MODELING OF NITRO-POLYCYCLIC AROMATIC HYDROCARBON FORMATION AND DECAYIN THE ATMOSPHERE

A reaction mechanism of polycyclic aromatic hydrocarbons (PAH) and nit ro-PAH (NPAH) in the gas and particle phase in the atmosphere has been further evaluated and modified using outdoor smog chamber experimenta l results. Diesel exhaust emissions were added to a 190 m3 outdoor smo g chamber and permitted to age under conditions of daylight and darkne ss. A sampling train consisting of an annular denuder, filter, and pol yurethane foam (PUF) or XAD resin was used for the collection of gas a nd particle phase PAH and NPAH. On the basis of the results, the curre nt denuder design has sufficient flow (20 l min-1) and adsorption char acteristics for collection of PAH and NPAH in the chamber studies. Out door smog chamber experiments with dilute diesel soot were conducted u nder different initial photochemical conditions. Ozone (O3), nitrogen oxides (NO(x)), and volatile hydrocarbons in the gas phase were monito red. Simulations for fluoranthene (FL) and pyrene (PY) in the gas phas e were close to chamber observations, but those for the particle behav ior 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. Mo no-nitro-pyrenes (NPYs) and nitro-fluoranthenes (NFLs) were almost exc lusively found in particle associated extracts. This implied that no o r non-detectable 2nitro-FL (2NF) or 2nitro-PY (2NP) distributed in the gas phase and that they deposited on particles immediately after form ation in the gas phase by the photochemical processes. Formation of 2N F was observed in the chamber, but 2NP degraded rapidly under photoche mical conditions. Reasonable simulation results were obtained for 2NP and 2NF. The addition of NO2 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.