Measurements of photooxidants made during summer 1994 in the southern part of Switzerland are compared with results of a one-dimensional Lagrangian model. The agreement between modeled and measured ozone concentrations is good. Concentrations of highly soluble species such as H2O2 and HNO3 are overestimated by the model. The model underestimates concentrations of isoprene, HCHO, and HONO. The impact of this discrepancy on the question of NOx versus VOC limitation of the ozone production is discussed. According to the model, reduction of the NOx emissions is more effective in reducing O3 than reduction of the VOC emissions for a large fraction of the area under investigation. This finding is in accordance with what we concluded from the data. The southern part of Switzerland is often impacted by a plume originating from Milan, which leads to additional ozone at the measuring site. We used the model to simulate the influence of this plume on the chemical regime. The plume emissions are picked up by the model at two different times. Both cases deliver the same concentrations for the end point of the calculations. If the emissions are picked up early in the morning, the ozone production becomes VOC sensitive for a few hours before switching back to being NOx sensitive. If the emissions are picked up later, the O3 production stays NOx sensitive all the time. Forcing the model to reproduce measured concentrations of HONO and isoprene shifts the O3 production toward being more NOx sensitive.¿ 1997 American Geophysical Union