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

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 --> X-2 + 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 10(16) molecules corresponding to one HCl monolayer condensed onto a typically 1 mum 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) cm(2) s(-1) and 2.5. 10(-15) -7.0. 10(-14) cm(2) 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.