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

Abstract. Fossil fuel CO₂ (CO_2ff) 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 (Δ¹⁴CO₂) 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 (R_CO) 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 (ΔCO) and the radiocarbon-based fossil fuel CO₂ mole fractions. The wintertime R_CO 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 ΔCO and ΔCO₂ 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 CO_2ff 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.