Molecular interaction between ammonium sulfate and secondary organic aerosol from styrene

Secondary organic aerosol (SOA) and inorganic aerosol (SIA) are two major constituents of PM_2.5, which are well-studied separately. However, the molecular-level investigation of mechanical interactions between them is still limited, which will affect the understanding of toxicity and optical properties of PM_2.5. Styrene contains a benzene ring and a highly reactive double bond, mainly contributing to urban SOA formation. Here, chamber experiments were conducted to explore the effects of the (NH₄)₂SO₄ seeds, as an example of SIA, on styrene SOA formation under varying relative humidity (RH) conditions. The particle-phase products were determined based on a high-resolution orbitrap mass spectrometer. The results showed that in the styrene-H₂O₂-hv systems, (NH₄)₂SO₄ seeds facilitated the gas-particle partitioning of SOA precursors, leading to slightly higher SOA yields than the unseeded systems at 8.5 % RH. While RH exceeded 44.3 %, the presence of (NH₄)₂SO₄ seeds increased the particle liquid water content, thus enhancing the partitioning of hydrophilic H₂O₂ to particle phase and the further oxidation of SOA, finally decreasing SOA yields. In the styrene-NO_x-hv systems, NH₄⁺ and SO₄^2- were involved in the particle-phase reactions, producing prominent nitrogen- and sulfur-containing compounds of C₈H₇O₂N and C₈H₉O₆NS. In the styrene-O₃ dark reaction systems, the existence of (NH₄)₂SO₄ seeds significantly changed the oligomer components under humid conditions. The particle-phase formation pathways of oligomers affected by (NH₄)₂SO₄ seeds were also illustrated based on tandem mass spectra data. This study emphasizes the importance of inorganic seed effects on particle-phase reactions and improves our understanding of the SOA formation pathways in the ambient atmosphere.