Photochemical production and aging of an urban air mass

Aircraft measurements of O-3, NO, NO2, NOy, HCHO, and H2O2 over the Swiss P lateau during 4 days in July 1993 were analyzed. Special emphasis was put o n the urban plume of Zurich. An effective photochemical age for an urban pl ume was introduced, which accounts for a background NOz concentration. Only the effective photochemical age of the urban air mass increased with the t ime of transport between Zurich and the flight leg, where the aircraft cros sed the plume. Ozone and NOz were strongly correlated, Ozone production rat es ranged from 2.4 to 6.5 ozone molecules produced per NOx processed. The p roduction of ozone per NOx molecule was lowest when the effective photochem ical age was lowest and vice versa. Good correlations between HCHO and NOy have been found in the urban plume of Zurich as well as over the Swiss Plat eau. Between H2O2 and NOz, a negative correlation was observed. On the basi s of Sillman's [1995] indicator species, the ozone production in the Zurich plume and other air masses over the Swiss Plateau is in the NOx-sensitive range. However, there remains some uncertainty in this approach regarding t he influence of biogenic emissions and the initial concentration of indicat or species. Taking the low emissions of biogenic hydrocarbons compared to S illman's model calculation into consideration, the central portion of the Z urich plume falls in the transition or even reactive organic gases (ROG)-li mited range of all indicators. Estimates of production rates of HNO3 and pe roxides support a ROG-sensitive ozone production in the most polluted porti on of the plume.