Photooxidation Products of Isoprene Epoxydiols (IEPOX) and IEPOX-Derived Secondary Organic Aerosol

Abstract:

Isoprene epoxydiol (IEPOX) has recently been identified as a key intermediate in the photooxidation of isoprene under low-NO conditions and in the formation of isoprene-derived secondary organic aerosol (SOA). IEPOX is generally expected to react with OH in the gas phase, where it has been found to form predominantly C₄O₃H₈ products, or undergo reactive uptake onto particles, where it is converted into 2-methyltetrols or organosulfates by acid- or ammonium-catalyzed mechanisms. The subsequent chemistry of these gas- and particle-phase products has not yet been explored. Using synthetic standards of IEPOX and its gas-phase products, we have performed environmental chamber and flow tube experiments to investigate the fate of IEPOX in both the gas and particle phases. To explore the gas-phase chemistry of IEPOX, three potential isomers of the C₄O₃H₈ products were synthesized and photooxidized by exposure to OH. Detection with CF₃O^- chemical ionization mass spectrometry (CIMS) allowed for determination of their oxidation rates, fractional yields from IEPOX oxidation, and major products. To explore the photooxidation of IEPOX-derived SOA, synthetic IEPOX was reacted with various salts and atomized into a flow tube, where it was photooxidized by exposure to OH. We will present results showing changes in gas- and particle-phase chemical composition, monitored during oxidation by CIMS and aerosol mass spectrometry, including their dependence on both seed particle composition and OH concentration. Preliminary data show that the photochemical loss of IEPOX-derived SOA mass may be an important consideration for predicting aerosol loading and gas phase oxidative chemistry in isoprene-rich environments.