Uptake of HNO 3 on water‐ice and coadsorption of HNO 3 and HCl in the temperature range 210–235 K

The uptake of HNO 3 on water‐ice films was investigated in a coated wall flow reactor under tropospheric temperature conditions. Experiments were performed in the “ice” region of the HNO 3 –H 2 O phase diagram. With HNO 3 partial pressures in the range of (0.3–2.0) × 10 −6 Torr, continuous uptake was observed below 215 K; whereas above 215 K, the uptake was time dependent. Using the geometric surface area of the ice film, the surface coverage at 218 K was 3.0 × 10 14 molecules/cm 2 , decreasing to 1.5 × 10 14 molecules/cm 2 at 233 K; the timescales for saturation were 700 and 800 s at 218 and 233 K, respectively. The surface coverage was found to change by a factor of 2 over a 10‐fold change in HNO 3 partial pressure. By assuming that the surface coverage can be represented by a Langmuir isotherm for dissociative adsorption, the enthalpy of adsorption of HNO 3 onto ice was found to be −(54.0 ± 2.6) kJ/mol. At a constant HNO 3 partial pressure, the maximum uptake coefficients, γ, were measured as a function of temperature, decreasing from 0.03 at 215 K down to 0.006 at 235 K. The uptake coefficients at 218 K were not significantly affected by changes in HNO 3 partial pressure. The uptake of HCl at 218 K on ice surfaces previously dosed with HNO 3 was found to be reversible, and the coadsorption of HNO 3 with HCl indicates that HCl is displaced from surface sites by HNO 3 molecules. Uptake of HNO 3 on HCl‐dosed surfaces showed that HNO 3 molecules displace ∼10 13 molecules/cm 3 of HCl. The efficiency of cirrus clouds in scavenging HNO 3 is discussed, as well as the implications for chlorine activation reactions under tropospheric temperature conditions.