A MODEL FOR STUDYING THE COMPOSITION AND CHEMICAL EFFECTS OF STRATOSPHERIC AEROSOLS

We developed polynomial expressions for the temperature dependence of the mean binary and water activity coefficients for H2SO4 and HNO3 sol utions. These activities were used in an equilibrium model to predict the composition of stratospheric aerosols under a wide range of enviro nmental conditions. For typical concentrations of H2O, H2SO4, HNO3, HC l, HBr, HF, and HOCl in the lower stratosphere, the aerosol compositio n is estimated as a function of the local temperature and the ambient relative humidity. For temperatures below 200 K, our results indicate that (1) HNO3 contributes a significant mass fraction to stratospheric aerosols, and (2) HCI solubility is considerably affected by HNO3 dis solution into sulfate aerosols. We also show that, in volcanically dis turbed periods, changes in stratospheric aerosol composition can signi ficantly alter the microphysics that leads to the formation of polar s tratospheric clouds. The effects caused by HNO3 dissolution on the phy sical and chemical properties of stratospheric aerosols are discussed.