Simulation of SOA formation from the photooxidation of monoalkylbenzenes in the presence of aqueous aerosols containing electrolytes under various NO_<i>x</i> levels

The formation of secondary organic aerosols (SOAs) from the photooxidation of three monoalkylbenzenes (toluene, ethylbenzene, and n-propylbenzene) in the presence of inorganic seeds (${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$–${\mathrm{NH}}_{\mathrm{4}}^{+}$–H₂O system) under varying NO_x levels has been simulated using the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model. The evolution of the volatility–reactivity distribution (mass-based stoichiometric coefficient, α_i) of oxygenated products, which were created by the near-explicit gas kinetic mechanism, was integrated with the model using the parameters linked to the concentrations of HO₂ and RO₂ radicals. This dynamic distribution was used to estimate the model parameters related to the thermodynamic constants of the products in multiple phases (e.g., the gas phase, organic phase, and inorganic phase) and the reaction rate constants in the aerosol phase. The SOA mass was predicted through the partitioning and aerosol chemistry processes of the oxygenated products in both the organic phase and aqueous solution containing electrolytes, with the assumption of organic–inorganic phase separation. The prediction of the time series SOA mass (12 h), against the aerosol data obtained from an outdoor photochemical smog chamber, was improved by the dynamic α_i set compared to the prediction using the fixed α_i set. Overall, the effect of an aqueous phase containing electrolytes on SOA yields was more important than that of the NO_x level under our simulated conditions or the utilization of the age-driven α_i set. Regardless of the NO_x conditions, the SOA yields for the three aromatics were significantly higher in the presence of wet electrolytic seeds than those obtained with dry seeds or no seed. When increasing the NO_x level, the fraction of organic matter (OM) produced by aqueous reactions to the total OM increased due to the increased formation of relatively volatile organic nitrates and peroxyacyl-nitrate-like products. The predicted partitioning mass fraction increased as the alkyl chain length increased but the organic mass produced via aerosol-phase reactions decreased due to the increased activity coefficient of the organic compounds containing longer alkyl chains. Overall, the lower mass-based SOA yield was seen in the longer alkyl-substituted benzene in both the presence and absence of inorganic-seeded aerosols. However, the difference of mole-based SOA yields of three monoalkylbenzenes becomes small because the highly reactive organic species (i.e., glyoxal) mainly originates from ring opening products without an alkyl side chain. UNIPAR predicted the conversion of hydrophilic, acidic sulfur species to non-electrolytic dialkyl organosulfate (diOS) in the aerosol. Thus, the model predicted the impact of diOS on both hygroscopicity and acidity, which subsequently influenced aerosol growth via aqueous reactions.