The uptake of HI and HBr on ice

A low pressure flow system equipped with mass spectrometric detection has b een employed to study the heterogeneous reactivity of HX (where X = I or Br ) on solid ice; HX(g) + H2O(s) --> products. HX uptake coefficients were me asured on ice films with HX concentrations in the range (1-30) x 10(12) mol ecule cm(-3) and over the temperature range 200-233 K. The steady state rea ctive uptake coefficient for HI was found to be constant at T > 212 K, gamm a = 0.02 +/- 0.004 and for HBr gamma = 0.03 +/- 0.005, independent of conce ntrations used. Below 212 K a large increase in the rate of uptake of HX on ice was observed with gamma greater than or equal to 0.1 for both HI and H Br. The apparent increase in the reactivity of ice was attributed to an inc rease in the surface area due to the increased porosity of ice at lower tem peratures.