Atmospheric CO₂ dynamics in a coastal megacity: spatiotemporal patterns, sea-land breeze impacts, and anthropogenic-biogenic emission partitioning

Abstract. Accurate quantification of urban carbon dioxide (CO₂) emissions is essential for evaluating the efficacy of urban climate mitigation policies. However, the complex interplay of anthropogenic emissions, biogenic fluxes, and meteorological processes in coastal megacities poses significant challenges to characterizing urban CO₂ dynamics. To address this, we present an observation-based framework that integrates high-precision CO₂ monitoring, meteorological analyses, and ΔCO/ΔCO₂ ratios (Rco) to resolve spatiotemporal CO₂ variations, quantify sea-land breeze (SLB) effects, and partition anthropogenic and biogenic contributions. Applied in Guangzhou, a coastal megacity, our approach captures a pronounced urban–rural gradient. The coastal site shows the largest seasonal amplitude (25.63 ppm), resulting from wintertime transport of urban emissions and summertime inflow of marine air. Diurnally, suburban CO₂ variations are dominated by biogenic activity (summer amplitude: 39.90 ppm), while urban signals reflect anthropogenic influence. SLB generally reduces coastal CO₂ by 5.87 ppm but leads to a summer accumulation (+2.08 ppm) under stable, low-wind conditions with shallow boundary layers. Regression-derived Rco values (urban: 7.45 ± 1.38 ppb ppm⁻¹) reflect improved combustion efficiency linked to clean-air policies. Importantly, our combined observational, modeling, and Rco framework reveals that biogenic fluxes offset 60.17 % of anthropogenic CO₂ emissions during summer afternoons. The framework is validated against emission inventories, Normalized Difference Vegetation Index data, and independent studies, demonstrating its robustness. This study enhances process-oriented understanding of coastal carbon cycling and underscores the integration of meteorological and biospheric dynamics in urban CO₂ assessments.