Impact of Regulations of Traffic Emissions on PAH Level in the Air 1 Introduction1.1 Air pollution with PAH and other mutagens from traffic and
other sources
1.1 Air pollution with PAH and other mutagens from traffic and other sourcesDefinition of PAH and POM Polycyclic aromatic hydrocarbons (PAH) consists of carbon and hydrogen and can be conceived as consisting of fused rings of benzene. PAH belongs to the group of polycyclic aromatic compounds (PAC). PAC cover azaarenes, oxaarenes, thiaarenes and transformation products of these and PAH, e.g. nitro derivatives and quinones besides PAH. Azaarenes, thiaarenes and oxaarenes can be conceived as a PAH, where a carbon atom in the ring system is replaced by a nitrogen, sulphur or an oxygen atom, respectively. The S- and O-PAC can be identified and determined in the PAH fraction of a sample as the chemical and physico-chemical properties of S- and O-PAC are very similar to those of PAH (Nielsen, 1983). The concern of the presence of PAH and other PAC in the environment is caused by the fact that several of them are carcinogens and present in polluted air. The compounds are formed by combustion, e.g. of petrol, oil and wood (Finlayson-Pitts and Pitts, 1986). Particle/gas distribution The larger PAH (5 - 7 rings), covering most of the carcinogens are associated with particles in the atmosphere. Part of the 3 and 4 rings PAH is also present in ambient air in vapour phase (Nielsen and Pilegaard, 1990). O- and S-PAC show similar distribution as their analogue PAH. The nitrogen atom in N-PAC causes a minor reduction in the vapour pressure, thus the gas phase to particulates ratio of three rings N-PAC (Mw = 179) corresponds to the ratio of fluoranthene and pyrene (Mw = 202) (Adams et al., 1982, Chen and Preston, 1997). Substituted PAH, e.g. nitro derivatives, have lower volatility than the parent PAH (Feilberg et al., 1998). Therefore, a larger proportion of the 3 and 4 ring members of these is associated with particles. The major part of the mutagens in ambient air has been shown to be particle-associated (Fenger et al., 1990). Indicator for carcinogenicity Previous investigations (Nielsen, 1989, Nielsen et al., 1995b and c and 1996, Ostenfeldt, 1989) have shown that PAH as well as mutagenic activity are applicable as indicators to investigate the content of carcinogens in ambient air. Several sources may contribute to the air pollution, e.g. traffic, oil heating, wood stoves, municipal incinerators and industry. In addition, long-range transport of pollution from other countries may occur. In order to determine the importance of different sources, the relative PAH composition of ambient air samples has been compared with that of exhaust and stack gases. Thus, Sawicki (1962) and Brasser (1983) applied the ratio of pyrene to benzo(a)pyrene (BaP), that of benzo(ghi)perylene to BaP and that of coronene to BaP to discriminate between the contributions from traffic sources and from domestic coal heating. The application of compositional differences to source identification has also been suggested by others (Daisey and Lioy, 1981, Daisey et al., 1986, Gordon and Bryan, 1973). In recent years, PAH have also replaced lead as a tracer for traffic pollution after the phase-out of lead in petrol (Daisey et al., 1986). In Denmark it is agreed that traffic, domestic heating and long-range transport are important sources of PAH and other mutagens (Fenger et al., 1990, Gudmundsson, 1988, Kemp, 1989, Nielsen, 1989, Nielsen and Pilegaard, 1990, Nielsen et al., 1993, Ostenfeldt, 1989). The traffic sources are, however, the dominant PAH sources in street air (Nielsen et al. 1995b and c, Nielsen 1996). In a smaller number of incidents PAH contribution from wood-stoves has been recognised in suburban areas (Nielsen, 1989, Nielsen and Pilegaard, 1990, Nielsen et al., 1993, Ostenfeldt, 1989). Objective The objective of this investigation was to determine whether the application of diesel fuel having a low distillation end point had affected the air levels of PAH and mutagens. These new diesel qualities are expected to reduce the emissions of particulates and soot (Karonis et al., 1998) and therefore, probably also the emissions of PAH and other mutagens (Westerholm and Egebäck, 1994). Most of the PAH in the diesel exhaust is carried over from the fuel and not formed by pyrosynthesis during the combustion process (Williams et al., 1989). After the introduction of the new diesel fuel quality a significant reduction in the levels of PAH and especially the mutagens was observed (Nielsen, 1996, Nielsen et al. 1995b and c). However, the results were not unambiguous, as the meteorological conditions were different in the two set of measurements, and the emissions of PAH and other mutagens also might have been affected by the ambient temperature (Nielsen, 1996, Nielsen et al. 1995b and c). 