Evaluation of 4 years of continuous δ13C(CO2) data using a moving Keeling plot method

Different carbon dioxide (CO2) emitters can be distinguished by their carbon isotope ratios. Therefore measurements of atmospheric delta C-13(CO2) and CO2 concentration contain information on the CO2 source mix in the catchment area of an atmospheric measurement site. This information may be illustratively presented as the mean isotopic source signature. Recently an increasing number of continuous measurements of delta C-13(CO2) and CO2 have become available, opening the door to the quantification of CO2 shares from different sources at high temporal resolution. Here, we present a method to compute the CO2 source signature (delta(S)) continuously and evaluate our result using model data from the Stochastic Time-Inverted Lagrangian Transport model. Only when we restrict the analysis to situations which fulfill the basic assumptions of the Keeling plot method does our approach provide correct results with minimal biases in delta(S). On average, this bias is 0.2 parts per thousand with an interquartile range of about 1.2 parts per thousand for hourly model data. As a consequence of applying the required strict filter criteria, 85% of the data points - mainly daytime values - need to be discarded. Applying the method to a 4-year dataset of CO2 and delta C-13(CO2) measured in Heidelberg, Germany, yields a distinct seasonal cycle of delta(S). Disentangling this seasonal source signature into shares of source components is, however, only possible if the isotopic end members of these sources - i.e., the biosphere, delta(bio), and the fuel mix, delta(F) - are known. From the mean source signature record in 2012, delta(bio) could be reliably estimated only for summer to (-25.0 +/- 1.0) % and delta(F) only for winter to (-32.5 +/- 2.5) parts per thousand. As the isotopic end members delta(bio) and delta(F) were shown to change over the season, no year-round estimation of the fossil fuel or biosphere share is possible from the measured mean source signature record without additional information from emission inventories or other tracer measurements.