Diurnal variability of CO₂ flux at coastal zone of Taiwan based on eddy covariance observation

In this study, we employed shore-based eddy covariance systems for a continuous measurement of the coastal CO₂ flux near the northwestern coast of Taiwan from 2011 to 2015. To ensure the validity of the analysis, the data was selected and filtered with a footprint model and an empirical mode decomposition method. The results indicate that the nearshore air–sea and air–land CO₂ fluxes exhibited a significant diurnal variability and a substantial day–night difference. The net air–sea CO₂ flux was −1.75 ± 0.98 μmol-C m⁻² s⁻¹, whereas the net air–land CO₂ flux was 0.54 ± 7.35 μmol-C m⁻² s⁻¹, which indicated that in northwestern Taiwan, the coastal water acts as a sink of atmospheric CO₂ but the coastal land acts as a source. The Random Forest Method was applied to hierarchize the influence of Chl-a, SST, DO, pH and U₁₀ on air-sea CO₂ fluxes. The result suggests that the strength of the diurnal air-sea CO₂ flux is strongly influenced by the local wind speed.