Estimation of the fossil fuel component in atmospheric CO₂ based on radiocarbon measurements at the Beromünster tall tower, Switzerland

Berhanu, Tesfaye; Szidat, Sönke; Brunner, Dominik; Satar, Ece; Schanda, Rüdiger; Nyfeler, Peter; Battaglia, Michael; Steinbacher, Martin; Hammer, Samuel; Leuenberger, Markus (2017). Estimation of the fossil fuel component in atmospheric CO₂ based on radiocarbon measurements at the Beromünster tall tower, Switzerland. Atmospheric chemistry and physics, 17(17), pp. 10753-10766. European Geosciences Union 10.5194/acp-17-10753-2017

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Fossil fuel CO₂ (CO₂ff) is the major contributor of anthropogenic CO₂ in the atmosphere, and accurate quantification is essential to better understand the carbon cycle. Since October 2012, we have been continuously measuring the mixing ratios of CO, CO₂, CH₄, and H₂O at five different heights at the Beromünster tall tower, Switzerland. Air samples for radiocarbon (∆¹⁴CO₂/ analysis have also been collected from the highest sampling inlet (212.5 m) of the tower on a biweekly basis. A correction was applied for ¹⁴CO₂ emissions from nearby nuclear power plants (NPPs), which have been simulated with the Lagrangian transport model FLEXPART-COSMO. The ¹⁴CO₂ emissions from NPPs offset the depletion in ¹⁴C by fossil fuel emissions, resulting in an underestimation of the fossil fuel component in atmospheric CO₂ by about 16 %. An average observed ratio (RCO/ of 13.4±1.3 mmol mol⁻¹ was calculated from the enhancements in CO mixing ratios relative to the clean-air reference site Jungfraujoch (1CO) and the radiocarbon-based fossil fuel CO₂ mole fractions. The wintertime RCO estimate of 12.5±3.3 is about 30% higher than the wintertime ratio between in situ measured CO and CO₂ enhancements at Beromünster over the Jungfraujoch background (8.7 mmol mol⁻¹) corrected for non-fossil contributions due to strong biospheric contribution despite the strong correlation between ∆CO and ∆CO₂ in winter. By combining the ratio derived using the radiocarbon measurements and the in situ measured CO mixing ratios, a high-resolution time series of CO₂ff was calculated exhibiting a clear seasonality driven by seasonal variability in emissions and vertical mixing. By subtracting the fossil fuel component and the large-scale background, we have determined the regional biospheric CO₂ component that is characterized by seasonal variations ranging between -15 and +30 ppm. A pronounced diurnal variation was observed during summer modulated by biospheric exchange and vertical mixing, while no consistent pattern was found during winter.

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