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

Abstract

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 β-pinene and 2-butene in the presence of SO₂ in the pressure range of 10 to 1000 mbar. For β-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.