a free-air carbon dioxide enrichment (face) array was assembled in a cotton field at yazoo city, ms, in the summer of 1987 to develop and evaluate control of co2 concentrations over extended periods in an agricultural setting. a variety of system configurations was tested in 17 case-runs, at co2 concentration set points of 500, 600 and 700 ppm. reliable data were obtained from 532 hours out of 602 hours of operation. in general, a better degree of control was obtained at the 500 and 600 ppm set points than at 700 ppm. in the worst case, 1-second observations differed by more than 20 of the 700 ppm set point for no more than 17 of the time. in the best case, 1-sec co2 concentration was 20 or more away from the 500 ppm set point for no more than 6 of the time. the coefficients of variation for daytime measurements at a 600 ppm set point were 0.16, 0.07, 0.03, and 0.02, for 1-second, 5-minute, 30-minute and 1-hour aggregations, respectively, indicating the effect of data aggregation on perceived performance. over all, the face system greatly exceeded performance criteria. analysis and discussion of factors influencing system performance are presented. field site selection and layout, advantages and disadvantages of the yazoo city field site, and the physical face system and its control equipment and software are described. 6 refs., 16 figs., 7 tabs. (era citation 14:027063)