Estimating regional fossil fuel CO2 concentrations from 14CO2 observations: challenges and uncertainties

Abstract

The direct way to estimate the regional fossil fuel CO₂ surplus (ΔffCO₂) at a station is by measuring the Δ¹⁴CO₂ depletion compared with a respective background. However, this approach has several challenges, which are (i) the choice of an appropriate Δ¹⁴CO₂ background, (ii) potential contaminations through nuclear ¹⁴CO₂ emissions and (iii) masking of ΔffCO₂ by ¹⁴C-enriched biosphere respiration. Here we evaluate these challenges and estimate potential biases and typical uncertainties of ¹⁴C-based ΔffCO₂ estimates in Europe. We show that Mace Head (MHD), Ireland, is a representative background station for the Integrated Carbon Observation System (ICOS) atmosphere station network. The mean ΔffCO₂ representativeness bias when using the MHD Δ¹⁴CO₂ background for the whole observation network is of order 0.1 ± 0.3 ppm. At ICOS sites, the median nuclear contamination leads to 25% low-biased ΔffCO₂ estimates if not corrected for. The ΔffCO₂ masking due to ¹⁴C-enriched heterotrophic CO₂ respiration can lead to similar ΔffCO₂ biases as the nuclear contaminations, especially in summer. Our evaluation of all components contributing to the uncertainty of ΔffCO₂ estimates reveals that, due to the small ffCO₂ signals at ICOS stations, almost half of the ¹⁴C-based ΔffCO₂ estimates from integrated samples have an uncertainty that is larger than 50%. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

Keywords: fossil fuel CO2; nuclear 14CO2 contamination; radiocarbon; Δ14CO2 background.

Figure 1.

Ratio of ΔffCO₂ when estimated with equation (2.4) versus equation (2.3) for C_photo/C_ff ratios ranging from −0.1 to −10.

Figure 2.

Abundance distribution for the endpoints of the STILT back-trajectories from nine ICOS sites (coloured crosses) in 2018. At each hour and each station, 100 particles were released in STILT, and their back-trajectories were calculated for 10 days backward in time. If the trajectory leaves the shown domain, its endpoint is defined as the grid cell where the trajectory leaves the domain the first time. STILT was driven with the 0.25° resolved ERA5 (European ReAnalysis 5) meteorology from the European Center for Medium-Range Weather Forecasts (ECMWF). The black cross indicates the position of the Mace Head (MHD) background site (back-trajectories were not calculated for MHD).

Figure 3.

TM3-simulated fossil fuel CO₂ concentration offsets relative to Mace Head caused by emissions outside of our target region (figure 2). (a,b) The offsets at a location at mid-latitudes of the eastern boundary, for all hours (a) and for 13 h UTC only (b). (c,d) Corresponding data for the ICOS station Křešín (KRE). Note the different scales in (a,b) and (c,d).

Figure 4.

ΔffCO₂ masking by ¹⁴CO₂ emissions from nuclear installations in relation to the ratio between ${\mathrm{\Delta }}_{\text{nuc}}^{14}$ and $({\mathrm{\Delta }}_{\text{bg}}^{14}-{\mathrm{\Delta }}_{\text{meas}}^{14})$.

Figure 5.

ΔffCO₂ masking by ¹⁴C-enriched respiration CO₂ plotted versus the difference between $({\mathrm{\Delta }}_{\text{resp}}^{14}-{\mathrm{\Delta }}_{\text{meas}}^{14})$ for C_resp/C_ff ratios between 0.1 and 6.

Figure 6.

Distribution of observed Δ¹⁴ signals between reference station (MHD) and ICOS stations ((a) for two-week integrated samples and (c) for the currently available flask samples). (b,d) The distributions of the estimated nuclear corrections of the samples.

Figure 7.

(a) Relative uncertainty of ΔffCO₂ in relation to the observed difference of Δ¹⁴ between background and station $({\mathrm{\Delta }}_{\text{bg}}^{\text{14}}-{\mathrm{\Delta }}_{\text{meas}}^{14})$. (b) The absolute uncertainty in ppm. Note, the x-axis starts at −2 to account for masking effects.

Figure 8.

Nuclear ¹⁴CO₂ emissions in Europe (annual means for 2018). The black crosses show the locations of the ICOS class-1 stations. See the text for a description of how this map was created.

Figure 9.

Simulated C_photo/C_ff (green) and C_resp/C_ff (red) ratios for the integrated samples collected at the two ICOS sites KRE (a) and OPE (b).