Abstract. Aromatic hydrocarbons can dominate the volatile organic
compound budget in the urban atmosphere. Among them, 1,2,4-trimethylbenzene
(TMB), mainly emitted from solvent use, is one of the most important
secondary organic aerosol (SOA) precursors. Although atmospheric SO2
and NH3 levels can affect secondary aerosol formation, the influenced
extent of their impact and their detailed driving mechanisms are not well
understood. The focus of the present study is to examine the chemical
compositions and formation mechanisms of SOA from TMB photooxidation
influenced by SO2 and/or NH3. Here, we show that SO2
emission could considerably enhance aerosol particle formation due to
SO2-induced sulfate generation and acid-catalyzed heterogeneous
reactions. Orbitrap mass spectrometry measurements revealed the
generation of not only typical TMB products but also hitherto unidentified
organosulfates (OSs) in SO2-added experiments. The OSs designated as being of
unknown origin in earlier field measurements were also detected in TMB SOA,
indicating that atmospheric OSs might also be originated from TMB
photooxidation. For NH3-involved experiments, results demonstrated a
positive correlation between NH3 levels and particle volume as well as
number concentrations. The effects of NH3 on SOA composition were slight
under SO2-free conditions but stronger in the presence of SO2. A
series of multifunctional products with carbonyl, alcohols, and nitrate
functional groups were tentatively characterized in NH3-involved
experiments based on infrared spectra and mass spectrometry analysis. Plausible formation
pathways were proposed for detected products in the particle phase. The
volatility distributions of products, estimated using parameterization
methods, suggested that the detected products gradually condense onto the
nucleation particles to contribute to aerosol formation and growth. Our
results suggest that strict control of SO2 and NH3 emissions might
remarkably reduce organosulfates and secondary aerosol burden in the
atmosphere. Updating the aromatic oxidation mechanism in models could result
in more accurate treatment of particle formation for urban regions with
considerable SO2, NH3, and aromatics emissions.
Evidence for the role of organics in aerosol particle formation under atmospheric conditions