Atmospheric Chemistry of Polycyclic Aromatic Compounds with Special Emphasis on Nitro Derivatives

Field measurements of nitro-polycyclic aromatic hydrocarbons (nitro-PAHs) and other polycyclic aromatic compounds (PAC) have been carried out at a semi-rural site and at an urban site. A combination of correlation analyses, PAC indicators, and PAC ratios has been used to evaluate the importance of various sources of nitro-PAHs in ambient air. A major source of nitro-PAHs in urban, heavily traffic-influenced air as well as semi-rural air is atmospheric transformation of PAHs initiated by OH radicals. Especially during long-range transport (LRT) of air pollution from Central Europe, the nitro-PAH composition in Denmark is dominated by nitro-PAHs formed in the atmosphere. Locally emitted nitro-PAHs are primarily from diesel vehicles. Levels of unsubstituted PAHs can also be strongly elevated in connection with LRT episodes. Particularly for non-urban areas, this has a profound effect on the air quality. The ratio of 2-nitrofluoranthene relative to 1-nitropyrene is proposed as a measure of the relative photochemical age of particulate matter. Using this ratio, the relative mutagenicity of particle extracts appears to increase with increasing photochemical age. In connection with the field measurements, a sensitive and selective method for measuring nitro-PAHs in particle extracts based on MS-MS mass spectrometric detection has been developed. The atmospheric chemistry of nitronaphthalenes has been investigated with a smog chamber system combined with simulation with photochemical kinetics software. A methodology to implement gas-particle partitioning in a model based on chemical kinetics is described. Equilibrium constants (KP) for partitioning of 1and 2nitronaphthalene between the gas phase and diesel exhaust particles have been determined. The gas-particle partitioning equilibrium of nitronaphthalenes, representing relatively volatile nitro-PAHs, is demonstrated to be maintained even if the compounds decay fast in the gas-phase. Mass transfer between the two phases (gas and particulate) appears to occur on a very short timescale. The rate of gas phase direct photolysis of the nitronaphthalenes depends upon the orientation of the nitro-substituent relative to the aromatic plane. Consequently, significantly faster photolysis of 1-nitronaphthalene than of 2-nitronaphthalene is observed. Finally, an approach to model the nitronaphthalene chemistry in ambient air is proposed. The photochemistry of nitro-PAHs, and to some extent other PAC, associated with organic aerosols, such as combustion aerosols, has been studied with chemical model systems simulating the organic fraction of ambient aerosols. Direct photolysis of particle-associated nitro-PAHs can not explain the degradation on combustion-generated aerosols. A number of aerosol constituents, including substituted phenols, benzaldehydes, and oxy-PAHs, are demonstrated to accelerate the photodegradation. A mechanism involving radical chain reactions initiated by electronically excited carbonylcontaining compounds is most consistent with smog chamber observations. The photodegradation of nitro-PAHs and other PAC associated with organic aerosols are strongly dependent on the physical state of the organic medium. Viscosity, temperature, and water content are of importance for the degradation rates. Nitro-PAHs are photochemically reduced to amino-PAHs under conditions of low O2 concentrations and high polarity as demonstrated by using glycerine as a surrogate for the organic fraction of ambient aerosols. Under several experimental conditions it is observed that the PAH decay is much slower than the nitro-PAH decay. Pulse radiolysis experiments have shown that reactions with OH radicals in atmospheric water droplets represent an important sink for water soluble PAC. Rate constants for reactions of N-PAC with OH radicals and for the subsequent reactions of the OHadducts with O2 have been determined. ISBN 87-550-2700-8 ISBN 87-550-2701-6(Internet) ISSN 0106-2840 Information Service Department, Riso, 2000