Characterization of Secondary Organic Aerosol Tracers over Tianjin, North China during Summer to Autumn

To characterize secondary organic aerosols (SOA) over the Tianjin region, we studied the SOA tracers derived from isoprene, alpha/beta-pinene, beta-caryophyllene, and aromatics in PM2.5 collected at an urban site and a background site over Tianjin, North China during summer to autumn 2018. Concentrations of total isoprene SOA tracers were twice those of alpha/beta-pinene in summer, while they were the opposite in autumn, followed by beta-caryophyllinic acid and 2,3-dihydroxy-4-oxopentanoic acid (2,3-DHOPA), the only tracers derived from beta-caryophyllene and toluene, respectively. The isoprene and alpha/beta-pinene SOA tracers were abundant in summer, whereas beta-caryophylinic acid and 2,3-DHOPA dominated in autumn. Temporal variations and linear relations of SOA tracers with ambient temperature and molecular markers (levoglucosan and hopanes) together with the air mass trajectories implied that the SOA derived from terrestrial vegetation and marine biota and biomass burning was high in summer and autumn, respectively, while that derived from fossil fuel combustion and its processing during long-range atmospheric transport were significant in both seasons. The estimated SOA derived from aromatics was found to be most abundant, followed by that from sesquiterpenes, monoterpenes, and isoprene, respectively. The contribution of total SOC to OC and WSOC varied between 4.16% and 23.7% and 6.28-30.7%, respectively, in which biogenic SOC accounted for about 50% in summer and 40% in autumn, indicating significant loading of SOA over Tianjin, North China.