The usefulness of the indicator H2O2/HNO3 to separate NOx-sensitive and VOC-sensitive regions in the polluted planetary boundary layer on the regional scale is investigated using numerical simulations. The results of box model runs are applied to highlight individual definitions of the NOx-VOC-ozone sensitivity. In particular, different ways to determine the transition value of the indicators H2O2/HNO3 and NOy are explained. Three-dimensional model runs are performed for two model domains in Germany and one domain in the Po valley (Italy). These domains differ significantly in the spatial distribution of the emissions, in the absolute values of the emissions, and in the composition of the VOC. The domains also differ in their meteorological conditions and hence in the involved transport processes. Nevertheless, the transition values of H2O2/HNO3 found for the individual domains differ from 0.2 by less than 15%. This transition value is applied to an emission reduction based on the assumption of all aromatic compound emissions being switched off. It shows that this transition value and the horizontal distribution of H2O2/HNO3 of the base case allow a forecast of those regions where ozone is affected by this emission reduction. This offers the possibility to apply the indicator H2O2/HNO3 and the transition value of 0.2 to observations.