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

Abstract

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 γ0 of HOBr on different types of pure H2O ice such as single-crystal ice, vapor-deposited ice, and samples frozen from liquid H2O range from 0.4 to 0.03 and reveal a pronounced negative temperature dependence with an activation energy of Ea = −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 ΔH0r = −9.4 ± 1.0 kcal/mol. Adsorbed HNO3 has no influence on the HOBr uptake coefficient on ice, even when the amount of HNO3 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 format...