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

Abstract

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 (²;FFCO²) mixing ratios. We investigate three different ¹⁴C calibration schemes to calculate²;FFCO²: (1) the long-term median²;CO/²;FFCO² ratio of ¹⁴.6 ppb ppm¹ (mean: 15.5 ± 5.6 ppb ppm¹), (²) individual (2-)week-long integrated CO/FFCO² ratios, which take into account the large week-to-week variability of±5.6 ppb ppm¹ (1ó; interquartile range: 5.5 ppb ppm¹), and (3) a calibration which also includes diurnal changes of the CO/FFCO² ratio.We show that in winter a diurnally changing CO/FFCO² ratio provides a much better agreement with the direct ¹⁴C-based hourly FFCO² estimates whereas summer values are not significantly improved with a diurnal calibration. Using integrated ¹⁴CO₂ samples to determine weekly mean ²;CO/FFCO₂ ratios introduces a bias in the CO-based FFCO² estimates which can be corrected for with diurnal grab sample data. Altogether our ¹⁴C-calibrated CO-based method allows determining FFCO² at a semi-polluted site with a precision of approximately ±25%.