Northern hemispheric ER-2 (NASA) data from stratospheric aerosol measurements during background conditions, periods disturbed by the influence of Mount Pinatubo, and polar stratospheric cloud (PSC) type I events are used to study the heterogeneous reactions of ClONO2 with H2O and of HOCl and ClONO2 with HCl in comparison to the gas phase reaction rate of OH with HCl. To calculate the reaction rates, the measured data of pressure, temperature, water vapor, and aerosol surface are utilized together with recent laboratory results for the heterogeneous reactive uptake coefficients. Because observations are limited, the mixing ratios of the gas phase species entering these rate calculations (i.e., ClONO2, HOCl, HCl, and N2O5) are taken from a two-dimensional model. It is found that in dense volcanic clouds at temperatures below 200 K the resulting heterogeneous reaction rates of chlorine activation can be of similar magnitude as the gas phase reaction rate. The heterogeneous rates in PSCs can exceed the gas phase rates by more than 2 orders of magnitude. For the ClONO2 and HOCl reactions the measured aerosol surfaces during the PSC events are treated both as liquid (e.g., ternary solution) droplets and as solid NAT to compare the effects of the different phases. The reaction rates on NAT are significantly lower than on liquid droplets. Indeed, this study shows that a transition from liquid ternary solutions to NAT is expected to reduce the rate of chlorine activation based on present chemical understanding and on observed aerosol surface areas. Additionally, the effect of temperature and surface area fluctuations on the heterogeneous reaction rates is discussed.¿ 1997 American Geophysical Union