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Polycyclic aromatic hydrocarbons are a complex class of organic compounds consisting of multiple aromatic rings. The simplest member of the PAH family is naphthalene, a two-ring compound which is found in the vapor phase in the atmosphere. Three-, four-, and five-ring compounds are found in both the vapor and particulate phases in the atmosphere. PAHs consisting of five or more rings tend to be solids adsorbed onto other particulate matter in the atmosphere. A widely recognized five-ring PAH compound is benzo(a)pyrene.
In previous studies, high localized concentrations of PAHs have been measured in the vicinity of emission sources such as wood burning stoves and fireplaces, agricultural waste burning, motor vehicle exhaust, cigarette smoking, and asphalt road and roofing operations. Recent analytical developments have improved our ability to speciate and quantify concentrations of PAHs and their derivatives in the atmosphere. PAHs are included on the Air Resources Board's list of suspected toxic air contaminants. |
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Seven sites representing different types of emissions sources were selected: Reseda (residential), Point Arguello (rural), Concord (industrial), Yuba City (agricultural burning), Mammoth Lakes (wood burning), Oildale (oil production) and Glendora (motor vehicle impact). Using appropriate sampling equipment to collect the PAHs and their derivatives in both the vapor and particulate states, the investigators quantified the ambient concentrations of 45 compounds with molecular weights ranging from 128 (naphthalene) to 302 (dibenzo[a,1]pyrene). The particulate samples were collected on filters modified to capture only respirable particles. Twelve hour sampling periods were used to characterize day-and nighttime concentrations, in order to measure the extent of atmospheric transformations resulting from chemical reactions with other substances in the atmosphere. Average values of ambient concentrations are shown in Table 1.
Naphthalene is by far the most predominant airborne PAH. Nighttime concentrations of naphthalene are greater than daytime concentrations because of the hydroxyl radical reaction mechanism which removes naphthalene from the atmosphere in daylight. As ring size increases, the effect of this removal mechanism is diminished. These gas phase reactions are important because they also lead to the formation of nitro-PAHs, which are direct acting mutagens. Direct acting mutagens produce high levels of bacterial mutagenicity in the Ames test procedure, without supplemental activations. Ambient mutagenicity levels, without (-S9) and with (+S9) supplemental activation, are shown in Table 2. |
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The findings present a characterization of the identity and ambient concentrations of two- to five-ring PAH compounds and their nitro-derivatives at various sites in California. The report also presents the mutagenic activity of the ambient particulate matter at each site. The authors conclude:
- The emission profiles of the PAHs differ from site to site, and no single PAH can be used as a marker compound for the range of PAHs observed in ambient air.
- Retene (1-methyl-7-isopropylphenanthrene) is a tracer for coniferous wood combustion.
- No obvious tracer compounds were observed for other specific combustion sources such as industrial emissions and emissions from motor vehicles.
- Significant differences in the ambient PAH concentrations and in the amounts of atmospheric transformations which had occurred between emission sources and sampler locations were observed among the different sites.
- The measured ambient mutagenicity densities correlate with the levels of nitro-PAHs present in the atmospheres of the locations sampled. However, the PAHs and their nitro-derivatives do not account for all of the mutagenicity measured in the atmosphere.
- Although mutagenicity testing of particulate matter form individual emission sources may provide an indication of relative source strengths, such testing is not a reliable measure of ambient mutagenicity at receptor sites because of atmospheric transformations which create additional mutagenicity during transport to the receptor sites.
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Currently, the Air Resources Board is sponsoring two research projects which build upon the data, analytical techniques and methodologies developed in this study. One project will identify and quantify indoor concentrations of PAHs to more completely assess the exposure of Californians to these pollutants. The second project will identify and quantify the more potent derivatives of PAHs responsible for mutagenicity in the atmosphere. |
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