Phase transformations in sulfuric acid aerosols: Implications for stratospheric ozone depletion

Activation reactions of benign chlorine species (HCl, ClONO 2 ) on aerosols in the winter polar stratosphere set the stage for the spring‐time catalytic destruction of ozone leading to the Antarctic ozone hole. Field observations have demonstrated the existence of both solid and liquid particles consisting of H 2 SO 4 , HNO 3 , and H 2 O. The exact freezing conditions and final composition of the solid aerosols remain the subject of investigations. We present laboratory observations of isolated individual sulfuric acid/water particles under stratospheric temperatures and water vapor pressures. Our experiments demonstrate that this binary system would not freeze unless temperatures were below the water‐ice frost point. Upon freezing, we observe H 2 SO 4 ·8H 2 O, not the generally invoked H 2 SO 4 ·4H 2 O. We suggest that the water‐rich octahydrate phase is likely to be one of the high relative humidity forms which is efficient in chlorine activation.