PHASE-TRANSFORMATIONS IN SULFURIC-ACID AEROSOLS - IMPLICATIONS FOR STRATOSPHERIC OZONE DEPLETION

Activation reactions of benign chlorine species (HCl, ClONO2) on aeros ols in the winter polar stratosphere set the stage for the spring-time catalytic destruction of ozone leading to the Antarctic ozone hole. F ield observations have demonstrated the existence of both solid and li quid particles consisting of H2SO4 HNO3, and H2O. The exact freezing c onditions and final composition of the solid aerosols remain the subje ct of investigations. We present laboratory observations of isolated i ndividual sulfuric acid/water particles under stratospheric temperatur es and water vapor pressures. Our experiments demonstrate that this bi nary system would not freeze unless temperatures were below the water- ice frost point. Upon freezing, we observe H2SO4 . 8H(2)O, not the gen erally invoked H2SO4 . 4H(2)O. We suggest that the water-rich octahydr ate phase is likely to be one of the high relative humidity forms whic h is efficient in chlorine activation.