We propose a new approach to estimate urban emission ratios that takes advantage of the enhanced local urban signal in the atmosphere at low wind speed. We apply it to estimate monthly ratios between CO$_2$, CO and some VOCs from several atmospheric concentration measurement datasets acquired in the centre of Paris between 2010 and 2014. We find that this approach is little sensitive to the regional background level definition and that, in the case of Paris, it samples all days (weekdays and weekends) and all hours of the day evenly. A large seasonal variability of the $\Delta$CO/$\Delta$CO$_ 2$ratio in Paris is shown, with a difference of around 60 % between the extreme values and a strong anti-correlation ($r^2$ = 0.75) with atmospheric temperature. The comparison of the ratios obtained for two short measurement campaigns conducted in two different districts and two different periods (fall and winter) shows differences ranging from -120 to 63 %. A comparison with a highly resolved regional emission inventory suggests some spatial variations of the ratio within the city, although most of these differences seem to be rather driven by the seasonal variability.