1.2 Reduction of emissionsIntroduction of catalysts Catalysts were introduced on vehicles in the seventies in USA. These catalysts were oxidation catalysts for vehicles equipped with petrol engines. The oxidation catalyst reduces the amount of CO and hydrocarbons in the exhaust gas. In the eighties 3-way catalysts were introduced, also for petrol engines. In 3-way catalysts there are neither oxidising nor reducing conditions. This is because CO and hydrocarbon removal needs oxidising conditions while NOx removal requires reducing conditions. Therefore, the 3-way catalyst has to operate in the border area. National legislation In October 1990 legislation was enforced in Denmark which required the closed loop 3-way catalysts on all new petrol driven passenger cars. In the EU similar legislation has been in force since January 1993. It is estimated that this new technology will reduce the emission of CO, HC (hydrocarbons) and NOx from the individual car by 70-80%. The use of catalysts will probably also result in a significant reduction in the emission of PAH (Rogge et al., 1993a). The proportion of petrol passenger cars equipped with catalysts was about 10% during the 1992 measurements. This proportion increased to about 15% during the 1993 measurements and to 40% in 1996. Petrol makes up about 55% of the fuel used for road transport in Denmark. Diesel oil is the second next dominating fuel. Another legislative change that might influence the PAH-emission from motor vehicles is the introduction of new diesel qualities as discussed in the end of the introduction. In July 1992 the sulphur content in autodiesel in Denmark was reduced from 0.2% to 0.05%. At the same time a special diesel quality with low distillation end point was introduced in buses. The new diesel qualities will reduce the emission of particulates and thereby probably the emission of PAH. The reduction in sulphur content to 0.05% has been mandatory in the EU from October 1996. Formation of PAH The introduction of catalytic converters raised the question to what extent unregulated compounds like PAH would be formed in the converter. The majority of the investigations show that the converters are very efficient and in almost every case produce a reduction of about 90%. However, most measurements are valid for new catalysts only. There are almost no data for used catalysts. Furthermore, it has been shown that engine technology like exhaust gas recirculation, which was introduced to reduce NOx emissions, will have a dramatic increasing effect on the PAH conversion efficiency of the catalyst. Therefore it seems not plausible to expect 90% conversion efficiency of a catalyst under real conditions. Furthermore, the application of catalysts appears to change the PAC composition in favour of oxygenated PAC (Rogge et al., 1993a). This could either be due to a lower efficiency of the catalyst towards oxy-PAH or because PAH are converted to oxy-PAH by the catalyst. 1.3 Recommended limit valuesVarious national and international authorities have established standards or limit values for air pollution components. With respect to the occurrence of carcinogenic PAH and other mutagens in air the regulators are faced with an extremely difficult situation as these compounds are present in complex mixtures with widely varying compositions and carcinogenic potencies depending on different sources and locations. Most often benzo(a)pyrene is used as a marker substance for the total carcinogenic potency present in ambient air. The Netherlands In the Netherlands a draft (annual average) tolerable level of Germany In Germany The Umwelt Bundes Amt has stated that "Since dose-effect relationships
for man do not exist, the recommended value is based on technical and economic
feasibility". In view of the concentrations occurring in Western European cities an
