Aircraft measurements of O₃, NO, NO₂, NO_y, HCHO, and H₂O₂ over the Swiss Plateau during 4 days in July 1993 were analyzed. Special emphasis was put on the urban plume of Zurich. An effective photochemical age for an urban plume was introduced, which accounts for a background NO_z concentration. Only the effective photochemical age of the urban air mass increased with the time of transport between Zurich and the flight leg, where the aircraft crossed the plume. Ozone and NO_z were strongly correlated. Ozone production rates ranged from 2.4 to 6.5 ozone molecules produced per NO_x processed. The production of ozone per NO_x molecule was lowest when the effective photochemical age was lowest and vice versa. Good correlations between HCHO and NO_y have been found in the urban plume of Zurich as well as over the Swiss Plateau. Between H₂O₂ and NO_z, a negative correlation was observed. On the basis of Sillman's [1995] indicator species, the ozone production in the Zurich plume and other air masses over the Swiss Plateau is in the NO_x-sensitive range. However, there remains some uncertainty in this approach regarding the influence of biogenic emissions and the initial concentration of indicator species. Taking the low emissions of biogenic hydrocarbons compared to Sillman's model calculation into consideration, the central portion of the Zurich plume falls in the transition or even reactive organic gases (ROG)- limited range of all indicators. Estimates of production rates of HNO₃ and peroxides support a ROG-sensitive ozone production in the most polluted portion of the plume.