Properties of the HCl/Ice, HBr/Ice, and H2O/Ice Interface at Stratospheric Temperatures (200 K) and Its Importance for Atmospheric Heterogeneous Reactions

Abstract

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 × 1011 to 1 × 1013 and from 5 × 1012 to 5 × 1014 molecule s-1 cm-2 for HCl and D2O, respectively. A positive activation energy for diffusional loss of EA = 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 → Cl2 + ΗΝΟ3 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 DHCl ranging from (4.0 ± 1.0) × 10-14 to (2.8 ± 1.0) × 10-12 cm2 s-1 for single-crystal and bulk ice, respectively.