Real-Time Kinetics of the Uptake of ClONO2 on Ice and in the Presence of HCl in the Temperature Range 160 K ≤ T ≤ 200 K

Pulsed dosing and steady state experiments of ClONO 2 on ice at 180 and 200 K studied in a low pressure flow reactor reveal a temperature-independent reactive uptake coefficient γ of 0.2 ± 0.05 at limiting doses and low flow rates of 10 14 molecules per pulse and 10 14 molecules s -1, respectively. The reaction involves the formation of a precursor in a slow process releasing HOCl. This precursor does not interact with HCl. The reaction of ClONO 2 with HCl was studied under pulsed, concurrent, and sequential flow conditions and was found to follow a direct mechanism. The formation of Cl 2 occurs promptly on the time scale of several tens of milliseconds. The reactive uptake coefficient at equivalent flow rates of ClONO 2 and HCl was measured to be 0.14 ± 0.05 and 0.26 ± 0.05 at 200 and 180 K, respectively. At a 3-fold excess of HCl γ increases to 0.24 ± 0.05 and 0.34 ± 0.05 at 200 and 180 K, respectively. HOCl is found to interact with ice at T >173 K and pressures of approximately 10 -6 Torr only up to the extent of 5% of a monolayer beyond which it saturates. Arguments are put forward in favor of an ionic displacement mechanism in both reactions. The difference between the precursor mechanism of ClONO 2 interaction on ice and the direct interaction of ClONO 2 with HCl on ice may have ramifications for atmospheric chemistry which are briefly discussed.