This work describes studies of the photodegradation mechanism of 1-nitropyrene (1-NO2Py) in a chemical model system consisting of an organic solvent and known constituents of an aerosol particle. Photoproducts such as 1-hydroxypyrene (1-OHPy), 1-hydroxy-x-nitropyrenes (1-OH-x-NO2Py), 1-nitrosopyrene, and 1,6- and 1,8-pyrenediones were identified by high-performance liquid chromatography (HPLC) and HPLC/mass spectrometry (HPLC/MS) techniques, and their quantum yields show a significant dependence on the type of solvent. The photodegradation quantum yield of 1-NO2Py, phi((-1-NO2Py)), was larger in toluene, benzene, and polar protic solvents (10(-3)) in comparison with nonpolar and polar aprotic solvents, where the yield is on the order of 10(-4). In solvents with an abstractable hydrogen atom, the products formed in higher yields were 1-OHPy and 1-OH-x-NO2Py. These represent 60-80% of the photodestruction yield and result from abstraction and recombination reactions of the pyrenoxy radical, an intermediate postulated to be formed as a result of a nitro-nitrite rearrangement in nitroaromatics. The small O-2 effect in the photodegradation yield and the quenching experiments with azulene demonstrate the small contribution of the (3)(pi,pi*) state in the 1-NO2Py photoreaction. The nitrosopyrene product was not observed under these conditions, demonstrating the participation of the (3)(pi,pi*) state in its formation. In the presence various phenol aerosol constituents, the photodegradation yield increased by 10-fold in all solvents. This effect is partly ascribed to the reaction of the (3)(pi,pi*) state with the phenol. The effect of water resulted in the reduction of the 1-NO2Py photodegradation yield and of its photoproducts. The phototodegradation of 1-NO2Py was also studied in a viscous solvent, hexadecane, and it was determined that this medium does not inhibit its photodecay.