Characteristics of wintertime VOCs in urban Beijing: Composition and source apportionment

Characteristics and sources of volatile organic compounds (VOCs) were investigated with highly time-resolved measurements by a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) at an urban site in Beijing in winter 2017. During the measurement period, high mixing ratios of VOCs (48.9 ppbv) and trace gases were observed, with alternating episodes of strong haze pollution and clean air. Ten VOC families showed clear dependence on the VOC concentration. Aromatics increased the most during haze, with significantly elevated benzene concentration at high VOC concentration, while C_xH_yO₃ and C_xH_y increased the least. The positive matrix factorization (PMF) receptor model was applied to the VOC mass spectra, yielding four major VOC factors: traffic emissions (21.0%), solid fuel combustion (SFC, 24.4%), and two oxygenated VOC (OVOC) factors (32.3% and 22.3%). Traffic and solid fuel combustion were dominant during the periods of high total VOC concentration, while the OVOC1 fraction was reduced. Comparisons with organic aerosol (OA) sources showed increased oxygenated organic aerosol (OOA) concentration during high VOC concentration periods, indicating the importance of OVOCs to secondary organic aerosol formation. Furthermore, trajectory analysis showed that most of the clean days were associated with northerly winds with high ratios of OVOC1. In contrast, the haze periods were not only due to high primary emissions under stagnant conditions, but also influenced by air masses from a more regional scale.