Properties of the HCl/ice, HBr/ice, and H2O/ice interface at stratospheric temperatures (200 K) and its importance for atmospheric heterogeneous reactions

The properties of the interface region of bulk, vapor-deposited, and single-crystal ice have been studied in a Knudsen cell flow reactor in the range 190-210 K using repetitive pulse experiments (RPEs) at variable frequency. Fluxes of surface-to-bulk loss in ice on single-crystal and bulk ice vary from 5 x 10(11) to 1 x 10(13) and from 5 x 10(12) to 5 x 10(14) molecule s(-1) cm(-2) for HCl and D2O, respectively. A positive activation energy for diffusional loss of E-A = 3.0 +/- 0.5 kcal/mol for HCl/ice and 5.3 +/- 0.7 kcal/mol for D2O/ice has been measured. Complementary measurements ("dope and probe" experiments) of the HCl/ice interface region using the titration reaction of ClONO2 + HCl --> Cl-2 + HNO3 are in good agreement with the diffusional loss measurements from RPEs. These experiments allowed the evaluation of the thickness h of the interfacial region, defined as the near-surface region of the ice where HCl is immediately available for titration at a high rate. We measured h = 100 +/- 10 nm for single-crystal ice, 200 +/- 50 nm for vapor-deposited ice, and 1000 +/- 200 nm for bulk ice samples. The modeling of our results according to the laws of diffusion leads to values of the HCl diffusion coefficient D-HCl ranging from (4.0 +/- 1.0) x 10(-14) to (2.8 +/- 1.0) x 10(-12) cm(2) s(-1) for single-crystal and bulk ice, respectively.