| Summary: | The uptake of HNO<sub>3</sub> and HCI on water-ice films has been investigated using a coated wall flow reactor at temperatures typical of the upper troposphere. The uptake was followed by sensitive detection of gaseous species using a quadruple mass spectrometer. The uptake coefficients and surface coverage have been measured as a function of temperature (205 – 250) and reactant partial pressure (~10<sup>-7</sup> Torr). Using a Langmuir absorption isotherm and the surface coverage data, the enthalpy and entropy of absorption have been evaluated. The uptake coefficients showed a negative temperature dependence in both cases. It was concluded that HNO<sub>3</sub> has a higher propensity for absorption to ice than HC1 under these conditions. Competitive absorption experiments confirmed this conclusion. HNO<sub>3</sub> was found to promote HC1 desorption when both gases were exposed to an ice surface simultaneously. The surface coverage of HC1 on HNO<sub>3</sub>-doped ice was found to be an order of magnitude lower and completely reversible compared to the uptake on bare ice. It was concluded that in the presence of HNO<sub>3</sub>, reactions which require absorbed HC1 may be slower than anticipated on the basis of experiments which do not account for competitive absorption. One such reaction is C1ONO<sub>2</sub> + HC1 → C1<sub>2</sub> + HNO<sub>3</sub>. The reactive uptake of C1ONO<sub>2</sub> has been studied at 218 and 228 K and at partial pressures ~ 10<sup>-7</sup> Torr. On bare ice, HOC1 was observed in the gas phase immediately upon exposure of C1ONO<sub>2</sub>. The uptake coefficient of C1ONO<sub>2</sub> was found to have a weak negative partial pressure dependence. On HC1-doped ice, C1<sub>2</sub> was observed immediately upon C1ONO<sub>2 </sub>exposure. In a final set of experiments, HC1 was taken up onto HNO<sub>3</sub>-doped ice. The surface coverage of HC1 was an order of magnitude lower than on bare ice.
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