The uptake of HNO 3 onto ice, NAT, and frozen sulfuric acid

The uptake of HNO 3 onto ice, HNO 3 ‐H 2 O layers, and solid H 2 SO 4 /H 2 O solutions were studied in a cylindrical flow tube in conjunction with a chemical ionization mass spectrometer detection system. An efficient loss of HNO 3 from the gas‐phase to these surfaces was observed and the uptake coefficient was ≥0.3. The HNO 3 ‐H 2 O layers formed, however, had HNO 3 vapor pressures significantly in excess of the expected values for NAT (nitric acid trihydrate). Supersaturations in HNO 3 of at least a factor of three and up to ten were observed suggesting a hindrance to the formation of NAT. These results imply that the condensation temperatures of Type I PSCs are 2–3 K colder than that expected for NAT.