Abstract CO 2 treatment level control and CO 2 use are reported for free-air carbon dioxide enrichment (FACE) facility operations at the University of Arizona's Maricopa Agricultural Center in 1990 and 1991. These are required for evaluation of the validity of biological experiments conducted in four replicates of paired experimental and control plots in a large cotton field and the cost-effectiveness of the plant fumigation facility. Gas concentration was controlled to 550 γmol mol -1 at the center of each experimental plot, just above the canopy. In both years, season-long (April–September) average CO 2 levels during treatment hours (05:00–19:00 h Mountain Standard Time) were 550 γmol mol −1 measured at treatment plot centers when the facility was operating. Including downtime, the season average was 548 γmol mol −1 in 1991. In 1990, the season averages for the four elevated CO 2 treatments varied from 522 to 544 γmol mol −1, owing to extended periods of downtime after lightning damage. Ambient CO 2 concentration during treatment was 370 γmol mol −1. Instantaneous measurements of CO 2 concentration were within 10% of the target concentration of 550 γmol mol −1 more than 65% of the time when the facility was operating, and 1 min averages were within 10% of the target concentration for 90% of the time. The long-term average of CO 2 concentration measured over the 20 m diameter experimental area of one array at the height of the canopy was in the range 550–580 γmol mol −1 during July 1991, with the higher values near the edges. In 1991, CO 2 demand averaged 1250 kg per array per 14 h treatment day, or 4 kg m −2 of fumigated plant canopy. The FACE facility provided good temporal and spatial control of CO 2 concentration and was a cost-effective method for large-scale field evaluations of the biological effects of CO 2.