Evaporation studies of model polar stratospheric cloud films

To investigate evaporation of polar stratospheric clouds (PSCs), mixed nitric‐acid/ice films with varying HNO 3 content were heated at 0.3 K min −1 in vacuum and monitored with transmission Fourier transform infrared (FTIR) spectiroscopy. The ability for HNO 3 inclusions to hinder ice evaporation was found to depend on the concentration of HNO 3 . Specifically, films with initial compositions of ∼80:1 and ∼150:1 H 2 O:HNO 3 evaporated ∼8 K and ∼3 K higher than pure ice, respectively. In addition, the surface coverage of nitric acid trihydrate (NAT) appears to control the overall evaporation rate. In the atmosphere, NAT may form on the surface of an ice PSC by direct HNO 3 adsorption or by evaporation of a PSC with HNO 3 in the bulk.