Identification of Anthropogenic CO2 Using Triple Oxygen and Clumped Isotopes.

Quantification of contributions from various sources of CO2 is important for understanding the atmospheric CO2 budget. Considering the number and diversity of sources and sinks, the widely used proxies such as concentration and conventional isotopic compositions (δ13C and δ18O) are not always sufficient to fully constrain the CO2 budget. Additional constraints may help in understanding the mechanisms of CO2 production and consumption. The anomaly in triple oxygen isotopes or 17O excess (denoted by Δ17O) and molecules containing two rare isotopes, called clumped isotopes, are two recently developed tracers with potentials to independently constrain some important processes that regulate CO2 in the atmosphere. The clumped isotope for CO2, denoted by Δ47, is the excess of 13C16O18O over a random distribution of isotopes in a CO2 molecule. We measured the concentrations of δ13C, δ18O, Δ17O, and Δ47 in air CO2 samples collected from the Hsuehshan tunnel (length: 12.9 km), and applied linear and polynomial regressions to obtain the fossil fuel end-members for all these isotope proxies. The other end-members, the values of all these proxies for background air CO2, are either assumed or taken as the values obtained over the tunnel and ocean. The fossil fuel (anthropogenic) CO2 end-member values for δ13C, δ18O, Δ17O, and Δ47 are estimated using the two component mixing approach: the derived values are -26.76 ± 0.25‰, 24.57 ± 0.33‰, -0.219 ± 0.021‰, and 0.267 ± 0.036‰, respectively. These four major CO2 isotope tracers along with the concentration were used to estimate the anthropogenic contribution in the atmospheric CO2 in urban and suburban locations. We demonstrate that Δ17O and Δ47 have the potential to independently estimate anthropogenic contribution, and the advantages of these two over the conventional isotope proxies are discussed.