Abstract
Abstract. 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 (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 (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 (ΔCO) 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 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 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.
Highlights
Fossil fuel CO2 (CO2ff) is the fundamental contributor to the increase in atmospheric CO2; its precise quantification is crucial to better understand the global carbon budget
Information on the response of the biosphere to climate variations can be obtained from atmospheric CO2 observations, but isolating the biospheric signal in the measured CO2 mixing ratios requires an accurate quantification of the fossil fuel component
Contribution from nuclear power plants (NPPs) and from 14-day integrated samplings at Jungfraujoch (b), CO2ff determined during this period applying Eq (4) with a mean CO2ff value of 4.3 ppm (c), the biospheric CO2 determined by simple subtraction of CO2bg and CO2ff from the CO2meas (d), and the temperature record during this period at the 212.5 m height level (e)
Summary
Fossil fuel CO2 (CO2ff) is the fundamental contributor to the increase in atmospheric CO2; its precise quantification is crucial to better understand the global carbon budget. One of the major uncertainties in the projections of climate change is the uncertainty in the future carbon budget due to feedbacks between terrestrial ecosystems and climate (Heimann and Reichstein, 2008). Information on the response of the biosphere to climate variations can be obtained from atmospheric CO2 observations, but isolating the biospheric signal in the measured CO2 mixing ratios requires an accurate quantification of the fossil fuel component. Berhanu et al.: Estimation of the fossil fuel component in atmospheric CO2 methods have been proposed for quantifying CO2ff, which are based on observations or models. A widely employed approach is to determine CO2ff with an atmospheric transport model that incorporates CO2ff emissions from a bottom-up emission inventory
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