Uptake of HNO₃ on ice under upper tropospheric conditions

The uptake of HNO₃ on thin ice 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 γ ≥ 0.005 which became progressively smaller until a steady-state uptake coefficient of γ = 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 γ ≥ 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.