Carbon isotopic characterization of hydrophobic dissolved organic carbon in rainwater

The C-14 and C-13 content of the hydrophobic fraction (C18 extractable) of dissolved organic carbon (DOC) in Wilmington North Carolina USA rainwater was determined for six rain events to elucidate potential sources. A two end member mass-balance calculation indicated that 42-61% of the hydrophobic DOC was of fossil fuel origin compared to previously published values (4-24%) for bulk rain DOC indicating a strong anthropogenic contribution to the hydrophobic component. All fossil fuel derived organic carbon in the bulk DOC (similar to 15% fossil fuel derived) could be accounted for in the hydrophobic fraction (similar to 52% fossil fuel derived) which represents approximately 1/3 of the bulk DOC. The delta C-13 values of the hydrophobic DOC were consistent (-28.8 +/- 0.9 parts per thousand) similar to that for terrestrial and fossil fuel derived organic carbon. This is in contrast to previously published bulk rain DOC delta C-13 values that displayed a strong influence of air-mass back-trajectory with delta C-13 values ranging from -20.8 parts per thousand (typical of marine organic carbon) for marine air-mass back-trajectory rain to -28.2 parts per thousand (typical of terrestrial or fossil fuel derived organic carbon) for terrestrial air-mass back-trajectory rain. The combination of C-14 and C-13 isotopic data strongly suggests that a large fraction of hydrophobic organic material in rain comes from incompletely combusted fossil fuels. Changes in energy usage patterns and efforts to reduce fossil fuel emissions will alter the abundance of this hydrophobic material in the atmosphere which may impact the spectral distribution of sunlight reaching the earth's surface as well as the degree of oceanic primary productivity. (c) 2012 Elsevier Ltd. All rights reserved.