An engineering model is presented which represents the distribution of CO₂ within and downwind of a FACE array in terms of superimposed gaussian plumes. Each plume emanates from an injection port in one of the FACE vertical ventpipes, as approximated by an upwind virtual source using straight-line trajectories. The virtual sources are positioned to correspond to the asymptotic jet injection trajectories, which are initially at oblique angles to the wind in both horizontal and vertical planes. Jet penetration distances and angles are estimated using empirical relationships from the literature. The virtual source locations also depend on the rates of turbulent dispersion, which are affected by canopy conditions. Initial results using this calculation scheme are in good agreement with the observations. These calculations indicate the importance of the initial jet configurations in establishing the desired vertical concentration profiles. During the 1989 field experiments at Maricopa, these configurations were changed as the crop matured. Our initial calculations indicate that the overall efficiency of CO₂ use in FACE is also sensitive to these initial conditions.