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, 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. 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 (NH)SO 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-HO-hv systems, (NH)SO 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 (NH)SO seeds increased the particle liquid water content, thus enhancing the partitioning of hydrophilic HO to particle phase and the further oxidation of SOA, finally decreasing SOA yields. In the styrene-NO-hv systems, NH and SO were involved in the particle-phase reactions, producing prominent nitrogen- and sulfur-containing compounds of CHON and CHONS. In the styrene-O dark reaction systems, the existence of (NH)SO seeds significantly changed the oligomer components under humid conditions. The particle-phase formation pathways of oligomers affected by (NH)SO 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.