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 used to expose a poplar plantation to elevated atmospheric CO2 concentrations in otherwise unaltered conditions, in the open. This system releases pure CO2 at high velocity, through a large number of small gas jets, causing rapid mixing between CO2 and air. The theoretical and practical aspects of this design are described, with emphasis on the fluid mechanics of air-CO2 mixing in sonic jets. Field performance data, including spectral analysis of short-term fluctuations in CO2 concentrations as well as temporal and spatial CO2 control, are reported for the European project POPFACE facility. Temporal and spatial performances of the operational POPFACE systems were adequate 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, with significant simplification of construction and reduced capital cost.