1.1 Identity, physical and chemical properties, and analytical methods: Nitro-polycyclic aromatic hydrocarbons (nitroPAHs) are derivatives of polycyclic aromatic hydrocarbons (PAHs), which contain two or more fused aromatic rings made of carbon and hydrogen atoms. NitroPAHs occur in the environment as a mixture together with parent PAHs and hundreds of other organic compounds. NitroPAHs are usually present in much smaller quantities than PAHs. NitroPAHs in the environment occur either in the vapour phase or adsorbed to particulate matter. NitroPAHs are insoluble or sparingly soluble in water but mostly soluble in organic solvents. The sampling of nitroPAHs is similar to that of PAHs. Ambient air is sampled by collecting particulate matter on special filters by means of high-volume samplers. Vapour-phase nitroPAHs are commonly collected on solid sorbents such as polyurethane foam. Solvent extraction is followed by cleanup using liquid chromatography with silica gel or alumina, high-performance liquid chromatography (HPLC) or solid-phase extraction. The nitroPAH fraction must be separated from the PAH fraction and oxygenated PAH fraction by HPLC on silica. Methods used for the separation and detection of nitroPAHs include gas chromatography with a variety of detectors, HPLC with fluorescence, chemiluminescence or electrochemical detector, and mass spectrometric techniques. Analysis is dependent on the standards available. Another approach to analysis of complex mixtures is bioassay-directed chemical analysis, where mutagenically active fractions are bioassayed and characterized until the major class or specific compounds potentially responsible for the mutagenicity are identified. The use of bacterial tester strains selectively sensitive to nitroarenes has led to the identification of nitroPAHs as potent mutagens in complex mixtures from diverse sources. Synthetic standards are required for this type of analysis. The nitroketone 3-nitrobenzanthrone and nitrolactones, such as 2-and 4-nitrodibenzopyranone, are nitro-oxy compounds, which have been detected together with nitroPAHs and are analysed by similar methods. 1.2 Sources of human and environmental exposure: NitroPAHs originate primarily as direct or indirect products of incomplete combustion. Only a few nitroPAHs are produced industrially; commercially produced nitronaphthalenes and 5-nitroacenaphthene, for example, are used primarily as chemical intermediates. NitroPAHs originate from PAHs (generally adsorbed on particulate matter and themselves products of incomplete combustion) by at least two distinct processes: (1) through nitration during combustion processes (e.g., in vehicle exhaust, particularly diesel, but also gasoline and aircraft emissions; industrial emissions; domestic residential heating/cooking; wood burning) and (2) through atmospheric formation from PAHs by either gas-phase reactions -daytime hydroxyl radical addition to the PAH followed by reaction with nitrogen dioxide and loss of a water molecule and nighttime nitrate radical addition to the PAH followed by reaction with nitrogen dioxide and loss of nitric acid - or heterogeneous gas-particle interaction of parent PAHs adsorbed onto particles with nitrating agents. The distribution of nitroPAH isomers in samples of ambient air has been found to be significantly different from that in direct emissions from combustion. 2-trofluoranthene and 2-nitropyrene are ubiquitous components of particulate matter, although they are not directly emitted from most combustion sources. The nitroPAH profile, or the relative quantities of certain "marker" PAHs, is a pointer to the source of formation of the nitroPAH. The most abundant nitro isomers of pyrene, fluorene and fluoranthene observed in diesel exhaust are 1-nitropyrene, 2-nitrofluorene and 3-nitrofluoranthene, whereas the isomers formed from the hydroxyl radical reactions of these PAHs are 2-nitropyrene, 3-nitrofluorene and 2-nitrofluoranthene. The majority of ambient nitroPAHs are now thought to be formed in the atmosphere from the gas-phase reactions of PAHs with four rings or less. Many mono- and some di- and trinitroPAH isomers have been identified and quantified in various samples of diesel exhaust, 1-nitropyrene usually being the most abundant. 1-Nitropyrene is the "marker" nitroPAH for diesel exhaust, and its presence in ambient air samples is a sign of pollution by diesel vehicle traffic. Diesel fuel, engine types and catalytic traps are continually being modified, so the various studies of nitroPAHs in diesel exhaust cannot be directly compared. In general, the mass emission of particles, emissions of particle-bound PAHs and nitroPAHs, and mutagenic activity levels generally decreased with the use of either particulate traps or catalytic converters. The concentration of 1-nitropyrene was much less in gasoline exhaust particles than in diesel exhaust particles, but the concentrations of 1,3-, 1,6- and 1,8-dinitropyrenes were found to be almost the same in gasoline and diesel exhaust particles. There is evidence of the presence of nitroPAHs in jet aeroplane exhaust. NitroPAHs have been detected in the emissions of kerosene heaters, fuel gas and liquefied petroleum gas (LPG) burners, which are used in many countries for heating and cooking at home. 3-Nitrobenzanthrone has been detected in diesel exhaust particulate and in urban air samples. 