[en] Field studies have confirmed O₃ depletion inside power plant plumes, due to the rate-limited cycle between NO₂ and O₃. A reactive power plant plume model has been developed that includes a description of the complex photochemical system and a dispersion mechanism for mass transfer in the horizontal cross-wind direction. In addition, the model uses a one dimensional diffusion module to simulate the vertical transport of chemical species based on diffusivity profiles which are dependent on meteorological conditions. The model was applied to the plume from Nanticoke thermal generating station (TGS) in Ontario, released at 7:00 am and travelling for 12 hours towards the north-north-east at 5 m/s. The preliminary results suggest that under the emission conditions that were obtained, O₃ in the plume centre is reduced compared to the case in which the plume is absent. The O₃ reduction within the plume increases as the NOx emission from Nanticoke TGS increases. The general conclusion is confirmed by relaxing the local emissions and the dispersion parameter σ_y over a reasonable range. Further investigation shows that the O₃ reduction is consistent with current knowledge of nonlinear photochemical systems. 32 refs., 22 figs