Direct quantification of fossil fuel CO2(CO2ff) in atmospheric samples can be used toexamine several carbon cycle and air quality questions. We collected in-situ CO2, CO,and CH4 measurements and flask samples in the boundary layer and free troposphere5 over Sacramento, California, USA, during two aircraft flights over and downwind ofthis urban area during spring of 2009. The flask samples were analyzed for ∆14CO2and CO2to determine the recently added CO2ff mole fraction. A suite of additionalgreenhouse gases including hydrocarbons and halocarbons were measured in thesame samples. Strong correlations were observed between CO2ff and numerous trace10 gases associated with urban emissions. From these correlations we estimate emissionratios between CO2ff and these species, and compare these with bottom-up inventoryderived estimates. Recent county level inventory estimates for carbon monoxide (CO)and benzene from the California Air Resources Board CEPAM database are in goodagreement with our measured emission ratios, whereas older emissions inventories15 appear to overestimate emissions of these gases. For most other trace species, thereare substantial differences between our measured emission ratios and those derivedfrom available emission inventories. For the first flight, we combine in situ CO measurements with the measured CO:CO2ff emission ratio of 14±2 ppbCO/ppmCO2to derivean estimate of CO2ff mole fraction throughout this flight, and also estimate the biospheric CO2 mixing ratio (CO2bio) from the difference of total and fossil CO220 . Theresulting CO2bio varies substantially between air in the urban plume and the surrounding boundary layer air. Finally, we use the in situ estimates of CO2ff mole fraction toinfer total fossil fuel CO2 emissions from the Sacramento region, using a mass balance approach. However the resulting emissions are uncertain to within a factor of25 two due to uncertainties in wind speed and boundary layer height. Nevertheless, thisfirst attempt to estimate urban-scale CO2ff from atmospheric radiocarbon measurements shows that CO2ff can be used to verify and improve emission inventories formany poorly known anthropogenic species, separate biospheric CO2, and indicates the potential to constrain the CO2ff emissions if transport uncertainties are reduced.