Gas-Phase and Particle-Phase Reaction and Kinetics of Epoxydiols from Photooxidation of Isoprene

The emission of isoprene from deciduous plants is estimated to be upwards of 500 Tg a year and plays an important role in tropospheric chemistry over large regions of the globe. Both laboratory and field measurements have found evidence of large amounts of isoprene derivatives in secondary organic aerosols (SOAs). SOA formation has been shown to be favored under low-NOx conditions, producing aerosols with high concentrations of tetrols, sulfate esters, and C5 repeating polymeric species. The exact gas phase chemical precursors of the organics found in these SOAs are still debated. Recently, it has been shown that under low-NOx conditions the photooxidation of isoprene will result in the formation of isoprene epoxydiols. We have studied the gas-phase, aerosol-phase, and liquid-phase reactivity of a number of alkane epoxydiols. Experiments on the production of epoxydiols from the photooxidation of isoprene and butadiene as well as the uptake of these species by aerosol seed particles were performed in the Caltech dual 28-m3 environmental chamber. The gas-phase loss of the epoxydiols by aerosol uptake was monitored by chemical ionization mass spectrometry along with the products of the photooxidation of the epoxydiols. The organic composition of aerosols from the uptake of epoxydiols as well as from the photooxidation of isoprene was determined by a number of mass spectrometry techniques from chamber filter samples. Finally, the products and kinetics of the liquid-phase acid hydrolysis of the epoxides was determined by NMR. The epoxydiols were found to be readily incorporated into acidic aerosols by reactive uptake as well as partitioning. The organic composition of the SOAs from uptake of epoxydiols or the photooxidation of isoprene by acidic seeds were found to be very similar to the products of the acidic hydrolysis of the epoxydiols consisting of tetrols, sulfate esters, and polymeric species.