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.