Quantification and radiocarbon source apportionment of black carbon in atmospheric aerosols using the CTO-375 method

To make progress towards linking the atmosphere and biogeosphere parts of the black carbon (BC) cycle, a chemothermal oxidation method (CTO-375), commonly applied for isolating BC from complex geomatrices such as soils, sediments and aquatic particles, was applied to investigate the BC also in atmospheric particles. Concentrations and C-14-based source apportionment of CTO-375 based BC was established for a reference aerosol (NIST RM-8785) and for wintertime aerosols collected in Stockholm and in a Swedish background area. The results were compared with thermal-optical (OC/EC) measurements. For NIST RM-8785, a good agreement was found between the BCCTO-375 concentration and the reported elemental carbon (EC) concentration measured by the "Speciation Trends Network - National Institute of Occupational Safety and Health" method (ECNIOSH) with BCCTO-375 of 0.054+/-0.002gg(-1) and ECNIOSH of 0.067+/-0.008gg(-1). In contrast, there was an average factor of ca. 20 difference between BC(CTO-37)5 and ECNIOSH for the ambient Scandinavian wintertime aerosols, presumably reflecting a combination of BCCTO-375 isolating only the recalcitrant soot-BC portion of the BC continuum and the ECNIOSH metric inadvertently including some intrinsically non-pyrogenic organic matter. Isolation of BCCTO-375 with subsequent off-line radiocarbon analysis yielded fraction modern values (fM) for total organic carbon (TOC) of 0.93 (aerosols from a Swedish background area), and 0.58 (aerosols collected in Stockholm); whereas the fM for BCCTO-375 isolates were 1.08 (aerosols from a Swedish background area), and 0.87 (aerosols collected in Stockholm). This radiocarbon-based source apportionment suggests that contribution from biomass combustion to cold-season atmospheric BCCTO-375 in Stockholm was 70% and in the background area 88%. (C) 2007 Elsevier Ltd. All rights reserved.