Seasonal variations of volatile and PM2.5 bounded n-alkanes in a central plain city, China: Abundance, sources, and atmospheric behaviour

Abstract

The alkane series is a valuable tracer to elucidate the origins and atmospheric behaviour of organic compounds. Volatile n-alkanes (C2–C11) and PM2.5-bounded n-alkanes (C8–C40) were measured simultaneously at an urban site in a central plain city, China from April 2019 to January 2020. The seasonal variations in characteristics and atmospheric processes of n-alkanes during the haze and non-haze days were discussed. n-Alkanes were most abundant in winter, and 75% of the winter sampling days were classified as haze days (PM2.5 level >75 μg m−3). The diagnostic ratios suggested the dominant effect of vehicular emissions on volatile organic compounds in four seasons; solvent usage made a noticeable contribution in summer and autumn. The liquefied petroleum gas-related sources jointly affected light volatile n-alkanes (<C5), especially on haze days. Particulate n-alkanes ranging between C8 and C24 were mainly attributed to vehicular emissions and coal combustion, whereas the biogenic sources contributed 42% of high molecular weight homologues (≥C25) in warm seasons due to the local intense greening efforts. A negligible effect of biomass burning was observed in the summer, but there was a potential impact in other seasons. Substantial semi-volatile organics partitioning to the gas phase and their faster oxidation processes in the atmosphere were important factors in explaining the lower abundance of semi-volatile n-alkanes in the particle phase in summer. This study provides more comprehensive information on the seasonal profiles of volatile and particle-associated n-alkanes and gives insights into the phase partitioning behaviour of intermediate-/semi-volatile organic compounds.