Microwave Limb Sounder
Contact: Lucien Froidevaux
Although HOCl is not a major chlorine gas in terms of the chlorine budget or ozone destruction, it does play some role in the chemical ozone destruction cycles and there are still significant uncertainties in its rate of formation (from ClO and HO2). Obtaining global measurements of this product could help constrain some of these uncertainties and help ascertain the role of this molecule in polar winter/spring, when its abundance may increase significantly (from enhancements in ClO and HO2). The standard product for HOCl is taken from the 640 GHz (Core+R4A) retrievals. Simulations indicate that the weak HOCl signals will lead to v1.5 retrievals that are accurate to no better than 20%, and that small abundances at pressures larger than 46 hPa will be difficult to obtain. Simulations indicate that enhanced lower stratospheric values of 0.5 to 1 ppbv may be tracked at the 20% level, in single profiles. Real data systematics for v1.5 HOCl, however, lead us to recommend a maximum pressure of 22 hPa. HOCl is a noisy product and its typical stratospheric abundance is expected to be less than ~200 pptv, except under heterogeneously-enhanced conditions in the lower stratospheric winter polar vortex, where larger abundances of order 0.5 - 1 ppbv may occasionally be found. We feel that zonal mean information (e.g., in 10°-wide bins) from one day's worth of MLS data is marginally useful, although monthly or bi-weekly averages will reduce the noise to more practical levels. Few results will exist on actual validation for HOCl, given the paucity of such data and the lack of single-profile sensitivity for MLS. Some balloon-borne profiles exist, and comparisons of MLS zonal means to these over the recommended vertical range(22 - 2.2 hPa) look encouraging. MLS values show a reasonable behavior in the upper stratosphere but have a tendency to be somewhat lower than other data (balloon data), and lower also (from a rough comparison) than the MIPAS published values (Von Clarmann et al., JGR, 2006) for Sep./Oct. 2002. More progress in the MLS retrievals for HOCl may come from (off-line) retrievals using averaged radiances or other methods in the future.
 2006
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