HETEROGENEOUS REACTIONS ON NITRIC-ACID TRIHYDRATE

HETEROGENEOUS reactions on the surfaces of polar stratospheric cloud particles1-4 are now known to have a crucial role in the springtime decrease of polar ozone5,6 by converting inactive chlorine reservoir species, ClONO2 and HCI, to photochemically active forms, Cl2 and HOCl, thus preparing the stratosphere for ozone destruction on the return of sunlight in the spring. There is abundant recent theoretical7-9 and indirect observational10-16 evidence for two main types of polar stratospheric cloud (PSC). The predominant form, type I, which comprises HNO3 and H2O in the form of frozen nitric acid trihydrate (NAT), can condense at 195 K in the stratosphere9. This is 5-7 K warmer than the condensation temperature of type II PSCs, which are composed mainly of water ice. To understand the surface chemistry of PSCs and to test current hypotheses concerning polar ozone loss, laboratory measurements of several key kinetic parameters are needed. Here we report the first direct measurements of the reaction probabilities at stratospheric temperatures for two important heterogeneous reactions on NAT. Additionally, we report the sticking coefficients and solubilities of HC1 and NAT, which are important in modelling physical-chemical processes in the stratosphere. Our results show that the conversion of the chlorine reservoir species can occur within a few days of the first appearance of type I PSCs during the polar winter. © 1990 Nature Publishing Group.