UC Berkeley

Here we report a new multiphase reaction mechanism by which Criegee intermediates (CIs), formed by ozone reactions at an alkene surface, convert SO₂ to SO₃ to produce sulfuric acid, a precursor for new particle formation (NPF). During the heterogeneous ozone reaction, in the presence of 220 ppb SO₂, an unsaturated aerosol (squalene) undergoes rapid chemical erosion, which is accompanied by NPF. A kinetic model predicts that the mechanism for chemical erosion and NPF originate from a common elementary step (CI + SO₂) that produces both gas phase SO₃ and small ketones. At low relative humidity (RH = 5%), 20% of the aerosol mass is lost, with 17% of the ozone-surface reactions producing SO₃. At RH = 60%, the aerosol shrinks by 30%, and the yield of SO₃ is <5%. This multiphase formation mechanism of H₂SO₄ by CIs is discussed in the context of indoor air quality and atmospheric chemistry.