On 2026-05-26, the most notable air quality issue in Basel was elevated ozone (O3) and fine particulate matter (PM10) concentrations. The maximum ozone level of 138.9 µg/m³ exceeded the regulatory threshold of 120.0 µg/m³, which is the maximum hourly average allowed under the Swiss Clean Air Ordinance. The maximum PM10 concentration of 54.9 µg/m³ was also elevated, though the daily average of 41.0 µg/m³ remained below the 50.0 µg/m³ standard. Nitrogen dioxide (NO2) levels were low, with a maximum of 19.4 µg/m³ and an average of 7.3 µg/m³, well below the 80.0 µg/m³ daily average limit.

The maximum ozone concentration on this day ranked 18th highest out of the last 366 days, placing it in the top 5% of recent measurements. The maximum PM10 concentration ranked 7th highest over the same period, indicating a notably high value. In the context of all data since 2018, the ozone maximum ranked 151st out of 2917 days, while the PM10 maximum ranked 41st, showing that while these values were high for the recent year, they were not unprecedented historically. The all-time maximum ozone concentration was 206.3 µg/m³, recorded on June 27, 2019, and the all-time maximum PM10 concentration was 165.1 µg/m³, recorded on August 24, 2023.

Compared to the average of the last 30 days, the mean ozone level of 93.7 µg/m³ was higher than the 30-day average of 73.0 µg/m³, and the maximum ozone of 138.9 µg/m³ was close to the 30-day maximum of 150.1 µg/m³. The mean PM10 level of 41.0 µg/m³ was significantly higher than the 30-day average of 10.7 µg/m³, and the maximum PM10 of 54.9 µg/m³ was lower than the 30-day maximum of 84.1 µg/m³. Compared to the seasonal averages for this year, the mean ozone was above the seasonal average of 64.3 µg/m³, and the mean PM10 was well above the seasonal average of 13.3 µg/m³.

Overall, the air quality on 2026-05-26 was unusual due to the simultaneous elevation of ozone and PM10. The elevated ozone levels, which exceeded the regulatory threshold, are typical for a warm, sunny day with strong photochemical activity. The high PM10 concentrations, ranking 7th in the past year, suggest a significant source of particulate matter, such as a local dust event, construction activity, or long-range transport of polluted air. The combination of high ozone and PM10 is characteristic of a summer smog episode, where stagnant meteorological conditions allow pollutants to accumulate and undergo chemical reactions.