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 aero sol "soot" in the Denver, Colorado metropolitan area. The study was mandate d by the Colorado State Legislature as a result of the continuing impact of aerosol carbon on visibility in the region. Apportionment of fossil and bi omass carbon was based on blank-corrected values of carbon mass concentrati ons (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 m um diameter) atmospheric aerosol samples collected during the summer of 199 6 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 uncer tainty of the final results, and in a few of the most extreme cases blank-c orrected f(M) data had to be designated as "indeterminate". Blank correctio n procedures and limitations will be illustrated with quantitative data fro m the NFRAQS study. (C) 2000 Published by Elsevier Science B.V. All rights reserved.