Free-air CO2 enrichment (FACE) of a poplar plantation: the POPFACE fumigation system

A new design of free-air CO2 enrichment (FACE) is presented that has been u sed to expose a poplar plantation to elevated atmospheric CO2 concentration s in otherwise unaltered conditions, in the open. This system releases pure CO2 at high velocity, through a large number of s mall gas jets, causing rapid mixing between CO2 and air. The theoretical an d practical aspects of this design are described, with emphasis on the flui d mechanics of air-CO2 mixing in sonic jets. Field performance data, includ ing spectral analysis of short-term fluctuations in CO2 concentrations as w ell as temporal and spatial CO2 control, are reported for the European proj ect POPFACE facility. Temporal and spatial performances of the operational POPFACE systems were a dequate with average long-term CO2 mole fractions on target. Averages over 1 min measured in the centre of the rings were within +/-20% and +/-10% of the target concentration for > 91% and > 75% of the time, respectively. The data presented provide convincing evidence that a pure-CO2 FACE system can achieve reliable control, in terms of the quality of the CO2 control, w ith significant simplification of construction and reduced capital cost.