Carbon isotopic characterization of hydrophobic dissolved organic carbon in rainwater

The 14C and 13C 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 (∼15% fossil fuel derived) could be accounted for in the hydrophobic fraction (∼52% fossil fuel derived) which represents approximately 1/3 of the bulk DOC. The δ13C values of the hydrophobic DOC were consistent (−28.8 ± 0.9‰) similar to that for terrestrial and fossil fuel derived organic carbon. This is in contrast to previously published bulk rain DOC δ13C values that displayed a strong influence of air-mass back-trajectory with δ13C values ranging from −20.8‰ (typical of marine organic carbon) for marine air-mass back-trajectory rain to −28.2‰ (typical of terrestrial or fossil fuel derived organic carbon) for terrestrial air-mass back-trajectory rain. The combination of 14C and 13C 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.