Heterogeneous Chemistry of HOBr on Different Types of Ice and on Ice Doped with HCl, HBr, and HNO3 at 175 K < T < 215 K

The uptake kinetics of HOBr on ice with and without HX has been measured in a Teflon-coated low-pressure flow reactor (Knudsen cell) at temperatures of 175−205 K. The values of the initial uptake coefficient γ 0 of HOBr on different types of pure H 2 O ice such as single-crystal ice, vapor-deposited ice, and samples frozen from liquid H 2 O range from 0.4 to 0.03 and reveal a pronounced negative temperature dependence with an activation energy of E a = −9.7 ± 1.0 kcal/mol. The rate of HOBr uptake is independent of the type of ice. HOBr is able to sustain an equilibrium vapor pressure above an ice surface in the range from 185 to 210 K when sufficient HOBr has been adsorbed and is associated with an enthalpy change Δ H 0 r = −9.4 ± 1.0 kcal/mol. Adsorbed HNO 3 has no influence on the HOBr uptake coefficient on ice, even when the amount of HNO 3 contaminating the surface of ice is as high as 10 formal monolayers. The interaction of HOBr with HX-doped ice in the temperature range from 180 to 215 K leads to rapid formation of BrX at values of γ 0 of HOBr being less temperature dependent than for HOBr adsorption on pure ice: E a = −6.6 ± 2.0 kcal/mol has been found for the narrow temperature range from 195 to 215 K. The mass balance for BrCl from the reaction HOBr + HCl/ice is closed, in contrast to Br 2, which is the primary product of the reaction HOBr + HBr/ice. The uptake rate coefficients for HOBr + HCl/ice and HBr/ice are not significantly different from each other over the range from 180 to 215 K and are equal to γ 0 = 0.3 from 180 to 195 K, after which they drop with increasing temperature, but less so than for pure ice. At 205 K, γ 0 for HOBr + HX is typically a factor of 4 higher than γ 0 for HOBr uptake on pure ice. The atmospheric implications of these results are briefly discussed.