Measurements of the mole fraction of the CO<sub>2</sub> and its isotopes were performed in Paris during the MEGAPOLI winter campaign (January–February 2010). Radiocarbon (<sup>14</sup>CO<sub>2</sub>) measurements were used to identify the relative contributions of 77% CO<sub>2</sub> from fossil fuel consumption (CO<sub>2</sub>ff from liquid and gas combustion) and 23% from biospheric CO<sub>2</sub> (CO<sub>2</sub> from the use of biofuels and from human and plant respiration: CO<sub>2</sub>bio). These percentages correspond to average mole fractions of 26.4 ppm and 8.2 ppm for CO<sub>2</sub>ff and CO<sub>2</sub>bio, respectively. The <sup>13</sup>CO<sub>2</sub> analysis indicated that gas and liquid fuel contributed 70% and 30%, respectively, of the CO<sub>2</sub> emission from fossil fuel use. Continuous measurements of CO and NO<sub>x</sub> and the ratios CO/CO<sub>2</sub>ff and NO<sub>x</sub>/CO<sub>2</sub>ff derived from radiocarbon measurements during four days make it possible to estimate the fossil fuel CO<sub>2</sub> contribution over the entire campaign. The ratios CO/CO<sub>2</sub>ff and NO<sub>x</sub>/CO<sub>2</sub>ff are functions of air mass origin and exhibited daily ranges of 7.9 to 14.5 ppb ppm<sup>−1</sup> and 1.1 to 4.3 ppb ppm<sup>−1</sup>, 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 CO<sub>2</sub>, we observed similar diurnal cycles with two maxima during rush hour traffic.