Clumped isotopes in near-surface atmospheric CO 2 over land, coast and ocean in Taiwan and its vicinity
Molecules containing two rare isotopes (e.g., 13C 18O 16O in CO 2), called clumped isotopes, in atmospheric CO 2 are powerful tools to provide an alternative way to independently constrain the sources of CO 2 in the atmosphere because of their unique physical and chemical properties. We presented clumped isotope data (Δ 47) in near-surface atmospheric CO 2 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 Δ 47 signature in atmospheric CO 2 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 δ 13C and δ 18O cannot provide. We also explored the potential of Δ 47 to identify/quantify the contribution of CO 2 from various sources. Atmospheric CO 2 over ocean was found to be in thermodynamic equilibrium with the surrounding surface sea water. Respired CO 2 was also in close thermodynamic equilibrium at ambient air temperature. In contrast, photosynthetic activity result in significant deviation in Δ 47 values from that expected thermodynamically. The disequilibrium could be a consequence of kinetic effects associated with the diffusion of CO 2 in and out of the leaf stomata. We observed that δ 18O and Δ 47 do not vary similarly when photosynthesis was involved unlike simple water–CO 2 exchange. Additionally we obtained Δ 47 values of car exhaust CO 2 that were significantly lower than the atmospheric CO 2 but higher than that expected at the combustion temperature. In urban and suburban regions, the Δ 47 values were found to be lower than the thermodynamic equilibrium values at the ambient temperature, suggesting contributions from local combustion emission.