Measurements of O3, NO2, HCHO, H2O2, and hydrocarbons were performed onMetAir's airborne sampling platform in the area of Milan within the Italian Po Basin. Theobjective was to characterize the O3 formation process and its dependence on ambient levelsof NOx and volatile organic compounds (VOCs). In the metropolitan area and downwind aregional plume developed of the order of 50 km wide and atop of this an urban plume 10--15km wide. In the Po Basin, ozone mixing ratios were around 80--130 ppb, while in the regionalplume, ozone levels were 15--30 ppb higher. In addition, the urban plume adds another 10--30ppb. Maximum afternoon ozone mixing ratios reached 195 ppb. Downwind of Milan, ozonestrongly increased (0.6--1 ppb km-1), whereas peroxide usually decreased. An observationbased budget analysis of the photooxidant formation yielded chemical ozone production ratesbetween 5 and 15 ppb h-1, occasionally exceeding 20 ppb h-1. The chemical peroxide production ratewas negligible in the urban and regional plume and small in the adjacent areas. Ozoneproduction rates derived from an observation-driven steady state model were mostly lowerthan calculated from the budget approach, while peroxide production rates were negligible.Missing VOCs due to incomplete sampling might be responsible for the discrepanciesbetween the model and measurements. The analysis indicates VOC-sensitive ozone formationwithin and downwind of the metropolitan area of Milan. This would still be the result of themodel calculation in the case of doubled VOC levels.