Clumped isotopes in near-surface atmospheric CO₂ over land, coast and ocean in Taiwan and its vicinity

Abstract. Molecules containing two rare isotopes (e.g., ¹³C¹⁸O¹⁶O in CO₂), called clumped isotopes, in atmospheric CO₂ are powerful tools to provide an alternative way to independently constrain the sources of CO₂ in the atmosphere because of their unique physical and chemical properties. We presented clumped isotope data (Δ₄₇) in near-surface atmospheric CO₂ from urban, suburban, ocean, coast, high mountain ( ∼  3.2 km a.s.l.) and forest in Taiwan and its vicinity. The primary goal of the study was to use the unique Δ₄₇ signature in atmospheric CO₂ to show the extents of its deviations from thermodynamic equilibrium due to different processes such as photosynthesis, respiration and local anthropogenic emissions, which the commonly used tracers such as δ¹³C and δ¹⁸O cannot provide. We also explored the potential of Δ₄₇ to identify/quantify the contribution of CO₂ from various sources. Atmospheric CO₂ over ocean was found to be in thermodynamic equilibrium with the surrounding surface sea water. Respired CO₂ was also in close thermodynamic equilibrium at ambient air temperature. In contrast, photosynthetic activity result in significant deviation in Δ₄₇ values from that expected thermodynamically. The disequilibrium could be a consequence of kinetic effects associated with the diffusion of CO₂ in and out of the leaf stomata. We observed that δ¹⁸O and Δ₄₇ do not vary similarly when photosynthesis was involved unlike simple water–CO₂ exchange. Additionally we obtained Δ₄₇ values of car exhaust CO₂ that were significantly lower than the atmospheric CO₂ but higher than that expected at the combustion temperature. In urban and suburban regions, the Δ₄₇ values were found to be lower than the thermodynamic equilibrium values at the ambient temperature, suggesting contributions from local combustion emission.