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We have investigated the structural properties of the near-surface region of various types of HX (X ¼ Cl, Br)doped H 2 O ice from a kinetic point of view by monitoring the time dependent [HX] in the interface region using the fast titration reaction XONO 2 + HX ! X 2 + HNO 3 . 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 mm thick ice sample, we measured I HCl ¼ 90 AE 9, 120 AE 15 and 90 AE 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 ¼ 20 AE 2, 30 AE 4 and 30 AE 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 5.1Á10 À14 -4.3Á10 À13 cm 2 s À1 and 2.5Á10 À15 -7.0Á10 À14 cm 2 s À1 , 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.

The rate of uptake of bromine nitrate (BrONO2) and dibromine monoxide (Br2O) on different types of ice, such as condensed (C), bulk (B), and single-crystal ice (SC) have been investigated in a Teflon-coated Knudsen flow reactor in the temperature range 180−210 K using mass spectrometric detection. For the whole temperature range the Br2O uptake kinetics is first order in [Br2O] with a mean initial uptake coefficient of γ0 = 0.24 ± 0.10, which leads to the exclusive formation of HOBr. The BrONO2 hydrolysis has been measured on B-, C-, and SC-type ice and leads to HOBr and Br2O on all types of ice. At a fixed temperature the rate law is first order in [BrONO2] with γ ≈ 0.3 at 180 K. The observed negative temperature dependence for the heterogeneous hydrolysis of BrONO2 on pure ice leads to E a of −2.0 ± 0.2, −2.1 ± 0.2, and −6.6 ± 0.3 kcal/mol on C-, B- and SC-type ice, respectively. Despite the high reactivity of BrONO2 on ice substrates, the kinetics of interaction of BrONO2 on ice nevertheless depends on the type of ice used. No saturation of the uptake coefficient has been observed during the BrONO2 hydrolysis on ice in contrast to the ClONO2/ice system. On ice samples doped with approximately 5 × 1016 molecules HBr per cm3 the kinetics of the interaction of BrONO2 with HBr leads to an uptake coefficient similar to that for BrONO2 hydrolysis. The interaction of BrONO2 with HBr occurs via the hydrolysis of BrONO2 to HNO3 and HOBr where the latter reacts with HBr in a fast secondary reaction to produce Br2 with E a = −1.2 ± 0.2 kcal/mol.

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