Synergistic Effects of SO 2 and NH 3 Coexistence on SOA Formation from Gasoline Evaporative Emissions.

Vehicular evaporative emissions make an increasing contribution to anthropogenic sources of volatile organic compounds (VOCs), thus contributing to secondary organic aerosol (SOA) formation. However, few studies have been conducted on SOA formation from vehicle evaporative VOCs under complex pollution conditions with the coexistence of NO, SO, and NH. In this study, the synergistic effects of SO and NH on SOA formation from gasoline evaporative VOCs with NO were examined using a 30 m smog chamber with the aid of a series of mass spectrometers. Compared with the systems involving SO or NH alone, SO and NH coexistence had a greater promotion effect on SOA formation, which was larger than the cumulative effect of the two promotions alone. Meanwhile, contrasting effects of SO on the oxidation state (OSc) of SOA in the presence or absence of NH were observed, and SO could further increase the OSc with the coexistence of NH. The latter was attributed to the synergistic effects of SO and NH coexistence on SOA formation, wherein N-S-O adducts can be formed from the reaction of SO with N-heterocycles generated in the presence of NH. Our study contributes to the understanding of SOA formation from vehicle evaporative VOCs under highly complex pollution conditions and its atmospheric implications.