Surface sensitive studies of the reactive uptake of chlorine nitrate on ice

The reactive uptake of chlorine nitrate (ClONO2) on ice surfaces (ClONO2 + H2O --> HOCl + HNO3) was studied with surface sensitivity using laser-induced thermal desorption (LITD) techniques. Thin films of vapor-deposited ice were exposed to ClONO2 vapor at substrate temperatures from 75 to 140 K. The reactive uptake of ClONO2 was directly measured by monitoring the hydrolysis reaction products, HOCl and HNO3, on the ice surface in real time. At low temperatures from 75 to 110 K, the HOCl coverage initially increased rapidly with ClONO2 exposure, indicating an efficient hydrolysis reaction. After longer ClONO2 exposures, the rate of HOCl production decreased and the HOCl reached a constant coverage. A reaction probability of gamma = 0.03 was calculated for the reactive uptake of ClONO2 on ice and was independent of temperature from 75 to 110 K. At temperatures greater than 110 K, the reaction probability decreased with increasing temperature and reached a value of gamma = 0.005 at 140 K. This decrease in the reaction probability with increasing substrate temperature is consistent with a precursor-mediated adsorption model. The good fits to the precursor-mediated adsorption model indicate that the ClONO2 hydrolysis reaction has a low activation barrier. The precursor-mediated adsorption model extrapolated to stratospheric temperatures predicts a reaction probability that is significantly lower than the accepted literature value of gamma similar to 0.3. This discrepancy may be caused by the higher pressures and the dynamic ice surface at stratospheric conditions that could enhance the reaction probability. The constant HOCl coverage reached after longer ClONO2 exposures is attributed to the poisoning of the ice surface by product HNO3. Calibrated HNO3 signals at 86 and 140 K revealed that the ClONO2 hydrolysis reaction is inhibited at a nitric acid coverage of similar to 7.5 x 10(14) molecules/cm(2) or similar to 1 monolayer. This coverage suggests that the hydrolysis reaction is limited to the surface or near-surface region of ice at these low temperatures.