The air-sea CO₂ flux in marginal seas is an important component of the global ocean carbon cycle. Long-term field observations are crucial for scientific estimation of the complex and variable air-sea CO₂ fluxes in marginal seas. Located between the East China Sea and the northern South China Sea, the southeastern coast of China has large potential of CO₂ sink, but studies of air-sea CO₂ fluxes in this area are very limited. By combining sea surface CO₂ partial pressure (pCO₂) and auxiliary parameter data from 51 cruises from 2001 to 2022, air-sea CO₂ fluxes in the southeastern coast of China were estimated. Surface water pCO₂ exhibited conspicuous spatial and temporal variabilities. Surface water was high in summer (373±34 μatm) and fall (390±26 μatm), but low in spring (359±19 μatm) and winter (349±20 μatm). Surface water pCO₂ was primarily modulated by vertical mixing and cooling during cold seasons, and coastal upwelling, biological productivity and other factors in warm seasons. Normalized to the average temperature, NpCO₂increased linearly with latitude the South of Taiwan Shoal. Air-sea CO₂ fluxes exhibited strong seasonal variations. It was a strong sink of -9.4±5.5 mmol m⁻² d^-1 in winter, a moderate sink of -3.4±3.7 mmol m⁻² d^-1 in spring and near equilibrium with the atmosphere in summer with air-sea CO₂ fluxes of -0.5±2.7 mmol m⁻² d^-1. However, it was a weak source of 3.4±6.7 mmol m⁻² d^-1 in fall. The area-weighted annual average air-sea CO₂ flux was -2.7±5.9 mmol m⁻² d^-1, indicating a moderate sink annually.

Southeastern coast of China;,Surface water pCO2;,Air-sea CO2 flux;,Seasonal variability;,Spatial variability