Cities are hotspots for fossil fuel carbon dioxide (ffCO₂) emissions, but it is challenging to monitor these emissions at local scales because of complex wind patterns and large natural CO₂ fluxes. The unique radiocarbon signature of fossil fuels provides a means to directly isolate the ffCO₂ contribution in the total CO₂ signal and is commonly achieved via radiocarbon analyses of urban samples. This study demonstrates that radiocarbon analyses of turfgrasses can capture variations in ffCO₂ emissions in and around the Los Angeles Megacity. We find that turfgrass-based estimates of ffCO₂ emissions matched the amounts of excess CO₂ in urban air measured by two atmospheric CO₂ observing networks. Our findings support the use of plants as ffCO₂ monitors in cities to guide and track progress in decarbonization efforts without needing to establish any permanent instrumentation.