Uptake of HNO3 on water-ice and coadsorption of HNO3 and HCl in the temperature range 210-235 K -: art. no. 4797

[1] The uptake of HNO3 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 HNO3-H2O phase diagram. With HNO3 partial pressures in the range of (0.3-2. 0) x 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 x 10(14) molecules/cm(2), decreasing to 1.5 x 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 HNO3 partial pressure. By assuming that the surface coverage can be represented by a Langmuir isotherm for dissociative adsorption, the enthalpy of adsorption of HNO3 onto ice was found to be (54.0 +/- 2.6) kJ/mol. At a constant HNO3 partial pressure, the maximum uptake coefficients, gamma, 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 HNO3 partial pressure. The uptake of HCl at 218 K on ice surfaces previously dosed with HNO3 was found to be reversible, and the coadsorption of HNO3 with HCl indicates that HCl is displaced from surface sites by HNO3 molecules. Uptake of HNO3 on HCl-dosed surfaces showed that HNO3 molecules displace similar to 10(13) molecules/cm(3) of HCl. The efficiency of cirrus clouds in scavenging HNO3 is discussed, as well as the implications for chlorine activation reactions under tropospheric temperature conditions.