Interaction of HCl with ice: Investigation of the predicted trihydrate, hexahydrate, and monolayer regimes

Knowledge of the interaction of hydrogen chloride (HCl) with ice is important for an understanding of heterogeneous reactions on polar stratospheric clouds. The interaction of HCl with ice as a function of ice temperature, HCl partial pressure, and ice film thickness was studied using laser-induced thermal desorption (LITD) techniques. Ice films were prepared by depositing H2O vapor onto a cooled Al2O3 substrate. The ice was then exposed to HCl partial pressures ranging from 1 x 10(-9) to 1 x 10(-6) Torr at ice temperatures from 140 to 186 K. HCl uptake by ice was monitored as a function of time using the HCl LITD signals, A doubly differentially pumped mass spectrometer allowed the LITD studies to be conducted at H2O partial pressures up to 1 x 10(-4) Torr. These high H2O partial pressures are required to maintain stable ice films at the higher ice temperatures. At temperatures below 148 K, the HCl and H2O LITD signals indicated that the ice films rapidly formed an HCl . 3H(2)O trihydrate film. Between 148 and 175 K, the HCl:H2O ratio changed continuously with no observed stable HCl . 6H(2)O hexahydrate. At stratospheric temperatures between 180 and 186 K, HCl uptake on ice varied from 4 x 10(14) to 2 x 10(16) molecules/cm(2). This variation was attributed to varying ice surface roughness with the roughest ice films displaying the highest HCl uptake. HCl uptake was limited to similar to 1 ML (1 ML = 1.15 x 10(15) molecules/cm(2)) on the smoothest ice films, in agreement with previous estimates of monolayer HCl uptake by stratospheric ice.