To prevent this, CO 2 is released from an arc that encompasses 12 VVPs over 135 degrees of the VVP circle. This arrangement was determined from empirical observations of the arrangement of open VVPs that produced the most even spatial distribution of CO 2 concentration under a wide range of wind speed and turbulence conditions. At wind speeds below the anemometer stall threshold, it will take at least 40 s for an adjustment in the rate of CO 2 release to traverse the air handling system, be emitted from the VPPs, and carried across the plot, to be detected at the control point in the center. Therefore, when wind speed dropped below that threshold for a 20 s period, directional control was terminated and every other VVP around the FACE ring was opened. The triplet emitter ports were spaced at 0.5 m vertical intervals from 2 m above the ground to the top of the pipe.
Air is sampled from a control point at the center of the array.
The sampled air is pumped at 2.5 l min -1 through approximately 10 meters of 4.3 mm diameter polypropylene tubing before it reaches the analyzer. The time for the sample to traverse this distance, along with measurement delays inherent in the analyzer, resulted in a total delay of 10 to 12 s between when the air sample entered the sampling tube and when the resulting concentration was delivered to the control program. The CO 2 signal attenuation in the tubing was evaluated by Farrow et al. The large CO 2 enriched volumes available at a FACE research facility encourages concurrent research by teams of investigators, who can study different aspects of ecosystem response to CO 2 enrichment. Since their studies are temporally and spatially co-located, the individual research studies can be integrated into a comprehensive view of the functioning of the ecosystem under elevated atmospheric CO 2 conditions.