| Popis: |
The uptake of HNO3 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 (~10-7 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 HNO3 has a higher propensity for absorption to ice than HC1 under these conditions. Competitive absorption experiments confirmed this conclusion. HNO3 was found to promote HC1 desorption when both gases were exposed to an ice surface simultaneously. The surface coverage of HC1 on HNO3-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 HNO3, 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 C1ONO2 + HC1 → C12 + HNO3. The reactive uptake of C1ONO2 has been studied at 218 and 228 K and at partial pressures ~ 10-7 Torr. On bare ice, HOC1 was observed in the gas phase immediately upon exposure of C1ONO2. The uptake coefficient of C1ONO2 was found to have a weak negative partial pressure dependence. On HC1-doped ice, C12 was observed immediately upon C1ONO2 exposure. In a final set of experiments, HC1 was taken up onto HNO3-doped ice. The surface coverage of HC1 was an order of magnitude lower than on bare ice. |
