CO, NO_x and ¹³CO₂ as tracers for fossil fuel CO₂: results from a pilot study in Paris during winter 2010

Abstract. Measurements of the mole fraction of the CO₂ and its isotopes were performed in Paris during the MEGAPOLI winter campaign (January–February 2010). Radiocarbon (¹⁴CO₂) measurements were used to identify the relative contributions of 77% CO₂ from fossil fuel consumption (CO₂ff from liquid and gas combustion) and 23% from biospheric CO₂ (CO₂ from the use of biofuels and from human and plant respiration: CO₂bio). These percentages correspond to average mole fractions of 26.4 ppm and 8.2 ppm for CO₂ff and CO₂bio, respectively. The ¹³CO₂ analysis indicated that gas and liquid fuel contributed 70% and 30%, respectively, of the CO₂ emission from fossil fuel use. Continuous measurements of CO and NO_x and the ratios CO/CO₂ff and NO_x/CO₂ff derived from radiocarbon measurements during four days make it possible to estimate the fossil fuel CO₂ contribution over the entire campaign. The ratios CO/CO₂ff and NO_x/CO₂ff are functions of air mass origin and exhibited daily ranges of 7.9 to 14.5 ppb ppm⁻¹ and 1.1 to 4.3 ppb ppm⁻¹, respectively. These ratios are consistent with different emission inventories given the uncertainties of the different approaches. By using both tracers to derive the fossil fuel CO₂, we observed similar diurnal cycles with two maxima during rush hour traffic.