HNO3 measurements from MLS on the UARS and EOS CHEM satellites

Revised data processing algorithms for the UARS Microwave Limb Sounder (MLS) have recently been released. The new "version 5" files will constitute the final archived MLS dataset. The various refinements in the retrieval algorithms have led to much better fits to the radiometric data; in addition, geophysical parameters are now reported at twice the vertical resolution of previous MLS datasets. We will present estimates of the accuracy and precision of the UARS MLS version 5 HNO3 retrievals, and we will show comparisons with contemporaneous Atmospheric Trace Molecule Spectroscopy (ATMOS) and UARS Cryogenic Limb Array Etalon Spectrometer (CLAES) HNO3 observations.

We have engaged in a number of studies of chemical processes in the lower stratosphere utilizing the HNO3 data from UARS MLS. The issues investigated include: the seasonal buildup, depletion, and recovery of HNO3 at high latitudes; the phase and composition of polar stratospheric cloud particles in both the Arctic and the Antarctic; the interannual and interhemispheric differences in the extent and magnitude of denitrification; and the coupling (or lack thereof) between denitrification and dehydration mechanisms. While much has been learned from these studies, they have often been hampered by interrupted high-latitude data coverage (due to yaw maneuvers), a lack of simultaneous measurements arising from a variety of factors (such as data outages, depletion of the UARS CLAES supply of stored cryogen, failure of the MLS 183-GHz radiometer used to measure H2O, or simply incompatible data sampling patterns), and insufficient accuracy and/or resolution.

EOS MLS, currently scheduled for launch on the CHEM satellite in December 2002, is a greatly enhanced version of the UARS MLS experiment, and many of the improvements will be of tremendous benefit to stratospheric polar process studies. EOS MLS will provide better spatial resolution and coverage (82N to 82S on each orbit), measurement of additional chemical constituents (including the dynamical tracer N2O), and better precision and an extended vertical range for many species. Details of the enhancements for HNO3 and other species relevant to polar process studies will be shown. We will briefly review some of the UARS-based studies and illustrate why revisiting these topics with the more extensive and better-quality EOS CHEM data sets will be fruitful.