Natural Abundance 13C and 14C Analysis of Water-Soluble Organic Carbon in Atmospheric Aerosols

Water-soluble organic carbon (WSOC) constitutes a large fraction of climate-forcing organic aerosols in the atmosphere, yet the sources of WSOC are poorly constrained. A method was developed to measure the stable carbon isotope (delta C-13) and radiocarbon (delta C-14) composition of WSOC for apportionment between fossil fuel and different biogenic sources. Synthetic WSOC test substances and ambient aerosols were employed to investigate the effect of both modern and fossil carbon contamination and any method-induced isotope fractionation. The method includes extraction of aerosols collected on quartz filters followed by purification and preparation for off-line delta C-13 and Delta C-14 determination. The preparative freeze-drying step for isotope analysis yielded recoveries of only similar to 70% for ambient aerosols and WSOC probes. However, the delta C-13 of the WSOC isolates were in agreement with the delta C-13 of the unprocessed starting material, even for the volatile oxalic acid probe (6.59 +/- 0.37 parts per thousand vs 6.33 +/- 0.31 parts per thousand; 2 sd). A C-14-fossil phthalic acid WSOC probe returned a fraction modern biomass of <0.008 whereas a C-14-modern sucrose sucrose standard yielded a fraction modern of >0.999, indicating the Delta C-14-WSOC method to be free of both fossil and contemporary carbon contamination. Application of the (delta C-13/Delta C-14-WSOC method to source apportion climate-affecting aerosols was illustrated be constraining that WSOC in ambient Stockholm aerosols were 88% of contemporary biogenic C3 plant origin.