Uptake of HNO sub(3) on ice under upper tropospheric conditions

The uptake of HNO sub(3) 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 HNO sub(3) uptake regimes were noted with the critical threshold being the supercooled H sub(2)O/HNO sub(3) liquid isotherm at a specific temperature. For H sub(2)O/HNO sub(3) partial pressures below the supercooled H sub(2)O/HNO sub(3) liquid isotherm, a time-dependent uptake of HNO sub(3) was observed until the HNO sub(3) surface coverage reached approximately one monolayer. The initial uptake coefficient of HNO sub(3) on the ice film was [gamma] greater than or equal to 0.005 which became progressively smaller until a steady-state uptake coefficient of [gamma]=0.0002 plus or minus 0.0001 was obtained after monolayer coverage of HNO sub(3). In contrast, when H sub(2)O and HNO sub(3) partial pressures were greater than the supercooled H sub(2)O/HNO sub(3) isotherm, unlimited uptake of HNO sub(3) was observed with an uptake coefficient [gamma] > 0.005. Under these conditions, the infrared spectra showed the formation and continual growth of a supercooled H sub(2)O/HNO sub(3) liquid. By using the best estimates of HNO sub(3) mixing ratios and ice surface area densities in the upper troposphere, two limiting cases of HNO sub(3) uptake are discussed in the context of upper tropospheric chemistry.