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

A 7-year-long data set of integrated high-precision 14CO(2) observations combined with occasional hourly 14CO(2) 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 14CO(2) data are used in combination with hourly carbon monoxide (CO) observations to estimate regional hourly fossil fuel CO2 (delta FFCO2) mixing ratios. We investigate three different 14C calibration schemes to calculate delta FFCO2: (1) the long-term median delta CO/delta FFCO2 ratio of 14.6 ppb ppm-1 (mean: 15.5 +/- 5.6 ppb ppm-1), (2) individual (2-)week-long integrated delta CO/delta FFCO2 ratios, which take into account the large week-to-week variability of +/- 5.6 ppb ppm-1 (1 Sigma; interquartile range: 5.5 ppb ppm-1), and (3) a calibration which also includes diurnal changes of the delta CO/delta FFCO2 ratio. We show that in winter a diurnally changing delta CO/delta FFCO2 ratio provides a much better agreement with the direct 14C-based hourly delta FFCO2 estimates whereas summer values are not significantly improved with a diurnal calibration. Using integrated 14CO(2) samples to determine weekly mean delta CO/delta FFCO2 ratios introduces a bias in the CO-based delta FFCO2 estimates which can be corrected for with diurnal grab sample data. Altogether our 14C-calibrated CO-based method allows determining delta FFCO2 at a semi-polluted site with a precision of approximately +/- 25%.