Abstract
ABSTRACTThe most commonly used tracers to probe the atmospheric and biogeochemical cycles of CO2 are 16O12C16O, 16O13C16O, and 16O12C18O. Considering the number and diversity of sources and sinks affecting CO2, these tracers are not always sufficient to constrain the fluxes of CO2 between the atmosphere and biosphere/hydrosphere. In this context, 16O12C17O species was introduced but has rarely been used due to difficulties associated with its accurate measurement in natural samples. This tracer, expressed as an abundance anomaly in 17O, defined by ∆17O = ln(1 + δ17O) – 0.516 × ln(1 + δ18O) can independently constrain the fluxes associated with the terrestrial processes. The advantage of utilizing ∆17O over δ18O alone lies on the sensitivity of the former to the rates of biogeochemical processes involving multiple water reservoirs with spatial and temporal isotopic heterogeneities. To employ all the three oxygen isotopes for estimating fluxes of CO2, sources and processes affecting their partitioning have to be identified and quantified. Here, we measured ∆17O values in near surface atmospheric CO2 from Taiwan in urban and semi-urban areas and over the South China Sea. Strong spatiotemporal variation was seen, with an average ∆17O value of 0.332‰ and a mean variation of 0.043‰ (relative to V-SMOW; 1-σ standard deviation for a total of 140 samples). The large variation reflects combinations of distinct air masses carrying CO2 from sources having different ∆17O values: negative from combustion emissions, positive from the stratosphere, and a positive water-CO2 equilibration value from isotope exchange with leaf/soil/ocean waters. We observed that the variation of the semi-urban ∆17O values is largely affected by local biogeochemistry and stratospheric intrusion with only minor influence from anthropogenic emissions. This is the first oxygen anomaly study for near surface CO2 covering diverse source characteristics and has enormous potential in air CO2 source identification and constraining the global carbon budget.
Highlights
About 90% of anthropogenic emissions of CO2 are due to fossil fuel burning and cement manufacturing, while land use change is responsible for the rest (IPCC, 2007, 2013)
The ∆17O value in this region is highly variable, with mean value significantly lower in areas dominated by anthropogenic emissions
We show from measurements of samples from Daan forest park that local biogeochemistry dominates in determining the ∆17O values
Summary
About 90% of anthropogenic emissions of CO2 are due to fossil fuel burning and cement manufacturing, while land use change is responsible for the rest (IPCC, 2007, 2013). The coefficient λ varies between 0.500 and 0.529 (Thiemens, 1999; Thiemens et al, 2014) and is taken to be 0.516 (λo) for atmospheric CO2 This value of λo is chosen because it corresponds to the fractionation that occurs in transpiration (Landais et al, 2006) at an average relative humidity of 75% (more details about the choice of λ values are given in the Supplementary Material). With this selection of λ, the formulation of the partitioning of the tripleoxygen isotopes of CO2 coming from exchanges with diverse waters (meteoric, vapor, soil, stem, and leaf) can be simplified since the triple isotope variations in these processes have similar λ values as λo (Landais et al, 2006; Liang and Mahata, 2015). Oxygen isotope anomaly, expressed as ∆17O, is used to quantify the deviation of the δ-values of a sample from the mass-dependent fractionation line and is defined by
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