High water vapor and associated signatures from MLS in the mid-latitude summer lowermost stratosphere: Implications for posited ozone destruction

Aura Microwave Limb Sounder 100-hPa, 83-hPa and 68-hPa water vapor (H2O) measurements show the Asian monsoon anticyclone (AMA) and North American monsoon anticyclone (NAMA) to be unusually wet, both in mean values and in outliers reaching 18 ppmv. The 10-year MLS record, with associated constituent measurements, provides context for the report by Anderson, et al. (A2012) of high values of convectively-injected H2O from in situ measurements in the North American (NA) summer lowermost stratosphere (LMS). A2012 contends that in such cold/wet conditions, heterogeneous chemistry on binary water-sulfate aerosols will activate chlorine, leading to catalytic ozone destruction. In this work, we examine the statistics of extreme AMA and NAMA H2O observations within the global context of the MLS record. We discuss the implications of MLS vertical resolution on observation of ~1-km layers of enhanced H2O such as are reported in A2012, and discuss the usefulness of MLS hydrochloric acid (HCl), ozone (O3) and chlorine monoxide (ClO) measurements in putting observational limits on chlorine activation and ozone loss. Monsoon LMS parcels that are cold enough (T < ~205 K) to be candidates for aerosol growth and chlorine activation typically have much less chlorine, in the form of HCl, to activate and much less O3 to destroy than is used in A2012 as model initial conditions, regardless of whether they are wetter or drier than A2012's activation threshold. Parcels that are wet enough for activation will have been diluted with tropospheric air, and so will have lower O3 and HCl than dry parcels of similar temperature. While cooling of the stratosphere with climate change may lead to greater chlorine activation and ozone loss in the future, A2012 almost certainly overstates the potential for chemical chlorine loss in the NA summer LMS under current climate conditions.