Abstract. Attributing observed carbon dioxide (CO2) to fossil-fuel emissions versus biogenic fluxes is essential for assessing urban mitigation, but in coastal megacities it is complicated by anthropogenic–biogenic coupling and sea–land breeze (SLB) circulation. Here we analyze Guangzhou using multi-site in situ CO2 and CO measurements (January 2023–September 2024), transport footprints, and a site-specific ΔCO / ΔCO2 (RCO) relationship to resolve spatiotemporal variability, quantify SLB effects, and partition fossil-fuel (CO2ff) and biogenic (CO2bio) contributions without assimilating emission inventories. Along a coastal–urban–suburban gradient, the coastal site shows the largest seasonal amplitude, the vegetated site exhibits strong summertime diurnal amplitude, and the urban core is combustion-dominated. These gradients reveal a “coastal CO2 dome” that – unlike urban domes often conceptualized as core-anchored – is seasonally displaced, with peak concentrations shifting away from the core due to the interplay of coastal ventilation and biogenic exchange. SLB effects are seasonal: SLB ventilates CO2 in spring–winter but promotes summertime accumulation (+2.08 ppm) under stable stratification, accompanied by pronounced CO enhancements, consistent with trapped/recirculated combustion plumes. Regression-derived urban RCO is consistent with post-2013 broad tightening of coal/industrial and vehicle-emission controls. Winter-afternoon urban CO2ff attribution remains robust to transport-model configurations and measurement/background uncertainty. Summer-afternoon CO2bio shows substantial biogenic uptake, offsetting ∼ 60 % of concurrent CO2ff. These results demonstrate that coastal dynamics and urban greening reshape observed CO2 signals, highlighting that biogenic–anthropogenic decoupling and SLB-aware sampling are essential for the robust evaluation of carbon mitigation in coastal megacities.
Zhang et al. (2026) studied this question.