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

Abstract

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–O₃ 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,∞=(1.2±0.3)×10^-11 exp(-(360±100) K/T) cm³ 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)×10^-32 (T/300 K)^-(3.8±0.5) exp(-(360±100)K/T) cm⁶ molecule^-2 s^-1. With this new falloff analysis we reanalyzed the recombination rate constants in the literature for various third bodies (Ar, N₂, O₂, CO₂, SO₂, H₂O and SF₆), reliably extrapolated towards the corresponding low pressure limiting rate constants, and determined their collision efficiencies and the average energies 〈ΔE〉_all transferred per collision. Implications for atmospheric chemistry are discussed.