Carbon isotopic (14C and 13C) characterization of fossil-fuel derived dissolved organic carbon in wet precipitation in Shandong Province, China

We present results from a 1-year study of radiocarbon and stable carbon isotope measurement of dissolved organic carbon (DOC) of wet precipitation samples collected in the Shandong province of China. A total of 32 rainfall and 4 snow samples were collected from two coastal cities (Qingdao and Yantai) and one inland site (Taian) during 2014. Concentrations of DOC in the rainwater and snow samples varied widely from 28 to 616 mu M. In general, rainwater originating from marine sources had relatively low DOC concentrations compared with continentally generated precipitation. Values of delta C-13-DOC and Delta C-14-DOC in the snow and rainwater ranged from -19.0 to -29.4 % and -23 to -494 %, with corresponding radiocarbon ages of 125 to 5410 years before present. The dominant DOC in the wet precipitation had a relatively old C-14 age (average 2841 years) and a depleted C-13 value (average -24.4 Years), indicating the mixing of contemporary organic carbon with C-14-depleted fossil fuel-derived soluble organic carbon in the atmosphere. Using a dual isotopic two-endmember model, we calculate that 7 % to 52 % of DOC in the snow and rainfall originated from fossil fuels. It is estimated that the flux of DOC in continental rainfall of China is 23 x 10(12) g C yr(-1) and of this, 7.1 x 10(12) g C yr(-1) could be fossil-fuel OC. On a global scale, the DOC flux in rainfall is estimated to be 357 Tg C yr(-1), and 110 Tg C yr(-1) could be fossil fuel-derived. Our study demonstrates that wet precipitation is an important removal process for old fossil fuel carbon from the atmosphere. This removal mechanism could play an important role in the carbon cycle, especially for the anthropogenically derived fraction, and it should be taken into consideration in global carbon cycle models.