RADIOCARBON DATING OF INDIVIDUAL CHEMICAL-COMPOUNDS IN ATMOSPHERIC AEROSOL - FIRST RESULTS COMPARING DIRECT ISOTOPIC AND MULTIVARIATE STATISTICAL APPORTIONMENT OF SPECIFIC POLYCYCLIC AROMATIC-HYDROCARBONS

Recent instrumental developments which allow radiocarbon (C-14) determ inations on microgram quantities of organic material have revolutioniz ed our ability to apportion fossil and biogenic sources of trace carbo naceous gases and aerosols in the environment. The chemical complexity of carbonaceous aerosol, however, presents a challenge in the interpr etation of its average isotopic composition. For this reason, there ha s long been an interest in radiocarbon measurements of specific classe s of compounds, or even individual trace compounds of particular envir onmental concern, such as the polycyclic aromatic hydrocarbons (PAHs). The PAHs hold special interest because they are produced at trace lev els in combustion processes; they exhibit compound-specific genotoxici ty; and they have the potential to serve as ''molecular markers'' or t racers for specific ambient aerosol sources, Prior to the work reporte d here, inferences from ambient studies concerning molecular tracer ca pabilities have been indirect, relying on circumstantial evidence and/ or multivariate statistical techniques. In the present study we demons trate, for the first time, the ability to measure the C-14 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). Separa tion of individual compounds in sufficient amounts for AMS was accompl ished by automated preparative capillary gas chromatography (PCGC). Re sults 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 b y motor vehicles, although this assessment must be viewed as tentative because of potential interference from other compounds incompletely r emoved from the purified PAH fraction.