The O3-NOy 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 NOy, which separates the low-NOx and the high-NOx regimes, to changes in emissions and in ambient conditions. It is shown that the transition value of NOy depends on the hydrocarbon and NOx 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 NOy. 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-NOy 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 NOx 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-NOx 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 NOx emissions than in the case of pure chemistry. ¿ 1999 American Geophysical Union