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 ab sorption (FTIR-RAS) spectroscopic probe. The gas-phase uptake and reaction products resulting from the exposure of hydrogen halides to ice surfaces ov er a wide range of temperatures (110-210 K), hydrogen halide partial pressu res (5-1000 x 10(-7) Torr), and ice film thicknesses (10-100 nm) are report ed. Studies of HCl and HBr showed efficient reactions on crystalline and am orphous microporous ice films at 110 K to form H3O+ until reaching coverage s ranging from (5-20) x 10(15) molecules cm(-2), after which the rate of re action dramatically decreased. The uptake of T-ICI on hexagonal crystalline ice at temperatures representative of the lower stratosphere and upper tro posphere (180-210 K) was found to depend strongly on HCl partial pressure. Over the temperature range studied, exposure of ice to HCl partial pressure s below the HCl equilibrium partial pressure at the liquid/ice coexistence point resulted in uptake limited to (3.5 +/- 1.6) x 10(15) molecules cm(-2) . In contrast, exposure to HCl pressures larger than the HCl equilibrium pa rtial pressure resulted in unlimited uptake. HBr and HI were efficiently an d continuously taken up by ice surfaces (gamma greater than or equal to 0.0 2) over a range of atmospherically relevant temperatures (180-210 K). Altho ugh crystalline hydrates of HX:H2O are stable over the temperature range ex amined, the incorporation of hydrogen halides into ice always resulted in t he formation of amorphous HX:H2O product layers with the exception of HBr u ptake at high flow rates (flow rate greater than or equal to 3.1 x 10(15) m olecules s(-1)) which resulted in the formation of a mixture of crystalline hydrates.