Interaction of HOBr and HCl on ice surfaces in the temperature range 205–227 K

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Abstract
The uptake of HOBr on ice surfaces as well as the interaction of HOBr with HCl‐doped ice surfaces has been studied in a wall‐coated tubular flow reactor coupled to a quadrupole mass spectrometer. The uptake coefficient for HOBr on ice (γHOBr) was measured between 205 and 227 K and was independent of HOBr concentration over the range (2–22) × 1011 mol cm−3. A negative temperature dependence of γ was observed with γ = 0.003 ± 0.001 at 227 K increasing to γ = 0.040 ± 0.007 at 205 K. The reaction of HOBr + HCl on ice was studied over a temperature range of 205–227 K using an HCl concentration of (4.0–8.0) × 1011 mol cm−3 and an HOBr concentration of (2–22) × 1011 mol cm−3. On exposing HOBr to the HCl‐doped ice surface, BrCl was observed as a reaction product. Under conditions with the HCl concentration in excess of the HOBr concentration, the rate of the surface reaction was found to be diffusion controlled at all temperatures, with an approximate lower limit of the uptake coefficient for HOBr + HCl on ice of γ ≥ 0.1. Under conditions with the HOBr concentration in excess of the HCl concentration, a change in uptake behavior of HOBr on HCl‐doped ice was noted. Experimental evidence suggests that the reaction follows an Eley–Rideal‐type mechanism. The atmospheric implications of these results on the chemistry of the upper troposphere are discussed.