Identification of Anthropogenic CO Using Triple Oxygen and Clumped Isotopes.

Quantification of contributions from various sources of CO is important for understanding the atmospheric CO budget. Considering the number and diversity of sources and sinks, the widely used proxies such as concentration and conventional isotopic compositions (δC and δO) are not always sufficient to fully constrain the CO budget. Additional constraints may help in understanding the mechanisms of CO production and consumption. The anomaly in triple oxygen isotopes or O excess (denoted by ΔO) and molecules containing two rare isotopes, called clumped isotopes, are two recently developed tracers with potentials to independently constrain some important processes that regulate CO in the atmosphere. The clumped isotope for CO, denoted by Δ, is the excess of COO over a random distribution of isotopes in a CO molecule. We measured the concentrations of δC, δO, ΔO, and Δ in air CO 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 CO, are either assumed or taken as the values obtained over the tunnel and ocean. The fossil fuel (anthropogenic) CO end-member values for δC, δO, ΔO, and Δ 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 CO isotope tracers along with the concentration were used to estimate the anthropogenic contribution in the atmospheric CO in urban and suburban locations. We demonstrate that ΔO and Δ have the potential to independently estimate anthropogenic contribution, and the advantages of these two over the conventional isotope proxies are discussed.