Trends in stabilisation of Criegee intermediates from alkene ozonolysis

Abstract

Criegee Intermediates (CI), formed in the ozonolysis of alkenes, play a central role in tropospheric chemistry as an important source of radicals, with stabilised CI (SCI) able to participate in bimolecular reactions, affecting climate through the formation of inorganic and organic aerosol. However, total SCI yields have only been determined for a few alkene systems, while speciated SCI yields from asymmetrical alkenes are almost entirely unknown. Here we report for the first time a systematic experimental exploration of the stabilisation of CH₂OO and (CH₃)₂COO CI, formed from ten alkene–ozone systems with a range of different sizes and structures, under atmospherically relevant conditions in the EUPHORE chamber. Experiments in the presence of excess SO₂ (an SCI scavenger) determined total SCI yields from each alkene–ozone system. Comparison of primary carbonyl yields in the presence/absence of SO₂ determined the stabilisation fraction of a given CI. The results show that the stabilisation of a given CI increases as the size of the carbonyl co-product increases. This is interpreted in terms of the nascent population of CI formed following decomposition of the primary ozonide (POZ) having a lower mean energy distribution when formed with a larger carbonyl co-product, as more of the energy from the POZ is taken by the carbonyl. These findings have significant implications for atmospheric modelling of alkene ozonolysis. Higher stabilisation of small CI formed from large alkenes is expected to lead to lower radical yields from CI decomposition, and higher SCI concentrations, increasing the importance of SCI bimolecular reactions.