Characterization of the photooxidant formation in the metropolitan area of Milan from aircraft measurements

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