Investigation of the heterogeneous reactivity of HCl, HBr, and HI on ice surfaces

The interactions of hydrogen halide gases (HX = HCl, HBr, and HI) with thin ice films representative of atmospheric aerosols have been studied using a Knudsen cell reactor coupled to a Fourier transform infrared-reflection absorption (FTIR-RAS) spectroscopic probe. The gas-phase uptake and reaction products resulting from the exposure of hydrogen halides to ice surfaces over a wide range of temperatures (110-210 K), hydrogen halide partial pressures (5-1000 × 10^-7 Torr), and ice film thicknesses (10-100 nm) are reported. Studies of HC1 and HBr showed efficient reactions on crystalline and amorphous microporous ice films at 110 K to form H₃O⁺ until reaching coverages ranging from (5-20) × 10¹⁵ molecules cm^-2, after which the rate of reaction dramatically decreased. The uptake of HCl on hexagonal crystalline ice at temperatures representative of the lower stratosphere and upper troposphere (180-210 K) was found to depend strongly on HCl partial pressure. Over the temperature range studied, exposure of ice to HCl partial pressures below the HCl equilibrium partial pressure at the liquid/ice coexistence point resulted in uptake limited to (3.5 ± 1.6) × 10¹⁵ molecules cm^-2. In contrast, exposure to HCl pressures larger than the HCl equilibrium partial pressure resulted in unlimited uptake. HBr and HI were efficiently and continuously taken up by ice surfaces (γ ≥ 0.02) over a range of atmospherically relevant temperatures (180-210 K). Although crystalline hydrates of HX:H₂O are stable over the temperature range examined, the incorporation of hydrogen halides into ice always resulted in the formation of amorphous HX:H₂O product layers with the exception of HBr uptake at high flow rates (flow rate ≥ 3.1 × 10¹⁵ molecules s^-1) which resulted in the formation of a mixture of crystalline hydrates.