Radiocarbon blank correction: Methodologies and limitations in a major urban study of carbonaceous aerosols

The Northern Front Range Air Quality Study (NFRAQS) was the latest and most ambitious of a series of efforts to apportion sources of carbonaceous aerosol "soot" in the Denver, Colorado metropolitan area. The study was mandated by the Colorado State Legislature as a result of the continuing impact of aerosol carbon on visibility in the region. Apportionment of fossil and biomass carbon was based on blank-corrected values of carbon mass concentrations (mug/m(3)) and C-14 data (f(M), fraction of modem carbon) of a selected subset of the samples collected in conjunction with this program. Over 100 C-14 measurements were made on size segregated (less than or equal to2.5 mum diameter) atmospheric aerosol samples collected during the summer of 1996 and the winter of 1996-1997. The reported f(M) values required correction for both the mass and f(M) of the overall carbon blank. Lack of direct f(M) data for the field blanks had a substantial effect on the estimated uncertainty of the final results, and in a few of the most extreme cases blank-corrected f(M) data had to be designated as "indeterminate". Blank correction procedures and limitations will be illustrated with quantitative data from the NFRAQS study. (C) 2000 Published by Elsevier Science B.V. All rights reserved.