Assessment of the Webb-Pearman-Leuning Correction Method for Estimating CO₂ Flux in a Tropical Coastal Sea

Abstract. CO₂ fluxes in coastal waters are vital for the global carbon cycle. The Eddy Covariance technique was used with open-path gas analyzers to estimate CO₂ fluxes. However, these analyzers can lead to overestimation due to water vapor and temperature effects, and the Webb-Pearman-Leuning (WPL) correction method was applied to improve the accuracy of the estimated CO₂ flux. This study investigates the application of the WPL correction method on CO₂ flux measurements over a tropical coastal sea location. The analysis reveals that the CO₂ flux in the coastal waters mainly functions as a sink, with the diel cycle showing smaller flux magnitudes during the day and increased uptake during the night. The application of the WPL correction can result in sign changes of CO₂ flux, indicating a shift from a CO₂ sink to a CO₂ source. These sign changes occur frequently, particularly during afternoon hours, and can significantly impact the implications regarding carbon exchange. The WPL correction parameters, especially those related to temperature and water vapor fluctuations, play crucial roles in influencing the CO₂ flux variations. The decrease in dry air molar density and increased vertical wind speed within the correction related to water vapor fluctuations are the major reasons for the sign change of the CO₂ flux. Based on the quality flagging of the WPL correction, the non-sign change CO₂ fluxes are predominantly considered reliable data, while most of the sign change fluxes should be specially checked.