Effects of adsorbed water and hydrocarbon reactivity on the nitration of polycyclic aromatic hydrocarbons adsorbed on silica gel and Fe₂O₃ particles with NO₂

Here, the nitration of polycyclic aromatic hydrocarbons (PAHs) adsorbed on SiO₂ and Fe₂O₃ particles, which are abundant in PM_2.5, with NO₂ (9.77 ppm) was studied using a fluidized-bed column to simulate the transformation of atmospheric PAHs at night. Silica gel was used as SiO₂, and anthracene, phenanthrene, pyrene, chrysene, fluoranthene, and perylene were used as PAHs. The effects of water (H₂O_ads) adsorbed on the substrates and the reactivity of PAHs on the transformation kinetics and mechanism were investigated. On the hydrated silica gel (H₂O_ads: 4.2 wt%), the most reactive PAH (perylene) was nitrated following a pseudo-first-order reaction, and moderately (anthracene and pyrene) and less reactive (chrysene) PAHs were nitrated by a H⁺-autocatalyzed reaction. However, on dry Fe₂O₃ (H₂O_ads: 0.02 wt%), the nitration of the moderately reactive PAHs proceeded following a pseudo-first-order reaction, and chrysene was nitrated by a H⁺-autocatalyzed reaction. On both substrates, the nitration of PAHs changed from pseudo-first-order to H⁺-autocatalyzed reactions as the reactivity of the PAHs decreased, indicating that the nitration kinetics and mechanism are affected by the concentration of H⁺ formed in H₂O_ads upon exposure to NO₂. On dry Fe₂O₃, perylene and the moderately reactive PAHs were nitrated by NO₂, while the moderately reactive PAHs and chrysene were nitrated by NO₂⁺ on the hydrated silica gel, which would be formed via a pre-equilibrium reaction between NO₂ and H⁺ formed by NO₂ exposure and released by nitration. The formation of NO₂⁺ is supported by the acceleration of the H⁺-autocatalyzed nitration on the H₂SO₄-adsorbed hydrated silica gel and by the detection of NO₂⁺ on hydrated borosilicate glass.