According to recent measurements HNO3 nearly saturates laboratory ice films in flow tubes for vapor pressures 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¿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. ¿ 1999 American Geophysical Union