Radiocarbon “dating” of individual chemical compounds in atmospheric aerosol: First results comparing direct isotopic and multivariate statistical apportionment of specific polycyclic aromatic hydrocarbons

Abstract

Recent instrumental developments which allow radiocarbon (14C) determinations on microgram quantities of organic material have revolutionized our ability to apportion fossil and biogenic sources of trace carbonaceous gases and aerosols in the environment. The chemical complexity of carbonaceous aerosol, however, presents a challenge in the interpretation of its average isotopic composition. For this reason, there has long been an interest in radiocarbon measurements of specific classes of compounds, or even individual trace compounds of particular environmental concern, such as the polycyclic aromatic hydrocarbons (PAHs). The PAHs hold special interest because they are produced at trace levels in combustion processes; they exhibit compound-specific genotoxicity; and they have the potential to serve as “molecular markers” or tracers for specific ambient aerosol sources. Prior to the work reported here, inferences from ambient studies concerning molecular tracer capabilities have been indirect, relying on circumstantial evidence and/or multivariate statistical techniques. In the present study we demonstrate, for the first time, the ability to measure the 14C content of individual, trace organic compounds (PAHs) in atmospheric aerosol. The research utilized a special, well characterized atmospheric reference material, where individual PAHs were radiocarbon “dated” using off-line gas chromatography/accelerator mass spectrometry (GC/AMS). Separation of individual compounds in sufficient amounts for AMS was accomplished by automated preparative capillary gas chromatography (PCGC). Results showed the PAHs dated to be primarily fossil in origin, and lent support to an earlier multivariate statistical study suggesting that benzo[ghi]perylene might serve as a useful tracer for soot generated by motor vehicles, although this assessment must be viewed as tentative because of potential interference from other compounds incompletely removed from the purified PAH fraction.