Effect of Aerosol Chemical Composition on the Photodegradation of Nitro-polycyclic Aromatic Hydrocarbons

The photodegradation of four ring nitro-PAHs in the presence of organic aerosol constituents has been investigated in model systems to determine their degradation mechanism under ambient air conditions. Light-induced radical chain reactions initiated by organic aerosol constituents, e.g., oxy-PAHs, is most likely to be the dominant degradation pathway for particle-associated nitro-PAH. The photodegradation rates were investigated in a chemical model system simulating the liquid film on particles from diesel exhaust and wood stove stack gases. Relative rates were obtained in pure solutions (direct photolysis) and in the presence of different classes of organic aerosol components comprising PAHs, hydroxy-PAHs, substituted phenols, benzaldehydes, and oxy-PAHs (including polycyclic aromatic quinones) (indirect photolysis). Members of the four latter compound classes are demonstrated to accelerate the photodegradation rate. The mechanisms proposed to explain these effects include both radical chain reactions and photoinduced hydrogen abstraction by nitro-PAHs. PAHs that are known sensitizers of singlet O2 did not accelerate the decay. Certain oxy-PAHs strongly accelerate the nitro-PAH decay, whereas others have no effect. This difference is related to the nature of the excited state of the oxy-PAH and the ability to initiate radical chain reactions. The relevance of the model system and the environmental implications of the results are discussed, and the results are compared with relative degradation rates from smog chamber studies with the same compounds.