Hydroxyl Radical-Initiated Oxidation of Isoprene Leads to a Cyclic Peroxyhemiacetal via 1,6-Hydrogen Shift Chemistry That Yields Secondary Organic Aerosol.

Isoprene, the largest nonmethane volatile hydrocarbon emitted into Earth's atmosphere, is rapidly oxidized by ambient hydroxyl radical (•OH) to yield secondary organic aerosol (SOA). Early generation gas-phase products of isoprene oxidation remain to be fully characterized. •OH addition at terminal C1/C4 followed by the addition of O₂, yields β- and δ-peroxy (ISOPO₂•) isomers. Under low-nitric oxide conditions, δ-ISOPO₂• isomers branch between bimolecular reaction with HO₂• and intramolecular H-shifts. Initial C₅H₈O₃ products of H-shifts reported in chamber experiments were assigned δ-hydroperoxyalkenal (HPALD) structures and predicted to photolyze rapidly. We observe by ion mobility spectrometry that a putative δ-HPALD isomer from •OH addition at C4 is the cyclic peroxyhemiacetal, 5-methyl dioxinol, and may be the relevant product in atmospheric chemistry. 5-Methyl dioxinol is robust to photolysis at tropospheric UV-vis wavelengths and is thus potentially a significant previously unrecognized source of SOA. In chamber experiments, we found that 5-methyl dioxinol forms SOA and determined that the steady-state ratio of the proximal major unimolecular/bimolecular products downstream of ISOPO₂• is ∼0.26. Using the total steady-state dioxinol level formed in the isoprene oxidation experiment, dioxinols may contribute ∼3.2 × 10⁹ g yr⁻¹, or 4% to isoprene-derived atmospheric carbon.