Abstract
<jats:p>Abstract. Urban net CO2 fluxes measured by eddy covariance (EC) cannot be treated as anthropogenic emissions without biogenic constraints, because they integrate fossil-fuel sources, biogenic processes, human respiration, and changing footprints. Existing partitioning approaches often require isotopes, tracers, multi-species fluxes, or clean vegetation/emission sectors unavailable at many urban sites. We developed a footprint-informed ecological parameter-migration framework to constrain anthropogenic CO2 fluxes (Fff) in mixed urban source areas of Guangzhou, a humid subtropical megacity. A highly vegetated, weakly disturbed suburban donor site provided ecological parameters, which were transferred to a mixed urban target site using dynamic footprints and footprint-weighted enhanced vegetation index (EVIfp). The migrated parameters produced plausible diurnal and seasonal gross primary productivity (GPP) and ecosystem respiration (Reco), supporting the donor–target constraint. At the target site, daytime biogenic uptake masked anthropogenic emissions and occasionally drove net CO2 flux toward neutral or weakly negative values. After subtracting Reco, GPP, and human respiration, Fff remained consistently positive throughout the day (1.80–5.14 μmol m−2 s−1). Fff showed morning and evening peaks consistent with NOx, CO, wind-sector source areas, and traffic indicators, while footprint-aligned inventories provided magnitude context and suggested spatial-proxy mismatch in a high-population urban functional zone with relatively low on-site combustion. Uncertainty was dominated by GPP light-response structure, with smaller effects from other perturbations. These results demonstrate the feasibility of extracting footprint-scale Fff from mixed urban source areas using conventional EC, remote-sensing, and footprint data where isotopic or multi-species flux observations are unavailable.</jats:p>