The uptake of HNO3 on thin ce films was investigated under upper tropospheric temperature and pressure conditions using a Knudsen cell reactor and FTIR reflection-absorption spectroscopy. Two different HNO3 uptake regimes were noted with the critical threshold being the supercooled H2O/HNO3 liquid isotherm at a specific temperature. For H2O/HNO3 partial pressures below the supercooled H2O/HNO3 liquid isotherm, a time-dependent uptake of HNO3 was observed until the HNO3 surface coverage reached approximately one monolayer. The initial uptake coefficient of HNO3 on the ice film was &ggr;≥0.005 which became progressively smaller until a steady-state uptake coefficient of &ggr;=0.0002¿0.0001 was obtained after monolayer coverage of HNO3. In contrast, when H2O and HNO3 partial pressures were greater than the supercooled H2O/HNO3 isotherm, unlimited uptake of HNO3 was observed with an uptake coefficient &ggr;≥0.005. Under these conditions, the infrared spectra showed the formation and continual growth of a supercooled H2O/HNO3 liquid. By using the best estimates of HNO3 mixing ratios and ice surface area densities in the upper troposphere, two limiting cases of HNO3 uptake are discussed in the context of upper tropospheric chemistry.¿ 1997 American Geophysical Union