Abstract
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.
Highlights
Secondary organic and inorganic aerosols have been observed to account for a considerable fraction of fine particulate matter during PM2.5 pollution events which have frequently occurred and lasted for days or even weeks in China during the last decade (Huang et al, 2014; Guo et al, 2014)
Inorganic perturbations on secondary organic aerosol (SOA) formation (Shrivastava et al, 2017) are partly responsible for these uncertainties, and they include the addition of mineral particles (Yu and Jang, 2019), nitrogen oxide (NOx) (Zhao et al, 2018), ammonia (NH3) (Hao et al, 2020), and sulfur dioxide (SO2) (Yang et al, 2020), which can engage in the gasor particle-phase chemistry and subsequently influence SOA formation and growth (Friedman et al, 2016; Ng et al, 2007; Na et al, 2006)
To evaluate the impacts of SO2 on aerosol formation and growth from TMB photooxidation, a series of experiments were conducted with various initial SO2 levels under both low- and high-NOx conditions
Summary
Secondary organic and inorganic aerosols have been observed to account for a considerable fraction of fine particulate matter (aerosol particles ≤ 2.5 μm in aerodynamic diameter, PM2.5) during PM2.5 pollution events which have frequently occurred and lasted for days or even weeks in China during the last decade (Huang et al, 2014; Guo et al, 2014). These particles can directly and indirectly impact regional and global climate (Kanakidou et al, 2005), air quality (Zhang et al, 2015), and human health (Lelieveld et al, 2015). SO2-induced acidic sulfate plays an active role in the production of OSs, which have been recognized as significant SOA tracers describing the enhancement in SOA by SO2 emission (Xu et al, 2015)
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