 1 mbar (N2O). The global fields show generally good spatial correlation and exhibit the major morphological and seasonal features seen in previous global field data. Several morphological features are painted out for regions and conditions for which there have been essentially no previous data. These include the differential behavior of the tracer isopleths near and inside the Antarctic winter vortex, and local maxima in the tropics in 1992, probably associated with the Mount Pinatubo sulfate aerosol layer. Overall, the results of this validation exercise indicate that the version 7 CH4 and N2O data sets can be used with good confidence for quantitative and qualitative studies of stratospheric and lower-mesospheric atmospheric structure and dynamics.