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

Abstract Here, the nitration of polycyclic aromatic hydrocarbons (PAHs) adsorbed on SiO2 and Fe2O3 particles, which are abundant in PM2.5, 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 (H2Oads) adsorbed on the substrates and the reactivity of PAHs on the transformation kinetics and mechanism were investigated. On the hydrated silica gel (H2Oads: 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 (H2Oads: 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 H2Oads 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.