Convectively injected water vapor in the North American summer lowermost stratosphere

Anderson et al. (2012) ( A2012 ) report in situ observations of convectively injected water vapor (H 2 O) in the North American (NA) summer lowermost stratosphere (LMS), occasionally exceeding 12ppmv. They contend that, in such cold/wet conditions, heterogeneous chemistry on binary water‒sulfate aerosols can activate chlorine, leading to catalytic ozone destruction. Aura Microwave Limb Sounder 100 hPa and 82.5 hPa H 2 O 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.