Fourier transform infrared studies of the interaction of HCl with model polar stratospheric cloud films

Abstract

Heterogeneous reactions involving hydrochloric acid adsorbed on the surfaces of polar stratospheric clouds (PSCs) are postulated to contribute to polar ozone loss. Using Fourier transform infrared (FTIR) spectroscopy to probe the condensed phase, we have examined the interaction of HCl with ice and nitric acid trihydrate (NAT) films representative of types II and I PSCs, respectively. For HCl pressures in the range of 10−7 to 10−5 Torr our FTIR studies show that a small amount of crystalline HCl·6H2O formed on or in ice at 155 K. However, for higher HCl pressures we observed that the entire film of ice rapidly converted into an amorphous 4:1 H2O:HCl mixture. From HCl‐uptake experiments with PHCl = 8 × 10−7 Torr, we estimate roughly that the diffusion coefficient of HCl in ice is around 2 × 10−12 cm2/s at 158 K. For higher temperatures more closely approximating those found in the stratosphere, we were unable to detect bulk HCl uptake by ice. Our experiments also detected no bulk uptake of HCl by α‐NAT or β‐NAT under various temperature and pressure conditions. Indirect evidence suggests that HCl adsorption onto the surface of model PSC films inhibited the evaporation of both ice and NAT by 3–5 K.