Carbon isotope signature of dissolved inorganic carbon (DIC) in precipitation and atmospheric CO2

This paper describes results of chemical and isotopic analysis of inorganic carbon species in the atmosphere and precipitation for the calendar year 2008 in Wroclaw (SW Poland). Atmospheric air samples (collected weekly) and rainwater samples (collected after rain episodes) were analysed for CO2 and dissolved inorganic carbon (DIC) concentrations and for delta C-13 composition. The values obtained varied in the ranges: atmospheric CO2: 337-448 ppm; delta C-13(CO2) from -14.4 to -8.4 parts per thousand; DIC in precipitation: 0.6-5.5 mg dm(-3); delta C-13(DIC) from -22.2 to +0.2 parts per thousand. No statistical correlation was observed between the concentration and delta C-13 value of atmospheric CO2 and DIC in precipitation. These observations contradict the commonly held assumption that atmospheric CO2 controls the DIC in precipitation. We infer that DIC is generated in ambient air temperatures, but from other sources than the measured atmospheric CO2. The calculated isotopic composition of a hypothetical CO2 source for DIC forming ranges from -31.4 to -11.0 parts per thousand, showing significant seasonal variations accordingly to changing anthropogenic impact and atmospheric mixing processes. (C) 2010 Elsevier Ltd. All rights reserved.