Impact of nitric acid on ice evaporation rates

Recent studies have suggested that nitric acid uptake by ice clouds may dec rease ice evaporation rates and thereby prolong the cloud lifetimes. To les t this suggestion, ice desorption rates were studied as a function of HNO3 partial pressure (10(-6)-10(-5) Torr), relative humidity (28-92%), and temp erature (192-204 K) using optical interference of a helium neon laser. Ice evaporation rates in the presence of 1 x 10(-6) Torr HNO3 were indistinguis hable from those of pure ice. Ln contrast, ice evaporation in the presence of 8 x 10(-6) Torr HNO3 resulted in lower evaporation rates by 33% relative to pure ice. Higher partial pressures of KNO3 result in a supercooled H2O/ HNO3 liquid layer over ice, which may freeze to form a sealed NAT coating. This causes a lowering of the ice evaporation rate and prolongs the lifetim e of ice. Ice exposed to lower partial pressures of HNO3 will not form a li quid layer and will thus evaporate at the same rate as pure ice.