The O₃-NO _y relation for a polluted planetary boundary layer is investigated using numerical simulations and observations. Box model runs are carried out to study the sensitivity of the transition value of NO _y , which separates the low-NO _x . and the high-NO _x regimes, to changes in emissions and in ambient conditions. It is shown that the transition value of NO _y depends on the hydrocarbon and NO _x emissions, on water vapor, on radiation, and on temperature. Increasing hydrocarbon emissions, increasing water vapor, increasing radiation, or increasing temperature shifts the transition value to higher levels. One-dimensional simulations, which include vertical mixing and dry deposition, show that increasing dry deposition reduces the transition value of NO _y . Sensitivity studies with a three-dimensional nonhydrostatic model system are carried out to investigate the effects of transport. A smoothing procedure is applied to reduce the large scatter of the O₃-NO _y relation which is found in these cases. After smoothing, a transition value can be identified. For the southwestern part of Germany it is shown that this transition value indicates areas where NO _x reduction leads to the most effective ozone reduction. Runs with various scenarios show the dependence of the transition value on emissions and ambient conditions. A normalization procedure is introduced. The normalization procedure is applied to the model results and to observations. Under high-NO _x conditions a large difference between the cases with and without advection is found. It shows that in the three-dimensional case the ozone concentration is less sensitive to changes in the NO _x emissions than in the case of pure chemistry.