Sulfur dioxide oxidation induced mechanistic branching and particle formation during the ozonolysis of beta-pinene and 2-butene

Recent studies have suggested that the reaction of stabilised Criegee Intermediates (CIs) with sulfur dioxide (SO2), leading to the formation of a carbonyl compound and sulfur trioxide, is a relevant atmospheric source of sulfuric acid. Here, the significance of this pathway has been examined by studying the formation of gas phase products and aerosol during the ozonolysis of beta-pinene and 2-butene in the presence of SO2 in the pressure range of 10 to 1000 mbar. For b-pinene at atmospheric pressure, the addition of SO2 suppresses the formation of the secondary ozonide and leads to highly increased nopinone yields. A complete consumption of SO2 is observed at initial SO2 concentrations below the yield of stabilised CIs. In experiments using 2-butene a significant consumption of SO2 and additional formation of acetaldehyde are observed at 1 bar. A consistent kinetic simulation of the experimental findings is possible when a fast CI + SO2 reaction rate in the range of recent direct measurements [Welz et al., Science, 2012, 335, 204] is used. For 2-butene the addition of SO2 drastically increases the observed aerosol yields at higher pressures. Below 60 mbar the SO2 oxidation induced particle formation becomes inefficient pointing to the critical role of collisional stabilisation for sulfuric acid controlled nucleation at low pressures.