Convectively injected water vapor in the North American summer lowermost stratosphere

Anderson et al. (2012) (A2012) report in situobservations of convectively injected water vapor (H2O) in the North American (NA) summer lowermost stratosphere (LMS), occasionally exceeding 12ppmv. They contend that, in such cold/wet conditions, heterogeneous chemistry on binary watersulfate aerosols can activate chlorine, leading to catalytic ozone destruction. Aura Microwave Limb Sounder 100 hPa and 82.5 hPa H2O measurements show that, indeed, the NA LMS is unusually wet, both in mean values and in outliers reaching 18ppmv. Using A2012's threshold, 4% (0.03%) of 100 hPa (82.5 hPa) NA July-August observations are cold/wet enough for activation. Cold parcels, whether wet or dry, typically have much less HCl to activate and O(3)to destroy than A2012's initial conditions. Slightly lower concentrations of HCl and O-3 in cold/wet parcels are attributable, at least in part, to dilution by tropospheric air. Alarming reductions in NA summer column O(3)suggested by A2012 are not seen in the current climate.