terrestrial-atmospheric carbon balances are currently poorly understood and are vital for predicting future trends in the global carbon budget. uncertainties in c exchange rates can best be reduced by realistic experiments to determine flux rates in managed and natural ecosystems. such experiments must minimize any artificiality imposed by the apparatus and should be capable of economical and reliable operation over entire growing seasons or longer. brookhaven national laboratory (bnl) has developed a system for exposing field-grown plants to controlled elevated concentrations of atmospheric gases, including co(sub 2), without use of confining chambers that alter important atmospheric exchange processes. this system, called face for free air co(sub 2) enrichment, has been proven in over 15000 hours of testing at three locations. face uses a 15--27 m. diameter array of vertical ventpipes and offers significant improvements over previous attempts at chamber-free fumigation. bnl's face arrays have demonstrated these capabilities in experiment on field-grown cotton. this paper reviews the design concepts of face and presents operational results from several recent field campaigns involving seasonal fumigation of field-grown cotton with co(sub 2) at concentrations ranging from 500--700 ppm. the use of chamber-free experiments in conjunction with mathematical models of plant growth is discussed as a paradigm for assessing climate change effects on vegetation. 14 refs., 6 figs., 1 tab. (era citation 16:018426)