Quantifying Fossil Fuel CO ₂ Enhancements Along an Urban‐Rural Gradient With Radiocarbon Analysis of Turfgrasses

Abstract Atmospheric measurements are needed to verify progress in reducing fossil fuel carbon dioxide (ffCO 2 ) emissions, especially in cities where most ffCO 2 is emitted. However, measurements of CO 2 enhancements alone cannot identify ffCO 2 signals due to complexities in atmospheric dynamics and large natural CO 2 fluxes. Analysis of the radiocarbon ( 14 C) content of urban annual plants can reveal ffCO 2 patterns and is more cost‐effective than air 14 CO 2 sampling, but its use has been limited because of uncertainty in the temporal integration period and because it has not been quantitatively evaluated against other approaches. Here, we analyze the 14 C content of managed perennial turfgrasses collected along an urban to rural gradient in the Greater Los Angeles area. We compare the turfgrass 14 C to measurements of surface CO 2 and total column CO 2 (XCO 2 ). We find that turfgrass 14 C is highly sensitive to local ffCO 2 emissions at the intra‐city scale and captures pronounced differences between urban to rural sites. Despite their different atmospheric footprints, we observe significant correlations between fossil fuel enhancements (C ff ) derived from turfgrass 14 C and total CO 2 enhancements from atmospheric CO 2 measurements. Furthermore, we combine the turfgrass 14 C and surface CO 2 measurements to quantify the portion of excess CO 2 attributable to biospheric fluxes (C bio ). We find that the turfgrass 14 C is dominated by a fossil fuel signal and shows minimal influence of biogenic CO 2 fluxes. We show that turfgrass 14 C analysis can become a useful tool for quantifying ffCO 2 trends in cities that lack permanent surface CO 2 and XCO 2 measurement infrastructure.