This is only true in some regions. The creation of Ozone in the troposphere is dependent on availability of NOx, and availablity of hydrocarbons. In the southeast US, forests produce large amounts of volatile hydrocarbons, so the system is NOx-limited. In California's Central Valley, excess NOx from nitrogen fertilizers makes ozone production hydrocarbon-limited (and also basic rain from the NH3). So from a pollution management standpoint, either could be important, but usually only one.