2-Nitrodibenzopyranone and 4-nitrodibenzopyranone as well as nitropyrene lactones have been observed in ambient particulate matter. 1.3 Environmental transport, distribution and transformation: 1.3.1 Environmental transport and distribution - NitroPAHs can be transported in the vapour phase or adsorbed onto particulate matter. Those with liquid-phase vapour pressures greater than 10-4 Pa at ambient air temperature (i.e., two- to four-ring PAHs and two-ring nitroPAHs) will exist at least partially in the gas phase. Owing to their low aqueous solubility or insolubility, nitroPAHs are not expected to be transported in water. Data available give high values for sorption coefficients (log Koc), suggesting that nitroPAHs, similar to PAHs, adsorb onto soil and sediments. Leaching into groundwater is thought to be negligible. Some nitroPAHs may be slowly biodegradable under certain conditions. The values for the n-octanol/water partition coefficient (log Kow) range from 2.5 for 1-nitronaphthalene to 6.3 for 3-nitroperylene, suggesting a potential for bioaccumulation. There were no data available on biomagnification. 1.3.2 Biotransformation - Many anaerobic and aerobic bacteria reduce nitroPAHs to mutagenic aminoPAHs. Nitroreduction by intestinal microflora plays a major role in the metabolism of nitroPAHs in mammals. Although a wide variety of bacteria, fungi and algae have been shown to degrade the parent PAHs containing two to five rings, nitro-substituted PAHs are only slowly degraded by indigenous microorganisms and may persist in soils and sediments. The recalcitrance of high molecular weight nitroPAHs is due in part to the strong adsorption to soil organic matter, low solubility, large molecular size and the polar character of the nitro group. Time course studies in microcosms showed that 1-nitropyrene was degraded slowly under aerobic and anaerobic conditions in estuarine sediments. Sphingomonas paucimobilis strain EPA 505 (a soil bacterium capable of utilizing fluoranthene as the sole source of carbon and energy) biodegraded 1-nitropyrene to 48.6% after 6 h. The filamentous fungus Cunninghamella elegans has been shown to oxidatively etabolize, via a cytochrome P450 monooxygenase, a number of nitroPAHs (1-nitropyrene, 2-nitrofluorene, 2- and 3-nitrofluoranthene, 6-nitrochrysene, 1-nitrobenzo[e]pyrene and 6-nitrobenzo[a]pyrene) to products that are less mutagenic than the nitroPAHs themselves. A plant cell culture derived from alligator weed (Alternanthera philoxeroides) detoxified 1-nitropyrene and 1,3-, 1,6- and 1,8-dinitropyrene, all direct-acting mutagens, when incubated with them, as shown by mutagenicity response in the Salmonella typhimurium TA98 assay. 1.3.3 Abiotic degradation - The photolysis of nitroPAHs has been studied under varied conditions of irradiation. The rate of photolysis depends not only on the conditions of radiation but also on whether the nitroPAH is in the gaseous phase (e.g., 1-and 2-nitronaphthalene), in solution (type of solvent) or bound to solids/particles. In the latter case, the type and age of the particle seem to influence the photochemistry of the respective nitroPAH. The rate of photodecomposition, identification of photolytic products and resulting loss or gain of metabolic activity as determined by the S. typhimurium assay have been the main end-points studied. Calculated atmospheric lifetimes of nitroPAHs due to photolysis and gas-phase reactions with hydroxyl and nitrate radicals and with ozone under atmospheric conditions show that the dominant loss process for nitroPAHs (e.g., 1- and 2-nitronaphthalene) is photolysis. Particle oxidation of nitroPAHs by ozone may be the main loss process at night. 1.4 Environmental levels and human exposure: NitroPAHs that have been detected in ambient air include 1- and 2-nitronaphthalene and methylnitronaphthalenes (predominantly in the vapour phase), 2-nitrofluorene, 9-nitroanthracene, 9-nitrophenanthrene, 2-, 3- and 8-nitrofluoranthene, 1- and 2-nitropyrene, 1,3-, 1,6- and 1,8-dinitropyrene and 6-nitrochrysene. At remote and forest sites, nitroPAHs were either not detected or detected in the low picogram per cubic metre range (e.g., 17 pg/m3 for 2-nitrofluoranthene; 4 pg/m 3 for 1-nitropyrene). The concentration of nitroPAHs in the atmosphere of urban regions depends on the season, the type of heating used and the number and regulation of traffic vehicles.