Molecular Activation by Surface Coordination:  New Model for HCl Reactivity on Water−Ice Polar Stratospheric Clouds

Results of studies of the uptake of HCl by the deuterated analogue of protonated water clusters are reported. The successive uptake of n HCl n = 1−4 is observed, n = 2−4 appearing in a stepwise manner with a ratio of 6:1 D 2 O/HCl for the bimolecular reaction products. This primary uptake scheme is observed over a range of pressures (0.24−0.46 Torr) and temperatures (130−170 K). However, for increased flows of HCl, enhanced uptake is observed at a lower ratio of D 2 O/HCl, a trend that is effected by an increased buffer gas pressure. Two distinctly dominant mechanisms of HCl uptake are operative: the bimolecular uptake of HCl in a 6:1 ratio with water and a subsequent association mechanism of HCl binding to water in a 3:1 ratio. The atmospheric implications are discussed along with a proposed molecular activation by surface coordination (MASC) model for HCl uptake and subsequent reactivity on polar stratospheric clouds.