The effect of sulfur dioxide on particle formation and growth by ozonolysis of three monoterpenes (α-pinene, β-pinene, and limonene) and isoprene was investigated in the presence of monodisperse ammonium sulfate seed particles and an OH scavenger in a flow tube under dry conditions. Without sulfur dioxide, new particle formation was not observed, and seed particle growth was consistent with condensation of low-volatility oxidation products produced from each organic precursor. With sulfur dioxide, new particle formation was observed from every precursor studied, consistent with sulfuric acid formation by reaction of sulfur dioxide with stabilized Criegee Intermediates. The presence of sulfur dioxide did not significantly affect seed particle growth rates from α-pinene and limonene ozonolysis, although chemical composition measurements revealed the presence of organosulfates in the particles following SO_2 exposure. Contrarily, the growth of seeds by β-pinene and isoprene ozonolysis was considerably enhanced by sulfur dioxide, and chemical composition measurements revealed that the enhanced growth was not due to additional organic material, suggesting that inorganic sulfate was likely responsible. The results suggest that a previously unconsidered particle-phase pathway to growth activated by sulfur dioxide may alter production of cloud condensation nuclei over regions with significant SO_2-alkene interactions.
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