Measurements made from the DOE G-1 aircraft are used to calculate the rate and efficiency of O₃ production down wind of an isolated coal-fired power plant. The plume was transected at a constant altitude 12 times, up to a distance of 65 km from its source (corresponding to an age of ~4 h assuming constant wind velocity). For NO_x, a loss rate of 0.5 h^-1 is calculated. If reaction with OH is the sole loss mechanism, then an [OH]=1.6 x 10⁷ molec/cm³ is implied, which is ~2 - 3X values calculated using a box model constrained by observations. Possible explanations for this discrepancy are discussed. O₃ production efficiency per molecule of NO_x approaches 6-8 after the plume has aged >3h. Peak O₃ concentrations were 15 ppbv above background. Dilution appears to limit the peak O₃ concentration despite the high production efficiency. Hydrocarbon samples indicate high levels of VOC reactivity (~8 s^-1) in the plume. The number concentration of accumulation mode particles increases significantly with plume age indicating a rapid formation of aerosol mass.