Multiphase Chemistry and Phase State Explain Nonlinear Effects in the Formation and Evaporation of SOA from Mixed...

A large fraction of airborne particulate matter consists of secondary organic aerosol (SOA), which can be highly viscous, leading to kinetic limitations in gas-particle partitioning and chemical reactivity. The underlying processes, however, have not been fully resolved and quantified in computational models. We use experimental data and the kinetic multilayer model of multiphase chemistry (KM3C) to investigate SOA formation by oxidation of limonene and α-pinene with the nitrate radical (NO₃). The model explicitly treats gas-wall loss, gas-particle partitioning, as well as gas- and particle-phase chemistry, and includes a novel method for parametrizing bulk diffusivity from particle composition. KM3C utilizes and reproduces the temporal evolution of particle mass and thermal desorption mass spectrometry data (FIGAERO-CIMS) obtained in chamber experiments. The model can explain the observed slow evaporation of limonene SOA through slow bulk diffusivity and predicts the formation of a viscous surface crust. In mixed-precursor experiments, KM3C attributes nonadditive SOA mass yields to cross-reactions between α-pinene- and limonene-derived intermediates, leading to accretion products. We conclude that particle phase state, oligomerization, and multiprecursor effects have to be resolved to accurately describe and predict atmospheric SOA formation.