Laboratory studies of the nitric acid trihydrate: Implications for the south polar stratosphere

Vapor pressures of HNO 3 and H 2 O have been measured over the trihydrate crystal, formed by vapor deposit on a glass surface. In the temperature range 190 K to 205 K the two phase‐equilibrium trihydrate/vapor was studied by adding and removing H 2 O. Coexistence equilibria vapor pressures of trihydrate/solid solutions of HNO 3 in ice (‘ice’) and of mono‐/trihydrate were also measured. Results show that for typical mixing ratios of H 2 O and HNO 3 found in the lower stratosphere (3 ppm H 2 O, 5 ppb HNO 3 ) the trihydrate would start to form at temperatures about 7 K higher than the ice point. The pressure of atmospheric HNO 3 would rapidly decrease as the atmosphere cools without large changes in partial pressures of H 2 O. These laboratory results provide information on the formation of polar stratospheric clouds containing H 2 O and HNO 3 .