Organosulfates from Pinene and Isoprene over the Pearl River Delta, South China: Seasonal Variation and Implication in Formation Mechanisms

Biogenic organosulfates (OSs) are important markers of secondary organic aerosol (SOA) formation involving cross reactions of biogenic precursors (terpenoids) with anthropogenic pollutants. Until now, there has been rare information about biogenic OSs in the air of highly polluted areas. In this study, fine particle (PM_2.5) samples were separately collected in daytime and nighttime from summer to fall 2010 at a site in the central Pearl River Delta (PRD), South China. Pinene-derived nitrooxy-organosulfates (pNOSs) and isoprene-derived OSs (iOSs) were quantified using a liquid chromatograph (LC) coupled with a tandem mass spectrometer (MS/MS) operated in negative electrospray ionization (ESI) mode. The pNOSs with MW 295 exhibited higher levels in fall (151 ± 86.9 ng m^–3) than summer (52.4 ± 34.0 ng m^–3), probably owing to the elevated levels of NOx and sulfate in fall when air masses mainly passed through city clusters in the PRD and biomass burning was enhanced. In contrast to observations elsewhere where higher levels occurred at nighttime, pNOS levels in the PRD were higher during the daytime in both seasons, indicating that pNOS formation was likely driven by photochemistry over the PRD. This conclusion is supported by several lines of evidence: the specific pNOS which could be formed through both daytime photochemistry and nighttime NO₃ chemistry exhibited no day–night variation in abundance relative to other pNOS isomers; the production of the hydroxynitrate that is the key precursor for this specific pNOS was found to be significant through photochemistry but negligible through NO₃ chemistry based on the mechanisms in the Master Chemical Mechanism (MCM). For iOSs, 2-methyltetrol sulfate ester which could be formed from isoprene-derived epoxydiols (IEPOX) under low-NOx conditions showed low concentrations (below the detection limit to 2.09 ng m^–3), largely due to the depression of IEPOX formation by the high NOx levels over the PRD