Direct measurement of the rate of reaction of the methyl radical with sulphur dioxide

Abstract

The kinetics of the reaction CH₃+ SO₂(+M)→CH₃SO₂(+M)(1) have been studied by flash photolysis of azomethane in the presence of sulphur dioxide. The decay in the methyl radical concentration was monitored by its absorption at 216.4 nm using a novel “split-beam” photoelectric method which permits the period available for monitoring the decay to be almost trebled to 350 µS. The second order rate constant, k₁, was measured to be (1.75 ± 0.25)× 10⁸ dm³ mol^–1 s^–1 independent of the total pressure both in argon and nitrogen over the pressure range 6.65–26.6 kN m^–2(50–200 Torr). The value is of the same order as the rate constant for the reaction of methyl radicals with oxygen and confirms the suggestion of previous workers that free radical attack on SO₂ could contribute significantly to SO₂ removal mechanisms and aerosol formation in polluted atmospheres.