Estimation of sea surface pCO₂ and air–sea CO₂ flux in the East China Sea using in-situ and satellite data over the period 2000–2016

The partial pressure of carbon dioxide (pCO₂) is an important variable to characterize the state of the seawater carbonate system. It is required to calculate the air-sea CO₂ flux (FCO₂) and for understanding ocean acidification. Surface ocean pCO₂ is influenced by thermodynamic process, physical mixing, biological activity and air-sea gas exchange. Knowledge of the surface pCO₂ in the East China Sea (ECS) is limited due to a lack of observations and well-informed models. This study employed in situ hydrographic (sea surface temperature and salinity) and carbonate system observations combined with satellite observations of chlorophyll a to develop a regional multiple non-linear regression (MNR) model to estimate pCO₂ for three different physical-biogeochemical domains of the ECS. Comparison of the pCO₂ from in situ observations with the MNR model had a root mean square error (RMSE) of 45.19 μatm and coefficient of determination (R²) of 0.87 in the transitional domain, an RMSE of 11.59 μatm and R² of 0.92 in the shelf-dominated domain and an RMSE of 3.73 μatm and R² of 0.97 in the open water domain. The reliability of the MNR model was also validated using an independent dataset with a good accuracy (RMSE of 9.15 μatm for domain II and RMSE of 7.71 μatm for domain III). The MNR model was applied to estimate the surface pCO₂ and FCO₂ for ten cruises on the period of 2013–2018, and also used to predict monthly surface pCO₂ by using Changjiang Biology Finite-Volume Coastal Ocean Model (FVCOM) outputs for the period of 2000–2016 on the ECS inner shelf, and analyzed the seasonal and interannual variability of surface pCO₂ and FCO₂. The seasonal dynamics of sea surface pCO₂ were mainly attributed to temperature changes, biological activities and water mixing process over the seasons. The time series of surface pCO₂ and FCO₂ on the ECS inner shelf showed an increasing trend of 3.32 ± 0.79 μatm yr⁻¹ and 0.018 ± 0.0125 mol CO₂ m⁻² yr⁻², and the region was a CO₂ sink with an CO₂ uptake of 0.12 ± 0.48 Tg C yr⁻¹.