Abstract. The formation of secondary organic aerosols (SOAs) from
the photooxidation of three monoalkylbenzenes (toluene, ethylbenzene, and
n-propylbenzene) in the presence of inorganic seeds
(SO42-–NH4+–H2O system) under varying NOx
levels has been simulated using the Unified Partitioning Aerosol Phase
Reaction (UNIPAR) model. The evolution of the volatility–reactivity
distribution (mass-based stoichiometric coefficient, αi) 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 αi set
compared to the prediction using the fixed αi set. Overall, the
effect of an aqueous phase containing electrolytes on SOA yields was more
important than that of the NOx level under our simulated conditions or
the utilization of the age-driven αi set. Regardless of the
NOx 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 NOx 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.