Implication of weekly and diurnal ¹⁴C calibration on hourly estimates of CO-based fossil fuel CO₂ ata moderately polluted site in southwestern Germany

A 7-year-long data set of integrated high-precision ¹⁴CO₂ observations combined with occasional hourly ¹⁴CO₂ flask data from the Heidelberg sampling site is presented. Heidelberg is located in the highly populated and industrialized upper Rhine valley in southwestern Germany. The ¹⁴CO₂ data are used in combination with hourly carbon monoxide (CO) observations to estimate regional hourly fossil fuel CO₂ (ΔFFCO₂) mixing ratios. We investigate three different ¹⁴C calibration schemes to calculate ΔFFCO₂: (1) the long-term median ΔCO/ΔFFCO₂ ratio of 14.6 ppb ppm^–1 (mean: 15.5 ± 5.6 ppb ppm^–1), (2) individual (2-) week-long integrated ΔCO/ΔFFCO₂ ratios, which take into account the large week-to-week variability of ±5.6 ppb ppm^–1 (1σ; interquartile range: 5.5 ppb ppm^–1), and (3) a calibration which also includes diurnal changes of the ΔCO/ΔFFCO₂ ratio. We show that in winter a diurnally changing ΔCO/ΔFFCO₂ ratio provides a much better agreement with the direct ¹⁴C-based hourly ΔFFCO₂ estimates whereas summer values are not significantly improved with a diurnal calibration. Using integrated ¹⁴CO₂ samples to determine weekly mean ΔCO/ΔFFCO₂ ratios introduces a bias in the CO-based ΔFFCO₂ estimates which can be corrected for with diurnal grab sample data. Altogether our ¹⁴C-calibrated CO-based method allows determining ΔFFCO₂ at a semi-polluted site with a precision of approximately ±25%.