Evaluation of Webb Correction on CO2 Flux by Eddy Covariance Technique Using Open-Path Gas Analyzer over Asphalt Surface

Abstract

Webb correction is indispensable when evaluating trace gas flux by the eddy covariance technique using an open-path gas analyzer. In the present study, direct measurements of CO2 flux by the eddy covariance technique using an open-path CO2/H2O gas analyzer were carried out over an asphalt surface where CO2 and latent heat fluxes were considered almost zero. The power spectrum of CO2 density fluctuation had similar frequency structure to those of vertical wind velocity, sound virtual temperature, and water vapor density fluctuations. The cospectrum of raw CO2 flux was negatively correlated with that of sensible heat flux. The cospectrum of latent heat flux was almost zero and poorly correlated with that of raw CO2 or sensible heat fluxes. Raw CO2 flux was apparently downward from the atmosphere to the asphalt surface during this observation period. The Webb CO2 correction term for sensible heat flux was upward and larger than the raw CO2 flux, although the Webb CO2 correction term for latent heat flux was almost zero. Total latent heat and CO2 fluxes were evaluated as significant upward fluxes. This result shows that Webb correction may cause an overestimated correction to CO2 flux by the eddy covariance technique using an open-path CO2/H2O gas analyzer over surfaces, especially where small CO2 flux by soil respiration or air-sea exchange process is observed.