Formation of polar stratospheric clouds on preactivated background aerosols

Abstract

Results of laboratory simulations of the growth of nitric acid trihydrate, HNO 3 ;•3H 2 O, on sulfuric acid tetrahydrate, H 2 SO 4 •4H 2 O, are presented. The observations reveal that under typical stratospheric conditions uptake of HNO 3 on a H 2 SO 4 •4H 2 O substrate results in a surface coverage of approximately one monolayer or less, and that initial HNO 3 •3H 2 O nucleation requires a large supersaturation. We also observe that a H 2 SO 4 •4H 2 O substrate, onto which a HNO 3 •3H 2 O film has been deposited and subsequently evaporated, exhibits a remarkable enhancement in its nucleation ability for this nitric acid hydrate. In the stratosphere, PSC particles may experience repeated cycles of evaporation and condensation of HNO 3 on preexisting background frozen sulfate aerosols. Hence, growth of HNO 3 •3H 2 O on preactivated aerosols provides one important mechanism for polar stratospheric cloud formation and denitrification.