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%.