SO2 and NH3 emissions enhance organosulfur compounds and fine particle formation from the photooxidation of a typical aromatic hydrocarbon

Abstract

Abstract. Aromatic hydrocarbons can dominate the volatile organic\ncompound budget in the urban atmosphere. Among them, 1,2,4-trimethylbenzene\n(TMB), mainly emitted from solvent use, is one of the most important\nsecondary organic aerosol (SOA) precursors. Although atmospheric SO 2 \nand NH 3 levels can affect secondary aerosol formation, the influenced\nextent of their impact and their detailed driving mechanisms are not well\nunderstood. The focus of the present study is to examine the chemical\ncompositions and formation mechanisms of SOA from TMB photooxidation\ninfluenced by SO 2 and/or NH 3 . Here, we show that SO 2 \nemission could considerably enhance aerosol particle formation due to\nSO 2 -induced sulfate generation and acid-catalyzed heterogeneous\nreactions. Orbitrap mass spectrometry measurements revealed the\ngeneration of not only typical TMB products but also hitherto unidentified\norganosulfates (OSs) in SO 2 -added experiments. The OSs designated as being of\nunknown origin in earlier field measurements were also detected in TMB SOA,\nindicating that atmospheric OSs might also be originated from TMB\nphotooxidation. For NH 3 -involved experiments, results demonstrated a\npositive correlation between NH 3 levels and particle volume as well as\nnumber concentrations. The effects of NH 3 on SOA composition were slight\nunder SO 2 -free conditions but stronger in the presence of SO 2 . A\nseries of multifunctional products with carbonyl, alcohols, and nitrate\nfunctional groups were tentatively characterized in NH 3 -involved\nexperiments based on infrared spectra and mass spectrometry analysis. Plausible formation\npathways were proposed for detected products in the particle phase. The\nvolatility distributions of products, estimated using parameterization\nmethods, suggested that the detected products gradually condense onto the\nnucleation particles to contribute to aerosol formation and growth. Our\nresults suggest that strict control of SO 2 and NH 3 emissions might\nremarkably reduce organosulfates and secondary aerosol burden in the\natmosphere. Updating the aromatic oxidation mechanism in models could result\nin more accurate treatment of particle formation for urban regions with\nconsiderable SO 2 , NH 3 , and aromatics emissions.