EVIDENCE FOR NITRIC-ACID PENTAHYDRATE FORMED UNDER STRATOSPHERIC CONDITIONS

Type I polar stratospheric clouds (PSC I) play an important role in oz one depletion reactions, and their composition may determine a key ste p in the heterogeneous chemistry. Formation of PSC I has been simulate d by vapor depositing nitric acid and water on different substrates un der conditions designed to approach those found in the polar stratosph ere. Composition measurements of the resulting solids suggest the exis tence of a previously unreported hydrate of nitric acid, the pentahydr ate. When PSC I particles condense from the vapor phase, the pentahydr ate may be the nitric acid solid whose formation is favored under stra tospheric temperature and pressure conditions.