Kinetics of a Criegee intermediate that would survive high humidity and may oxidize atmospheric SO2

摘要

Criegee intermediates are thought to play a role in atmospheric chemistry, in particular, the oxidation of SO2, which produces SO3 and subsequently H2SO4, an important constituent of aerosols and acid rain. However, the impact of such oxidation reactions is affected by the reactions of Criegee intermediates with water vapor, because of high water concentrations in the troposphere. In this work, the kinetics of the reactions of dimethyl substituted Criegee intermediate (CH3)2COO with water vapor and with SO2 were directly measured via UV absorption of (CH3)2COO under near-atmospheric conditions. The results indicate that (i) the water reaction with (CH3)2COO is not fast enough (kH2O < 1.5 × 10⁻¹⁶ cm³s⁻¹) to consume atmospheric (CH3)2COO significantly and (ii) (CH3)2COO reacts with SO₂ at a near–gas-kinetic-limit rate (k_SO2 = 1.3 × 10⁻¹⁰ cm³s⁻¹). These observations imply a significant fraction of atmospheric (CH₃)₂COO may survive under humid conditions and react with SO₂, very different from the case of the simplest Criegee intermediate CH₂OO, in which the reaction with water dimer predominates in the CH₂OO decay under typical tropospheric conditions. In addition, a significant pressure dependence was observed for the reaction of (CH₃)₂COO with SO₂, suggesting the use of low pressure rate may underestimate the impact of this reaction. This work demonstrates that the reactivity of a Criegee intermediate toward water vapor strongly depends on its structure, which will influence the main decay pathways and steady-state concentrations for various Criegee intermediates in the atmosphere.