Ground-level ozone is a summer pollutant. Unlike fine particles it is not emitted directly: it forms when nitrogen oxides and volatile organic compounds react under strong sunlight and high temperatures. Concentrations therefore peak on hot, sunny afternoons and fall back overnight, and hot summers produce more ozone than cool ones. Because ozone is destroyed by fresh traffic emissions, NO₂ and ozone often move in opposite directions: NO₂ is at its lowest in summer, exactly when ozone is at its highest. Ozone irritates the airways, reduces lung function and aggravates asthma, and it also damages crops and forests. The Swiss Ordinance on Air Pollution Control sets a limit of 120 µg/m³ for the hourly mean, which may be exceeded on at most one day per year. This insight summarizes ozone during the past summer and compares it with historic data since 2018. For more information, check the data source reference or wikipedia

In summer 2026, the average ozone concentration at Binningen reached 87 µg/m³, compared with a baseline of 54 µg/m³ for the rest of the same year. The maximum hourly value was 176 µg/m³, against 153 µg/m³ in the baseline. The number of days above the 120 µg/m³ limit was 48, far above the baseline figure of 17 days and far above the legal allowance of one exceedance day per year. The summer also brought a maximum temperature of 38.5 °C, higher than the 32.2 °C recorded in the rest of the year. Figures 1 and 2 show daily maximum and average values for ozone and NO₂. Ozone runs clearly higher in summer, while NO₂ runs lower, with a summer average of only 7 µg/m³ compared with 14 µg/m³ in the baseline. This inverse pattern is expected because fresh nitrogen oxide emissions from traffic destroy ozone locally, so lower NO₂ in summer allows ozone to accumulate more strongly.

Looking at the yearly record since 2018, summer average ozone has fluctuated without a clear long-term trend. It fell from 85 µg/m³ in 2018 to a low of 63 µg/m³ in 2021, then rose again to 80 µg/m³ in 2022, dipped to 68 µg/m³ in 2024, and climbed to 87 µg/m³ in 2026. Summer maximum ozone has also varied, from 206 µg/m³ in 2019 down to 161 µg/m³ in 2021 and back up to 176 µg/m³ in 2026. The number of days above the 120 µg/m³ limit has followed a similar pattern. It dropped from 53 days in 2018 to 18 days in 2021, then rose to 43 days in 2022, fell to 16 days in 2024, and increased again to 30 days in 2025 and 48 days in 2026. The most recent two summers therefore show a renewed upward tendency in exceedance days, even though the long-term picture remains variable rather than steadily increasing or decreasing.