Kinetics of the gas phase reaction of the Criegee intermediate CH₂OO with SO₂as a function of temperature

Abstract

The kinetics of the gas phase reaction of the Criegee intermediate CH₂OO with SO₂ have been studied as a function of temperature in the range 223-344 K at 85 Torr using flash photolysis of CH₂I₂/O₂/SO₂/N₂ mixtures at 248 nm coupled to time-resolved broadband UV absorption spectroscopy. Measurements were performed under pseudo-first-order conditions with respect to SO₂, revealing a negative temperature dependence. Analysis of experimental results using the Master Equation Solver for Multi-Energy well Reactions (MESMER) indicates that the observed temperature dependence, combined with the reported lack of a pressure dependence in the range 1.5-760 Torr, can be described by a reaction mechanism consisting of the formation of a pre-reaction complex leading to a cyclic secondary ozonide which subsequently decomposes to produce HCHO + SO₃. The temperature dependence can be characterised by <i>k</i>_CH2OO+SO2 = (3.72 ± 0.13) × 10^-11 (<i>T</i>/298)^{(-2.05±0.38)} cm³ molecule^-1 s^-1. The observed negative temperature dependence for the title reaction in conjunction with the decrease in water dimer (the main competitor for the Criegee intermediate) concentration at lower temperatures means that Criegee intermediate chemistry can play an enhanced role in SO₂ oxidation in the atmosphere at lower temperatures.