Synergistic Effects of SO2 and NH3 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 NOx, SO2, and NH3. In this study, the synergistic effects of SO2 and NH3 on SOA formation from gasoline evaporative VOCs with NOx were examined using a 30 m3 smog chamber with the aid of a series of mass spectrometers. Compared with the systems involving SO2 or NH3 alone, SO2 and NH3 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 SO2 on the oxidation state (OSc) of SOA in the presence or absence of NH3 were observed, and SO2 could further increase the OSc with the coexistence of NH3. The latter was attributed to the synergistic effects of SO2 and NH3 coexistence on SOA formation, wherein N-S-O adducts can be formed from the reaction of SO2 with N-heterocycles generated in the presence of NH3. Our study contributes to the understanding of SOA formation from vehicle evaporative VOCs under highly complex pollution conditions and its atmospheric implications.