Pressure correction to the long-term measurement of carbon dioxide flux

Fluctuations in atmospheric CO2 density can arise from static pressure fluctuations but their effect on the long-term eddy covariance (EC) CO2 flux measurement is poorly known. In this paper, we report the results of a 1-year direct measurement of the static pressure fluctuations and the velocity–pressure covariance over a mixed forest in Northeast China. The results show that the pressure–vertical velocity covariance was primarily controlled by friction velocity and air stability. Without the pressure correction, the open-path EC measurement of the nighttime ecosystem respiration was biased low and that of the daytime photosynthetic CO2 uptake was biased high. Over the 1-year measurement period, the cumulative pressure correction was 40 gCm−2, which was about 20% of the annual net ecosystem production of this forest. Using the friction velocity data found in the literature, we estimated the magnitudes of the pressure correction for the major ecosystem types in the long-term global EC network (FluxNet).