Mechanisms for midlatitude ozone loss: Heterogeneous chemistry in the lowermost stratosphere?

The question of midlatitude ozone erosion by chlorine free radical catalysi s is examined. We present and analyze simultaneous, high-resolution observa tions of ClO, H2O, tropopause height, particle reactive surface area, and i ce saturation occurrence frequency obtained from the NASA ER-2 aircraft. Th e objective is to test the hypothesis that the existence of cirrus clouds o r cold aerosols in the first few kilometers above the tropopause at midlati tudes is responsible for increasing the ratio of chlorine free radicals to total inorganic chlorine, thus amplifying the rate of catalytic ozone destr uction. The observations reveal a sharp decrease in ice saturation frequenc y at the tropopause, a marked degree of undersaturation just above the trop opause, a corresponding sharp gradient in the product of cold aerosol react ive surface area and reaction probability, yS,, and, finally, the consisten t absence of enhanced concentrations of ClO immediately above the tropopaus e. These results suggest that midlatitude ozone loss is not controlled in s itu by the mechanism of cirrus cloud and/or cold aerosol enhancement of chl orine radicals in the vicinity of the tropopause.