annual average of US-EPA The US-EPA in 1984 has proposed to regulate PAH in the outdoor air by means of emission limits instead of determining a recommended value for PAH in the outdoor air. WHO The WHO (1987) states that because of the carcinogenic properties of PAH a safe level
cannot be recommended. Various risk assessments are given using benzo(a)pyrene as an
indicator. Based on benzene soluble fractions of cokeoven emissions, a risk of lung cancer
is given of 9x10-5 per ng benzo(a)pyrene per Denmark The Danish Environmental Protection Agency has not established standards for PAH in ambient air. As PAH are carcinogenic compounds the levels should be as low as possible, and the Danish EPA regulates PAH in the outdoor air by means of emission limits for the various sources. 1.4 Toxicological evaluation of PAH and other mutagens in airSingle compounds The variety of complex mixtures of PAH and other mutagens in ambient air at different locations are composed of many hundreds of different mutagenic and presumably carcinogenic compounds with varying potencies. Besides the limited number of PAH measured in this study, many other PAH and a variety of other mutagenic compounds contribute to the overall mutagenicity of ambient air samples. These may include alkylated PAH, oxygenated PAH such as PAH-ketones, carboxaldehydes and quinones, heterocyclic PAH such as azaarenes and thiophenes, and nitro-PAH (and their oxygenated derivatives) which has attracted interest due to their exceptionally high specific mutagenicity in the Ames test. However, when it comes to hard facts on the toxicological properties of these compounds, only a few of the PAH, some heterocyclic PAH and some nitro PAH have been studied to any greater extent. One compound, namely benzo(a)pyrene, which is one of the most carcinogenic PAH known, has been used in numerous biochemical and toxicological studies of mainly mechanistic nature, as a prototype of a chemical carcinogen. In spite of the many studies available on benzo(a)pyrene it is noteworthy that no adequate long-term carcinogenicity studies conducted according to modern standard and quality have been published yet. Potency and risk A summary of the toxicological properties of PAH with emphasis on mutagenicity and carcinogenicity has been given previously (Nielsen et al. 1995c and 1996). Potency estimates relative to the carcinogenic potency of benzo(a)pyrene were presented for those compounds for which sufficient data were available. In addition various risk assessments of benzo(a)pyrene were discussed. Complex mixtures Complex mixtures of PAH and other mutagens are inherent constituents of many heavier petroleum fractions and coal liquefaction products. For ambient air the significant sources of complex mixtures of PAH and other mutagens are the solid and gaseous combustion products of practically any organic product including fuels (gasoline, diesel, kerosene, heavy fuel oils, wood, alcohols), plastics, tobacco, organic wastes etc. The composition of these mixtures vary considerably depending on their source, the temperatures at which they are formed and several other variables. A large number of reported experimental work has been concerned with the biological effects of POM, especially the relation between the known constituents and the mutagenicity and carcinogenicity of POM from various sources. As discussed previously (Nielsen et al., 1995c and 1996) the 4-7 ring PAH fraction of condensate from car exhaust (petrol, diesel), domestic coal stove emissions and tobacco smoke contains nearly all the carcinogenic potential of PAH. This has been found after skin painting, subcutaneous injection and intrapulmonary implantation of fractions (WHO, 1987). The role of diesel particulates in the possible lung carcinogenicity of diesel exhaust was also discussed (Nielsen et al. 1995c and 1996). Potency and risk A summary of potency and risk assessment methods was given for complex mixtures of PAH
(Nielsen et al. 1995c and 1996). Four different approaches were discussed. The
specific marker substance approach, the maximum potency approach and the
relative potency approach all use benzo(a)pyrene as a hall-mark in the expression of
the carcinogenic potency of a given complex mixture of PAH. The comparative potency
approach use data from human studies which have been correlated to data from animal
bioassays and indirectly to results from in vitro mutagenicity tests, such as the
Ames test.
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