Effects of NOx and SO2 on the secondary organic aerosol formation from the photooxidation of 1,3,5-trimethylbenzene: A new source of organosulfates.

1,3,5-Trimethylbeneze (TMB) is an important constituent of anthropogenic volatile organic compounds that contributes to the formation of secondary organic aerosol (SOA). A series of chamber experiments were performed to probe the effects of NOx and SO2 on SOA formation from TMB photooxidation. The molecular composition of TMB SOA was investigated by ultra-high performance liquid chromatography/electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-Q-TOFMS). We found that the SOA yield increases notably with elevated NOx concentrations under low-NOx condition ([TMB]0/[NOx]0 > 10 ppbC ppb-1), while an opposite trend is observed in high-NOx experiments ([TMB]0/[NOx]0 < 10 ppbC ppb-1). The increase in SOA yield in low-NOx regime is attributed to the increase of NOx-induced OH concentrations. The formation of low-volatility species might be suppressed, thereby leading to a lower SOA yield in high-NOx conditions. Moreover, SOA formation was promoted in experiment with SO2 addition. Multifunctional products containing carbonyl, acid, alcohol, and nitrate functional groups were characterized in TMB/NOx photooxidation, whereas several organosulfates (OSs) and nitrooxy organosulfates were identified in TMB/NOx/SO2 photooxidation based on HR-Q-TOFMS analysis. The formation mechanism relevant to the detected compounds in SOA were proposed. Based on our measurements, the photooxidation of TMB in the presence of SO2 may be a new source of OSs in the atmosphere. The results presented here also deepen the understanding of SOA formation under relatively complex polluted environments.