The nature of the interface and the diffusion coefficient of HCl/ice and HBr/ice in the temperature range 190–205 K

Abstract

We have investigated the structural properties of the near-surface region of various types of HX (X = Cl, Br)-doped H₂O ice from a kinetic point of view by monitoring the time dependent [HX] in the interface region using the fast titration reaction XONO₂ + HX → X₂ + HNO₃. These “dope and probe” experiments reveal that HX is located throughout a well-defined region of thickness I readily available for titration. At 190 K and a dose of 10¹⁶ molecules corresponding to one HCl monolayer condensed onto a typically 1 μm thick ice sample, we measured I^HCl = 90 ± 9, 120 ± 15 and 90 ± 30 nm for single crystalline (SC), condensed (C) and frozen liquid water or bulk (B) ices, respectively, with I^HCl increasing by up to a factor of two with a tenfold increase in HCl dose. The corresponding values for HBr at the same temperature are I^HBr = 20 ± 2, 30 ± 4 and 30 ± 4 nm. In addition, we have determined the bulk diffusion coefficient in bulk ice for HX, D_HX, by applying Fick's second law of diffusion that results in values for D_HCl and D_HBr in the range 5.1·10⁻¹⁴–4.3·10⁻¹³ cm² s⁻¹ and 2.5·10⁻¹⁵–7.0·10⁻¹⁴ cm² s⁻¹, respectively, depending on the type of ice in the temperature range 190–205 K. The consequences of these results for heterogeneous reactions on ice particles such as Cirrus clouds, contrails and PSC's type II are discussed.