Ozone and hydrogen peroxide during summer smog episodes over the Swiss Plateau: Measurements and model simulations

As part of the POLLUMET (Pollution and Meteorology in Switzerland) study, measurements of O 3 , NO 2 , and H 2 O 2 were made at ground stations, by aircraft, and manned hydrogen balloons to characterize the photochemical processes prevailing during times of high ozone production. During summer smog episodes, elevated ozone concentrations were found over the whole area of the Swiss Plateau, which is a densely populated and industrialized region. However, NO 2 concentrations were generally at a low level in the afternoon. An increase of hydrogen peroxide concentration in the mixing layer during photochemical episodes was observed. To understand better the photochemical processes which control ozone formation over the Swiss Plateau, calculations with the Harvard photochemical model and the Harwell photochemical trajectory model were conducted. A comparison of model results with measurements of the triad O 3 , NO 2 , and H 2 O 2 indicates an ozone formation under low NO x conditions.