Source specificity and atmospheric processing of airborne PAHs: Implications for source apportionment

Polycyclic aromatic hydrocarbons (PAHs) are emitted to the atmosphere from a variety of sources. Though classified as persistent organic pollutants (POPs), their levels are affected by atmospheric removal and transformation processes. Efforts have been made to conduct receptor modelling of PAHs for over 25 years, whereby ambient measurement data are manipulated to compare relative amounts of compounds to those expected in relevant sources. These relative amounts, which can be based on particle or total (gas + particle) concentrations, are typically presented as diagnostic ratios of two isomeric species or as profiles representing several species at once.This review examines two of the assumptions necessary for conventional ratio- or profile-based source apportionment methods to be valid. The term “conventional” refers to the direct comparison of source and ambient data without accounting for alterations that occur in the atmosphere. These assumptions, namely source specificity and species conservation, do not generally hold for PAHs as a class. 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. In an effort to contribute to the reversal of this trend, a set of conditions under which conventional source apportionment may be valid is presented herein. Research relating to emissions' measurement analysis, numerical modelling and atmospheric processing is also suggested.