Identification and quantification of IEPOX in ambient aerosols, using electron and chemical ionization sources GC/MS...

Isoprene is the most abundant non-methane hydrocarbon (NMHC) emitted by vegetation, and the precursor that makes the greatest contribution to secondary organic aerosols (SOA) in the troposphere. Atmospheric oxidation of isoprene produces a series of reactive intermediates, isoprene epoxydiols (IEPOX). The reactive uptake of IEPOX is the significant formation pathway of atmospheric SOA. In this work, four isomers of IEPOX were synthesized, derivatized by silylation reagents, and measured by gas chromatography/mass spectrometry (GC/MS). The electron-impact (EI) and methane chemical ionization (CI) sources mass spectra of trimethylsilyl ester (TMS) derivatives of IEPOX isomers were obtained and the fragmentation behaviors of the derivatives were examined. Moreover, the hydrogen nuclear magnetic resonance (H-NMR) spectra of IEPOX isomers were also obtained and the peak intensities in the H-NMR spectra were analyzed. Based on the standard spectra, IEPOX isomers were identified in ambient PM_2.5 aerosols in the Gongga Mountain (China). The peak sequence of TMS derivatives of IEPOX isomers in GC/MS chromatogram was δ4-IEPOX, δ1-IEPOX, cis-β-IEPOX and trans-β-IEPOX. The isomers with the highest concentrations were δ1-IEPOX (threo- and erythro-). The mass ratios of IEPOX to 2-methyltetrols were 0.02-6.0 and the concentrations of IEPOX were 0.8-41.6 ng/m³ in the PM_2.5 aerosols. The current study verified the core roles of IEPOX as active intermediates in photo oxidation of isoprene in ambient atmosphere.