Global-scale analysis of the effect of gas-phase Criegee intermediates (CIs) on sulphate aerosol formation: general trend and the importance of hydroxy radicals decomposed from vibrationally excited CIs†

Abstract

Stabilised Criegee intermediates (sCIs), which are formed in the atmosphere through the ozonolysis of alkenes, are known precursors of sulphate aerosols (SO₄²⁻(p)). Several previous studies have focused on the kinetics of sCI-related chemistry using both experimental and theoretical methods. Nonetheless, detailed evaluations of how the sCI affects global-scale SO₄²⁻(p) formation using chemical transport models (CTMs) have rarely been conducted. In this study, the impact of sCIs on SO₄²⁻(p) and other particulate matter was estimated using a global CTM by implementing approximately 100 chemical reactions associated with CI chemistry. The results suggest that sCIs contribute maximally less than 0.5% in remote areas, such as Amazon rainforests, Central Africa, and Australia. This value is lower than the previously estimated value, despite certain kinetic parameters related to CI chemistry being provisional due to insufficient data. Future work should focus on obtaining these kinetic parameters through experimental studies or theoretical calculations. The sCI that contributed the most to SO₄²⁻(p) formation was E-methyl glyoxal-1-oxide, which was generated by the ozonolysis of methyl vinyl ketone owing to its low-rate coefficient for the loss reaction of unimolecular decomposition and water vapour. The change in SO₄²⁻(p) enhanced the formation of secondary organic aerosols, whereas the reactions of the sCIs with NO₂ decreased the formation of nitrate radicals. The results of the sensitivity analyses showed that in highly industrialised sites in China and India, OH radicals formed by the unimolecular decomposition of vibrationally excited CIs (vCIs) contributed to SO₄²⁻(p) formation, which maximally accounted for nearly ten times more than that of sCIs, whereas the contribution of vCIs and sCIs to SO₄²⁻(p) formation was estimated to be almost equal in rural and remote sites. The estimated sCI loss by HNO₃ and organic acids was comparable to that of the unimolecular decomposition of sCIs and scavenging by water. This study provides full insight into the impact of gas-phase CI chemistry on a global scale.