Constraining urban biogenic CO₂ fluxes: Composition, seasonality and drivers from radiocarbon and inventory analysis

Abstract. Urban areas play a pivotal role in achieving net-zero emissions to limit global warming to 1.5 °C, given their high carbon footprint and mitigation potential. Accurate quantification of urban CO₂ sources is essential for effective carbon budgeting and targeted climate action. While fossil fuel CO₂ (CO_2ff) emissions are extensively studied, biogenic CO₂ (CO_2bio) dynamics remain poorly constrained. Here, we separate fossil and biogenic contributions to CO₂ enhancements above background using Δ¹⁴C and CO₂ measurements in Shenzhen, a humid subtropical Chinese megacity potentially subject to substantial biomass burning influence. We calculate human/livestock metabolic emissions (CO_2HLM) at 9.32 Mt/6.22 kt per year from population/livestock data and respiratory/excretory rates, and estimate biomass burning emissions (CO_2BB) at 5.05 Mt/yr using an inventory encompassing both open and domestic combustion. The residual CO_2bio component is attributed to the terrestrial biosphere (CO_2bio'). Integrating Δ¹⁴C with multi-source data reveals annual CO_2bio contributions relative to fossil fluxes: CO_2HLM (17.8 ± 3.1 %), CO_2BB (9.2 ± 1.5 %), and CO_2bio' (73.0 ± 3.5 %). Key findings demonstrate the terrestrial biosphere component acts as a year-round net carbon sink with significant seasonality (11.5 ppm amplitude), driven primarily by atmospheric temperature (1–2 months lag; r = –0.80, p = 0.01) rather than precipitation. This study establishes human metabolic emissions as the dominant biogenic CO₂ source (17.8 % vs. 9.2 % from BB) in megacities, yet shows that concurrent biospheric sequestration can offset 63 % of fossil emissions during growing seasons, advancing understanding of urban carbon budgets.