Light-induced transformations of aza-aromatic pollutants adsorbed on models of atmospheric particulate matter: Acridine and 9(10-H) acridone

The effect of the characteristics of the surface on the phototransformation of acridine, one of the most abundant azapolycyclic compounds encountered in urban atmospheres, and of one of its principal photoproducts, acridone, was studied when adsorbed onto models of the atmospheric particulate matter. For this purpose, relative photodegradation rates were determined from absorption or emission intensities as a function of irradiation times, and some products were isolated and characterized. The relative photodegradation rates of adsorbed acridine show the tendency (NH4)2SO4>MgO>Al2O3>SiO2. In general, the rates decrease as the fraction of protonated acridine species on the surface increases in MgO, Al2O3, and SiO2, except for (NH4)2SO4 where a fast surface reaction occurs. Oxygen reduces the photodestruction rates by as much as 40–60% when compared to an inert atmosphere, implying the participation of an acridine triplet state in the transformation processes on all surfaces except on (NH4)2SO4. Acridone, a major product, undergoes a photo-induced tautomerization to 9-hydroxy acridine. The formation of a dihydrodiol, another photoproduct of acridine, is suggested by comparison to reported spectral properties of these compounds. This is formed through a singlet oxygen reaction. Photoproducts showing the absence of the narrow absorption band of 250 nm, characteristic of the π→π* transition in tricyclic aromatics, were detected in small yields but not identified. These results suggest possible photochemical transformation pathways that could lead to the ultimate fate of these pollutants in the environment.