Nitro Derivatives of Polycyclic Aromatic Hydrocarbons (NO₂-PAH)

Like the PAH, Nitro-PAH are universally distributed and predominantly emitted through anthropogenic activities such as combustion processes: Exhaust gases from motor vehicles are regarded as being the main source. Although present in smaller quantities than the corresponding PAH, they deserve special attention because of their biological activity: a number of them have proved to be mutagenic and carcinogenic in animal testing. Nitro-PAH are not only emitted directly but can also be formed indirectly through reactions of PAH with gaseous air pollutants. Nitration of PAH is a very complex reaction in which the formation rate of the nitro derivatives is influenced by the medium and the presence of NO₂, H⁺, hydroxyl radical, HNO₃, HNO₂, and metal ions with a high oxidation potential. Additional parameters are the surface characteristics and grain size of the particles on which the PAH are adsorbed and where they take part in further reactions. The nitro-PAH concentrations in particles from diesel and gasoline exhaust gases lie in the same order of magnitude whereas the emission rate from diesel engines is higher, however, because they expel more particles per unit distance traveled than gasoline engines. Vehicles equipped with a catalytic converter produce substantially lower amounts of nitro-PAH. Dinitropyrene, a mutagenic, particle-bound component in emissions from incomplete combustion processes, causes between 20 and 40% of the mutagenicity. Nitro-PAH are direct mutagens in the Ames-test. Dinitro-PAH, in general, appear to be stronger mutagens than mononitro-PAH. The most important transformation of nitro-PAH is photodecomposition to the respective arylnitrite followed by the formation of the phenoxyl radical with the elimination of dinitrogen monoxide. It further reacts to nitrohydroxy derivatives and quinones. The arylnitrite can also hydrolyze to hydroxy derivatives and nitric acid. Sampling techniques for automobile exhaust have been developed and analytical methods for clean-up, separation, and detection of nitro-PAH are given.