Direct quantification of PM2.5 fossil and biomass carbon within the Northern Front Range Air Quality Study's domain

Radiocarbon (C-14) analyses of PM2.5 (particulate matter with an aerodynamic diameter of 2.5 mu m or less) of both ambient and source samples from the Northern Front Range Air Quality Study (NFRAQS) in Colorado were performed. The C-14 analyses were undertaken to provide direct fossil vs modern (biomass) carbon source discrimination data for a subset of summer and winter 1996-1997 samples collected within the Denver metropolitan area. Samples were prepared for C-14 accelerator mass spectrometry measurements using techniques specially developed for small samples, i.e., <100 mu g C. For the days and sampling periods analyzed the median and interquartile range of the winter blank corrected fraction of modern carbon was 23% (16-34%) at Welby and 27% (25-37%) at Brighten. The summer samples exhibited a more mixed signature with a median and interquartile range of 47% (9-70%). Source samples yielded C-14 signatures consistent with expectation. We conclude fossil-derived sources contribute substantially in both seasons and at both locations; however, the biomass carbon component dominates episodically in the summer.