Simulation of Secondary Organic Aerosol Formation Using Near-Explicitly Predicted Products from Naphthalene Photooxidation in the Presence of NOx

Abstract

The atmospheric oxidation of naphthalene, found in automobile exhaust and biomass burning smoke, forms a secondary organic aerosol (SOA) with a high yield. In this study, a near-explicit gas mechanism for the photooxidation of naphthalene in the presence of NO_x was derived using a box model platform. The naphthalene oxidation initiated by an OH radical produces various products, including naphthols, nitronaphthols, naphthoquinones, ring-opening products, and organonitrates. The resulting gas mechanism was applied to the UNIfied Partitioning Aerosol-phase Reaction (UNIPAR) model to predict SOA formation via multiphase reactions of naphthalene. Semiexplicitly predicted products were sorted to construct volatility-reactivity-based two-dimensional (2D) lumping species, which were used to process multiphase partitioning of organics and their heterogeneous chemistry to form SOA. The performance of the gas mechanism and the SOA model was demonstrated with data obtained from the photooxidation of naphthalene under varying conditions (NO_x levels, humidity, temperature, and seed types) in a large outdoor photochemical smog chamber. Major products predicted from gas mechanisms were compared with products tentatively identified using proton transfer reaction-mass spectrometry. The simulated organic-to-carbon ratio (0.72) using predicted SOA functional groups was compared with the ratio (0.70 ± 0.7) constructed from the analysis of chamber-generated SOA using Fourier transform infrared spectrometry. Among environmental variables, NO_x and temperature are influential in naphthalene SOA formation. A strong negative relationship appeared between SOA and NO_x levels under hydrocarbon (HC)-limited regions (HC ppbC/NO_x ppb <5) but a weakly positive relationship at NO_x-limited regions. The impact of aqueous reactions on naphthalene SOA growth was insignificant regardless of inorganic seed types (inorganic aerosol liquid water content and seed aerosol acidity) due to poor solubility of naphthalene oxidation products in the inorganic aqueous phase. Under high NO_x levels, SOA growth is dominated by organic-phase heterogeneous reactions of reactive, low-volatile multifunctional aldehydes. Both partitioning and heterogeneous reactions are, however, influential in naphthalene SOA formation under the NO_x-limited regions.