The heterogeneous reaction of HONO and HBr on ice and on sulfuric acid

Abstract

AbstractThe heterogeneous reactions of HONO with HBr on ice, solid and supercooled liquid H2SO4 solution (40–95 wt%) were studied in the temperature range 180 to 200 K for the solid substrates (ice, H2SO4) and 210 to 270 K for the liquid solutions of H2SO4 using a Knudsen flow reactor.The uptake coefficient of HONO onto frozen aqueous HBr solutions at 190 K was determined and resulted in a mean value of γ = (2.3 ± 1.2)·10−2. On ice the HBr uptake rate is high with a mean value of γ = 0.32±0.12 between 180 and 200 K and is unaffected by the presence of HONO. The uptake of HONO onto ice is a function of the HBr concentration in the condensed phase and for the highest HBr flows used an uptake coefficient of γ = 2.2·10−2 was found.On frozen H2SO4 solutions the uptake coefficient of HBr varies with the concentration of the solution from about 0.25 at 10 wt% to less than 1·10−4 in the absence and 5·10−4 in the presence of HONO at 95 wt%. The uptake of HONO in the presence of HBr was found to vary with the HBr concentration and to be approximately a factor of two lower than on the ice surface.On liquid H2SO4 the uptake coefficients of HBr and HONO both strongly depended on the concentration of the solution. HONO shows the greatest interaction at 95 wt% with γ = 2·10−2 and decreases to about 5·10−3 at 40 wt%. In comparison HBr only shows a weak interaction of γ of approximately 5·10−4 at 95 wt% and a strong interaction of γ = 4·10−2 at 40 wt%.On all three substrates the concurrent uptake of HONO and HBr was found to be reactive forming the only identified bromine containing reaction product BrNO. The variation of the substrates from strong acid liquid to solid neutral provides information about the nature of the heterogeneous interaction.