HETEROGENEOUS REACTIONS ON STRATOSPHERIC BACKGROUND AEROSOLS, VOLCANIC SULFURIC-ACID DROPLETS, AND TYPE-I POLAR STRATOSPHERIC CLOUDS - EFFECTS OF TEMPERATURE-FLUCTUATIONS AND DIFFERENCES IN PARTICLE PHASE

Northern hemispheric ER-2 (NASA) data from stratospheric aerosol measu rements during background conditions, periods disturbed by the influen ce of Mount Pinatubo, ana polar stratospheric cloud (PSC) type I event s 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 r ate of OH with HCl. To calculate the reaction rates, the measured data of pressure, temperature, water vapor, and aerosol surface are utiliz ed together with recent laboratory results for the heterogeneous react ive 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 heterogen eous rates in PSCs can exceed the gas phase fates by more than 2 order s of magnitude. For the ClONO2 and HOCl reactions the measured aerosol surfaces during the PSC events are treated both as liquid (e.g., tern ary solution) droplets and: as solid NAT to compare the effects of the different phases. The reaction rates on NAT are significantly lower t han on liquid droplets. Indeed, this study shows that a transition fro m liquid ternary solutions to NAT is expected to reduce the rate of ch lorine activation based on present chemical understanding and on obser ved aerosol surface areas, Additionally, the effect of temperature and surface area fluctuations on the heterogeneous reaction rates is disc ussed.