A surface chemistry model for nonreactive trace gas adsorption on ice: Implications for nitric acid scavenging by cirrus

According to recent measurements HNO3 nearly saturates laboratory ice films in flow tubes for vapor pressures ka above 10(-7) torr and temperatures below 230 K. In the upper troposphere the HNO3 vapor pressure is in the range of 1 to 3 x 10(-8) torr and therefore the current laboratory estimates of saturation may not hold in the atmosphere. Here, with the aid of a surface chemistry model and laboratory data we estimate that the binding energy for HNO3 adsorbed on ice is about -14.2 +/- 0.2 kcal mol(-1). Using the surface model, we explore the sensitivity of HNO3 scavenging efficiency to cirrus cloud temperature, ice number density, and HCl competitive coadsorption.