It is shown from the mass conservation and the continuity equations that the net ecosystem exchange of a scalar constituent with the atmosphere should consist of three terms: storage below the height of flux observation, eddy flux, and a mass flow component arising from horizontal flow divergence/convergence or a non-zero mean vertical velocity at the height of flux observation. The last term, unaccounted for in previous studies of surface-air exchanges, becomes important over tall vegetation and at times when the vertical gradient of the atmospheric constituent is large, as is the case with carbon dioxide in forests at night. Experimental evidence is presented to support the postulation that the mass flow component is in large part responsible for the large run-to-run variations in eddy fluxes, the lack of energy balance closure and the apparent low eddy fluxes at night under stable stratifications.
Three mechanisms causing the non-zero mean vertical velocity are discussed. Of these, drainage flow on undulating terrain is the most important one for long-term flux observations because only a small terrain slope is needed to trigger its occurrence. It is suggested from the data obtained at a boral deciduous forest that without proper account of the mass flow component, the assessment of annual uptake of carbon dioxide could be biased significantly towards higher values. It is argued that quantifying the mass flow component is a major chanllenge facing the micrometeorological community.