The pressure and temperature dependence of the recombination reaction HO•+SO2+M→HOSO2•+M

The recombination of the HO. radical with SO2 was studied in the bath gas helium at temperatures between 220 and 400 K and in the pressure range between 1 and 96 bar. HO. radicals were generated by laser flash photolysis of CH4-O-3 mixtures at 248 nm and their decay was detected using saturated laser induced fluorescence (SLIF) following excitation at 281 nm. Thermal second order rate constants were extracted from the SLIF-time profiles under pseudo-first order conditions. Due to our high pressure experiments we accessed a wide part of the falloff range which allowed for a reliable extrapolation towards the high pressure limiting rate constant. We constructed complete falloff curves at 220, 300 and 400 K incorporating the low-pressure experiments from the literature. From the temperature dependence of the high pressure limiting rate constant k(1, infinity) = (1.2 +/- 0.3) x 10(-11) exp(-(360 +/- 100) K/T) cm(3) molecule(-1) s(-1) we conclude on a small barrier for the recombination process. Taking into account this barrier, we express the temperature dependence of the low pressure limiting rate constants as k(1,0) = [He] (2.5 +/- 0.7) x 10(-32) (T/300 K)-(3.8+/-0.5) exp(-(360 +/- 100)K/T) cm(6) molecule(-2) s(-1). With this new falloff analysis we reanalyzed the recombination rate constants in the literature for various third bodies (Ar, N-2, O-2, CO2, SO2, H2O and SF6), reliably extrapolated towards the corresponding low pressure limiting rate constants, and determined their collision efficiencies and the average energies [Delta E](all) transferred per collision. Implications for atmospheric chemistry are discussed.