Atmospheric carbon dioxide (CO2) concentrations observed in urban and continental areas and the surrounding areas often show short-term elevations on a timescale from several hours to several days. These variations are considered to be attributed to the CO2 emissions from biotic activities and burning of fossil fuels (coal, oil and natural gas). If the contribution rate from each CO2 source is clearly determined, the uncertainty of the CO2 emissions estimated from atmospheric inversion calculations would be reduced. In this study, we develop a method to evaluate the contribution rate from individual sources based on measurements of carbon stable isotope (13C), radiocarbon (14C) and O2 concentration as well as CO2 concentration in the atmosphere. The −O2:CO2 exchange ratios of the fossil fuel burning are different for the fuel types because the ratios are stoichiometrically related to the elemental compositions of the individual fuels. The 14C measurements is useful to separate the fossil fuel emissions from the biotic emissions because the fossil fuel-derived CO2 contain no 14C. In addition, values of 13C depend on source of CO2. Therefore, the combination of CO2 measurements with O2, 13C and 14C measurements allow us to estimate the contribution rates of the individual CO2 source.
In this presentation, we show preliminary results of the atmospheric measurements which were conducted at Tsukuba in July-August 2015 to assess the usefulness of the above method. In the experiment, the atmospheric CO2 and O2 concentrations were continuously measured, and the air samples were collected in the glass flasks to measure the carbon isotopes. The relations between CO2 concentrations and Δ14C for the observed CO2 change suggest that 60–70% of the CO2 change are attributed to the fossil fuel-derived CO2. Taking into account of the –O2:CO2 ratio for land biotic processes of 1.1, we can obtain the −O2:CO2 ratio for the fossil fuel component of the observed changes of 1.37–1.41, which is close to the exchange ratio for oil burring (R=1.44). The relation between CO2 and 13C is also consistent with the above result.