Secondary Organic Aerosol Formation from the Photooxidation of Naphthalene and Benzene Mixtures under Different Reaction Conditions

The atmospheric oxidation of aromatic hydrocarbons is a major source of secondary organic aerosol (SOA). This study investigates the OH-initiated oxidation of naphthalene (Nap) and benzene (Bz) mixtures under a variety of reaction conditions. Nap and Bz are common air pollutants produced from fuel combustion and are also simultaneously present at high concentrations in asphalt pavement emissions.

Experiments were conducted in the Irish Atmospheric Simulation Chamber, a 27 m³ Teflon reactor, where Nap and Bz were exposed to OH radicals generated via H₂O₂ photolysis. The photooxidation of Nap and Bz was investigated individually and in combination under varying NOx, SO2 and relative humidity (RH) conditions. Gas phase products were monitored using a Time-of-Flight Chemical Ionization Mass Spectrometer (ToF-CIMS) coupled to a Filter Inlet for Gases and AEROsol (FIGAERO) for particle composition analysis using both toluene and iodide as reagent ions. Aerosol number and mass evolution were measured using a Scanning Mobility Particle Sizer (SMPS), while NOx, SO2 and O3 concentrations were monitored with automated gas analysers and a custom-designed cavity enhanced absorption setup.

Analysis of experiments under high and low NOx conditions have so far shown that Nap + OH and Bz + OH reactions yielded C10H8O+ (m/z 144) and C6H6O⁺ (m/z 94) as the main primary products, respectively, corresponding to the addition of a hydroxyl group. Intense signals at m/z 160 and m/z 110 were subsequently observed, i.e. the addition of a second OH group. Experiments on Nap/Bz mixtures resulted in the same products, even in the presence of NO, which also produced nitroaromatic compounds like C10H7NO3+ (m/z 189) and C6H5NO3+ (m/z 139). A range of C12, C16 and C20 compounds were also identified and assigned to dimers produced from self- and cross-reactions of C6 and C10 radicals produced during the photooxidation process. The photooxidation of Bz alone (up to 120 ppbv) did not produce SOA. In contrast, SOA formation from Nap (30 ppbv) was rapid and affected by the reaction conditions. RH strongly influenced SOA formation, with lower RH delaying particle growth and reducing total mass. Introducing NO (55–144 ppbv) to the Nap + OH system enhanced SOA formation, while adding Bz suppressed SOA formation.

First results of SO2 experiments show that the addition of SO2 (17 ppbv) to the Nap + Bz + OH mixture significantly accelerated SOA formation, nearly doubling the SOA mass, particularly in the presence of NO. Future work will focus on completing the experimental matrix to deepen our understanding of chemical mechanisms leading to the formation of the detected products.