ReviewSource specificity and atmospheric processing of airborne PAHs: Implications for source apportionment
Introduction
Source apportionment is an exercise crucial to the determination of control strategies for environmental pollutants. Beginning with the work of Daisey et al. (1979), numerous studies have used diagnostic ratios of PAH concentrations (typically isomers) to infer sources of airborne particulate PAH content (Neilsen, 1996, Dickhut et al., 2000, Kavouras et al., 2001, Yassaa et al., 2001, Park et al., 2002, Vasconcellos et al., 2003, Guo et al., 2003, Sienra et al., 2005, Bourotte et al., 2005, Dallarosa et al., 2005, Fang et al., 2006, Ding et al., 2007, Esen et al., 2008). The concept behind the diagnostic ratio has often been extended to profiles or signatures for which the relative amounts of multiple species are examined using multivariate methods (Harrison et al., 1996, Schauer et al., 1996, Dickhut et al., 2000, Guo et al., 2003, Bourotte et al., 2005, Dallarosa et al., 2005, Robinson et al., 2006, Wan et al., 2006, Lee and Kim, 2007). A limited number of studies have examined total (gas + particle) concentrations (Simcik et al., 1999, Bi et al., 2003, Larsen and Baker, 2003, Ravindra et al., 2006, Battelle, 2007, Esen et al., 2008) in an attempt to account for the semivolatility of lower molecular weight PAH species.
The validity of these approaches rests on many assumptions (Watson, 1984), two of which may be particularly problematic for PAHs. Firstly, each suspected source or source type is assumed to be associated with relative proportions of the species in question that are unique. Secondly, the relative proportions of the species in question are assumed to be conserved between each emission source and the downwind point of measurement.
Herein it is argued that these assumptions only hold for PAHs under a limited set of conditions. As such, the approach of using PAH ratios and/or profiles as if they are unique and conserved should be limited to those cases where the validity of the assumptions can be demonstrated. Though concerns over conventional source apportionment have been expressed for some time, studies continue to appear in the literature that do not account for its limitations (Bourotte et al., 2005, Dallarosa et al., 2005, Ravindra et al., 2006, Ding et al., 2007, Lee and Kim, 2007, Battelle, 2007, Ravindra et al., 2008a, Ravindra et al., 2008b, Esen et al., 2008). Similar concerns have recently been expressed for multimedia fate studies (Zhang et al., 2005) and source apportionment of PAHs in sewage sludges (Katsoyiannis et al., 2007).
Section snippets
PAH source signatures are not unique by source type
When using diagnostic ratios or profiles to apportion atmospheric PAH sources, studies typically examine broad categories of potential source types. Combustion of coal, oil, natural gas and wood as well as coke oven emissions and vehicle exhaust (sometimes separated into gasoline and diesel components) have been used in atmospheric studies (Harrison et al., 1996, Schauer et al., 1996, Simcik et al., 1999, Dickhut et al., 2000, Vasconcellos et al., 2003, Guo et al., 2003, Larsen and Baker, 2003,
Relative PAH concentrations are not conserved in the atmosphere
PAHs are semivolatile and partition between the gas and particle phases of ambient air (Yamasaki et al., 1982). The vast majority of source apportionment studies examining PAHs limit their analysis to particle-phase compounds whereas studies examining the sum of gas and particle concentrations are relatively few. As discussed below, either representation of PAH concentrations leads to relative amount conservation under only limited conditions.
Some atmospheric fate processes relevant to PAHs are
Refining the approach to conventional airborne PAH source apportionment
The arguments presented above demonstrate that neither assumption required for conventional ratio- or profile-based source apportionment of PAHs holds true in all situations. Several studies have tried to adapt the diagnostic ratio or profile approach by accounting for the lack of conservation of species concentrations, particularly with respect to reactivity (Duval and Friedlander, 1982, Pistikopoulos et al., 1990b, Li and Kamens, 1993, Venkataraman and Friedlander, 1994, Schauer et al., 1996
Acknowledgements
The author thanks Terry Bidleman, Pierrette Blanchard, Tom Harner, Sunling Gong and Leiming Zhang for helpful comments and suggestions.
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