Simulation of aromatic SOA formation using the lumping model integrated with explicit gas-phase kinetic mechanisms and aerosol-phase reactions

Abstract

The Unified Partitioning-Aerosol phase Reaction (UNIPAR) model has been developed to predict the secondary organic aerosol (SOA) formation through multiphase reactions. The model was evaluated with aromatic SOA data produced from the photooxidation of toluene and 1,3,5-trimethylbenzene (135-TMB) under various concentrations of NOx and SO2 using an outdoor reactor (University of Florida Atmospheric PHotochemical Outdoor Reactor (UF-APHOR) chamber). When inorganic species (sulfate, ammonium and water) are present in aerosol, the prediction of both toluene SOA and 135-TMB SOA, in which the oxygen-to-carbon (O : C) ratio is lower than 0.62, are approached under the assumption of a complete organic/electrolyte-phase separation below a certain relative humidity. An explicit gas-kinetic model was employed to express gas-phase oxidation of aromatic hydrocarbons. Gas-phase products are grouped based on their volatility (6 levels) and reactivity (5 levels) and exploited to construct the stoichiometric coefficient (αi,j) matrix, the set of parameters used to describe the concentrations of organic compounds in multiphase. Weighting of the αi,j matrix as a function of NOx improved the evaluation of NOx effects on aromatic SOA. The total amount of organic matter (OMT) is predicted by two modules in the UNIPAR model: OMP by a partitioning process and OMAR by aerosol-phase reactions. The OMAR module predicts multiphase reactions of organic compounds, such as oligomerization, acid-catalyzed reactions, and organosulfate (OS) formation. The model reasonably simulates SOA formation under various aerosol acidities, NOx concentrations, humidities and temperatures. Furthermore, the OS fractions in the SOA predicted by the model were in good agreement with the experimentally measured OS fractions.