A study of type I polar stratospheric cloud formation

Abstract

Mechanisms for the formation of Type I (nitric acid‐based) polar stratospheric clouds (PSCs) are discussed. If the pre‐existing sulfate aerosols are liquid prior to PSC formation, then nitric acid particles (Type Ib) form by HNO 3 dissolution in aqueous H 2 SO 4 solution droplets. This process does not require a nucleation step for the formation of HNO 3 aerosols, so most pre‐existing aerosols grow to become relatively small HNO 3 ‐containing particles. At significantly lower temperatures, the resulting supercooled solutions (Type Ib) may freeze to form HNO 3 ice particles (Type Ia). If the preexisting sulfate aerosols are initially solid before PSC formation, then HNO 3 vapor can be deposited directly on the frozen sulfate particles. However, because an energy barrier to the condensation exists a nucleation mechanism is involved. Here, we suggest a unique nucleation mechanism that involves formation of HNO 3 /H 2 O solutions on the sulfate ice particles. These nucleation processes may be highly selective, resulting in the formation of relatively small number of large particles.