Ozone, nitric oxides, and total reactive nitrogen, measured at two stations in the vicinity of Milan, Italy, are used to infer information on the reactive organic gas (ROG) versus NOx sensitivity of ozone production. At the rural station, ozone production per NOx consumed is less efficient when the advected air masses originate from Milan. This lower efficiency is indicative of ROG-sensitive ozone production. Furthermore, the indicator variables H2O2/NOz, HCHO/NOy, and O3/NOz are applied to both sets of data. The data set with lower ozone production efficiency consistently shows lower indicator values. These findings corroborate the interpretation that ozone production within the Po basin has a tendency to be NOx sensitive and that ROG-sensitive ozone production is only possible within the Milan urban plume. Important links to model calculations for the domain are highlighted.