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.