Multi-decadal ozone air quality and the role of temperature in Switzerland during summertime

Abstract. Tropospheric ozone (O₃) is a greenhouse gas and air pollutant. Despite efforts to control O₃ precursor emissions, O₃ levels frequently exceed the Swiss air quality standards. We present multi-decadal summertime measurements of O₃ and its precursors across Switzerland from 12 NABEL (Nationales Beobachtungsnetz für Luftfremdstoffe) stations, which are representative of traffic, (sub)urban, rural and background conditions. Average O₃ levels have decreased at rural and background sites, remained constant at (sub)urban sites and increased under traffic conditions over the past two decades. Traffic, (sub)urban and rural sites exhibited a pronounced weekend effect at the beginning of the century, which has weakened over time and only persists under traffic conditions today, suggesting that O₃ formation is becoming more NO_x-sensitive. O₃ exhibits a strong dependence on temperature (dO₃/dT), which has weakened uniformly at all site types over time. At polluted sites, this effect could be associated with the decreasing influence of titration. While reductions of precursor levels have shifted the probability of O₃ exceedances to higher temperatures, O₃ is still frequently exceeded on hot summer days and the number of days exceeding 30 °C has tripled since 2000. Ozone formation has been suppressed due to the titration by NO in many locations in the past but is dominated by NO_x-sensitive O₃ chemistry in background, rural, and (sub)urban environments today. Ozone titration remains dominant under traffic conditions, where O₃ levels are currently increasing with NO_x and will likely increase for several years before emissions reductions will become effective.