THE HETEROGENEOUS REACTION OF HONO AND HBR ON ICE AND ON SULFURIC-ACID

The heterogeneous reactions of HONO with HBr on ice, solid and superco oled 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 2 70 K for the liquid solutions of H2SO4 using a Knudsen flow reactor. T he uptake coefficient of HONO onto frozen aqueous HBr solutions at 190 K was determined and resulted in a mean value of gamma = (2.3 +/- 1.2 ) 10(-2). On ice the HBr uptake rate is high with a mean value of gamm a = 0.32+/-0.12 between 180 and 200 K and is unaffected by the presenc e of HONO. The uptake of HONO onto ice is a function of the HBr concen tration in the condensed phase and for the highest HBr flows used an u ptake coefficient of gamma = 2.2.10(-2) was found. On frozen H2SO4 sol utions 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 ab sence and 5.10(-4) in the presence of HONO at 95 wt%. The uptake of HO NO 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 strongl y depended on the concentration of the solution. HONO shows the greate st interaction at 95 wt% with gamma = 2.10(-2) and decreases to about 5.10(-3) at 40 wt%. In comparison HBr only shows a weak interaction of gamma of approximately 5.10(-4) at 95 wt% and a strong interaction of gamma = 4.10(-2) at 40 wt%. On all three substrates the concurrent up take of HONO and HBr was found to be reactive forming the only identif ied bromine containing reaction product BrNO. The variation of the sub strates from strong acid liquid to solid neutral provides information about the nature of the heterogeneous interaction.