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Summary A new design of free‐air CO 2 enrichment (FACE) is presented that has been used to expose a poplar plantation to elevated atmospheric CO 2 concentrations in other‐wise unaltered conditions, in the open. This system releases pure CO 2 at high velocity, through a large number of small gas jets, causing rapid mixing between CO 2 and air. The theoretical and practical aspects of this design are described, with emphasis on the fluid mechanics of air–CO 2 mixing in sonic jets. Field performance data, including spectral analysis of short‐term fluctuations in CO 2 concentrations as well as temporal and spatial CO 2 control, are reported for the European project POPFACE facility. Temporal and spatial performances of the operational POPFACE systems were adequate with average long‐term CO 2 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‐CO 2 FACE system can achieve reliable control, in terms of the quality of the CO 2 control, with significant simplification of construction and reduced capital cost.
Miglietta et al. (Tue,) studied this question.