Chemical Composition of Gas- and Aerosol-Phase Products from the Photooxidation of Naphthalene

Abstract

The current work focuses on the detailed evolution of the chemical composition of both the gas- and aerosol-phase constituents produced from the OH-initiated photooxidation of naphthalene under low- and high-NOₓ conditions. Under high-NOₓ conditions ring-opening products are the primary gas-phase products, suggesting that the mechanism involves dissociation of alkoxy radicals (RO) formed through an RO₂ + NO pathway, or a bicyclic peroxy mechanism. In contrast to the high-NOₓ chemistry, ring-retaining compounds appear to dominate the low-NOₓ gas-phase products owing to the RO₂ + HO₂ pathway. We are able to chemically characterize 53−68% of the secondary organic aerosol (SOA) mass. Atomic oxygen-to-carbon (O/C), hydrogen-to-carbon (H/C), and nitrogen-to-carbon (N/C) ratios measured in bulk samples by high-resolution electrospray ionization time-of-flight mass spectrometry (HR-ESI-TOFMS) are the same as the ratios observed with online high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS), suggesting that the chemical compositions and oxidation levels found in the chemically-characterized fraction of the particle phase are representative of the bulk aerosol. Oligomers, organosulfates (R-OSO₃), and other high-molecular-weight (MW) products are not observed in either the low- or high-NOₓ SOA; however, in the presence of neutral ammonium sulfate seed aerosol, an organic sulfonic acid (R-SO₃), characterized as hydroxybenzene sulfonic acid, is observed in naphthalene SOA produced under both high- and low-NOₓ conditions. Acidic compounds and organic peroxides are found to account for a large fraction of the chemically characterized high- and low-NOₓ SOA. We propose that the major gas- and aerosol-phase products observed are generated through the formation and further reaction of 2-formylcinnamaldehyde or a bicyclic peroxy intermediate. The chemical similarity between the laboratory SOA and ambient aerosol collected from Birmingham, Alabama (AL) and Pasadena, California (CA) confirm the importance of PAH oxidation in the formation of aerosol within the urban atmosphere.

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