Efficiencies of free-air gas fumigation devices

One of the key uncertainties relative to future increases in atmospheric CO{sub 2} is the extent to which growth in future emissions will be accommodated by increased uptake by terrestrial vegetation, the so-called fertilization'' effect. Research on this issue is currently pursued by many research groups around the world, using various experimental protocols and devices. These range from leaf cuvettes to various types of enclosures and glass-houses to various types of open-field gas enrichment or fumigation systems. As research priorities move from crops to forests and natural ecosystems, these experimental devices tend to become large and enrichment gas (i.e., CO{sub 2}) requirements and costs become a major factor in experimental design. This paper considers the relative efficiencies of gas usage for different types of systems currently in use. One of these is the Free Air CO{sub 2} Enrichment System (FACE) designed and developed at Brookhaven National Laboratory (BNL). In this paper, we develop some nondimensional groups of parameters for the purpose of characterizing performance, i.e., enrichment gas usage. These nondimensional groups are then used as figures of merit and basically allow the required flow rates of CO{sub 2} to be predicted based on the geometry of the device, wind speed, and the incremental gas concentration desired. The parameters chosen to comprise a useful nondimensional group must not only have the correct dimensions, they must also represent an appropriate physical relationship