Real-time kinetics of the uptake of ClONO2 on ice and in the presence of HCl in the temperature range 160K<=T<=200K

Pulsed dosing and steady state experiments of CIONO2 on ice at 180 and 200 K studied in a low pressure flow reactor reveal a temperature-independent reactive uptake coefficient gamma 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 CIONO2 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 CIONO2 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 gamma 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 CIONO2 interaction on ice and the direct interaction of CIONO2 with HCl on ice may have ramifications for atmospheric chemistry which are briefly discussed.