Modeling of nitro-polycyclic aromatic hydrocarbon formation and decay in the atmosphere

Abstract

A reaction mechanism of polycyclic aromatic hydrocarbons (PAH) and nitro-PAH (NPAH) in the gas and particle phase in the atmosphere has been further evaluated and modified using outdoor smog chamber experimental results. Diesel exhaust emissions were added to a 190 m 3 outdoor smog chamber and permitted to age under conditions of daylight and darkness. A sampling train consisting of an annular denuder, filter, and polyurethane foam (PUF) or XAD resin was used for the collection of gas and particle phase PAH and NPAH. On the basis of the results, the current denuder design has sufficient flow (20 ℓ min −1) and adsorption characteristics for collection of PAH and NPAH in the chamber studies. Outdoor smog chamber experiments with dilute diesel soot were conducted under different initial photochemical conditions. Ozone (0 3), nitrogen oxides (NO x ), and volatile hydrocarbons in the gas phase were monitored. Simulations for fluoranthene (FL) and pyrene (PY) in the gas phase were close to chamber observations, but those for the particle behavior of FL and PY were not as good. This may occur because PAH and NPAH inside of the particle are not available for reaction in sunlight. Mono-nitro-pyrenes (NPYs) and nitro-fluoranthenes (NFLs) were almost exclusively found in particle associated extracts. This implied that no or non-detectable 2nitro-FL (2NF) or 2nitro-PY (2NP) distributed in the gas phase and that they deposited on particles immediately after formation in the gas phase by the photochemical processes. Formation of 2NF was observed in the chamber, but 2NP degraded rapidly under photochemical conditions. Reasonable simulation results were obtained for 2NP and 2NF. The addition of NO 2 to the gas phase adduct of FL + OH or PY + OH was the main reaction for NPAH formation. Photodecomposition was the main loss pathway for NPAH in the atmosphere.