Gas-Phase Oxidation, Particle Uptake, and Product Studies of Isoprene Epoxydiols

Isoprene epoxydiols (IEPOX), discovered in 2009 as low-NOx atmospheric oxidation products of the ubiquitous biogenic precursor isoprene, are key intermediates in the formation of secondary organic aerosol (SOA) from biogenic emissions. Recent studies carried out in the Caltech environmental chambers have elucidated the gas-phase OH oxidation rates and products, particle uptake rates, and particle-phase OH oxidation products of three isomers of IEPOX synthesized in-house. Gas-phase oxidation studies were conducted in 1 m3 chambers at both high- and low-NOx conditions, and were monitored by GC-FID, two types of chemical ionization mass spectrometry (CIMS), and GC-CIMS. Comparisons with previous studies on isoprene showed that the two β-IEPOX isomers dominate over the ∂ isomers under atmospheric conditions, and the use of propene as an internal standard provided a robust estimate of OH oxidation rates between 0.99*10-11 and 1.67*10-11 cm3molec-1s-1 for the three isomers. Particle uptake and oxidation studies were conducted in 28 m3 chambers, and were monitored by GC-FID, CIMS, an aerosol mass spectrometer (AMS), and a differential mobility analyzer (DMA). Particle uptake showed a strong dependence on humidity, with no uptake on dry seed, and was faster on ammonium sulfate seed than sodium chloride seed. Particle-phase oxidation gave similar products to gas-phase oxidation by OH radicals.