Apparent downward CO₂ flux observed with open-path eddy covariance over a non-vegetated surface

Summary

The open-path eddy covariance (EC) method often shows unlikely downward CO₂ fluxes in late winter and early spring over drained paddy fields with few active plants. To understand why, we carried out intensive measurements in a bare paddy field from 9 to 11 April 2003, simultaneously using open- and closed-path EC methods; aerodynamic and dynamic closed-chamber methods were also used. During this period, the open-path EC method showed downward daytime CO₂ fluxes ranging from 0 to −5.9 µmol m⁻² s⁻¹, even after application of the WPL correction (density correction) and ordinary quality control tests. Because the closed-path EC and aerodynamic methods showed upward CO₂ fluxes, the downward CO₂ fluxes observed with open-path EC appear not to represent true CO₂ transport. Diurnal variations in the downward daytime CO₂ fluxes appeared to be correlated with increases in solar radiation during the day, and also with increases in sensible heat flux in weak winds but not under strongly windy conditions. The daytime 10-Hz time series data of vertical wind and CO₂ mixing ratio demonstrated that updrafts were CO₂ depleted in the open-path system, whereas the same updrafts were CO₂ enriched in the closed-path system. Careful examination of the discrepancies between the open- and the closed-path EC measurements revealed that the amplitudes of the 10-Hz temperature signals from the sonic anemometer and the resultant sensible heat fluxes were too small to compensate for the discrepancies observed during the daytime. The open-path EC method with the conventional application of the WPL correction is not necessarily appropriate for measuring small magnitudes of CO₂ flux (≤5 µmol m⁻² s⁻¹) under such surface and atmospheric conditions that the magnitude of the WPL correction is as great as that of the uncorrected CO₂ flux itself.