Photooxidation of cyclohexene in the presence of SO 2 : SOA yield and chemical composition

Secondary organic aerosol (SOA) formation from cyclohexene/NOx system with various SO 2 concentrations under UV light was studied to understand the effects of cyclic alkenes on the atmospheric environment in polluted urban areas. A clear decrease at first and then increase of the SOA yield was found with increasing SO 2 concentrations. The lowest SOA yield was obtained when initial SO 2 concentration was in the range of 30–40 ppb, while higher SOA yield compared to that without SO 2 could not be obtained until the initial SO 2 concentration was higher than 85 ppb. SOA formation was enhanced by the acid-catalyzed heterogeneous reactions, which lead to an increase in the total organic aerosol mass. The competitive reaction of OH radicals with SO 2 and VOCs was the reason for the SOA yield decrease even under acidic conditions. The competitive reaction was an important factor for SOA yield and it should not be neglected in photooxidation, especially when acid-catalyzed mechanism could not significantly improve SOA yield. The composition of organic compounds in SOA was measured using several complementary techniques including Fourier transform infrared (FTIR) spectrometer, ion chromatograph (IC) and electrospray ionization high-resolution quadrupole mass spectrometer (ESI-HR-MS). We present the first evidence that organosulfates were produced from the photooxidation of cyclohexene in the presence of SO 2 .