Atomspheric oxidation mechanism of fluorene initiated by the OH radicals

Polycyclic aromatic hydrocarbons (PAHs) are a large class of pollutants widely present in the environment and are the earliest discovered and studied carcinogens.The oxidation of PAHs can generate oxygenated PAHs (OPAHs) and nitrated PAHs (NPAHs), which are key precursors for the formation of SOA. However, the formation mechanism of OPAHs and NPAHs are unclear. Fluorene (FLN) is one of the most abundant PAHs discovered, and several studies have been conducted on the reaction of FLN. In this article, we investigated the oxidation mechanism of FLN initiated by OH radicals through high level quantum chemistry and chemical kinetics calculations. The oxidation starts by forming adducts FLN-n-OH (FLNn in short, n = 1, 2, 3, 4, 10, 11) which then react with O₂ and NO₂ in the atmosphere. When the overall reaction rate coefficient of FLNn with O₂ are slow, FLNn mainly react with NO₂ to form n-nitrofluorene in the atmosphere. Effective reaction rates of FLN1 with O₂ in the atmosphere are fast at of ∼1.90 × 10⁴ s⁻¹, suggesting the negligible reaction between FLN1 adduct and NO₂. Therefore, the main fate of FLN1 is reaction with O₂ followed by subsequent unimolecular isomerization routes, forming OPAHs only. The rate coefficient for reaction of FLN2, FLN4 and FLN10 with O₂ are obtained at of 7.44, 26.4 and 1.07 s⁻¹, respectively, suggesting the likely formation of 2-nitrofluorene 4-nitrofluorene and 10-nitrofluorene. In addition, the hydrogen atom abstraction from FLN and the generation of alkoxy groups from FLNn-i-OO resulted in formation of unsaturated compounds, which subsequently reacted with O₂ to form a large number of unsaturated oxygen-containing compounds and further transform to highly oxidized products.