On August 9, 2026, the air quality in Basel was primarily characterized by elevated ozone levels. The maximum ozone concentration reached 138.0 µg/m³, which exceeded the regulatory threshold of 120.0 µg/m³. This value ranked as the 34th highest daily maximum in the past 365 days. In contrast, nitrogen dioxide and particulate matter remained well within typical bounds, with maximums of 12.2 µg/m³ and 22.4 µg/m³, respectively, and neither exceeded their respective regulatory limits. The daily average for ozone was 103.6 µg/m³, while nitrogen dioxide averaged 7.0 µg/m³ and PM10 averaged 12.8 µg/m³.

Comparing the day’s measurements to historical data since 2018, the ozone maximum of 138.0 µg/m³ was within the normal range of summer variability but did not approach the all-time high of 206.3 µg/m³ recorded on June 27, 2019. The seasonal average maximum for ozone is 175.8 µg/m³, and the 30-day average maximum is 175.3 µg/m³, placing the current reading below these recent peaks. Nitrogen dioxide and PM10 values were unremarkable, with their maximums falling well below historical extremes. The all-time maximum for NO₂ is 83.2 µg/m³ from February 15, 2019, and for PM10 it is 165.1 µg/m³ from August 24, 2023.

Overall, the air quality on this day was consistent with historical summer patterns, with the exception of the ozone exceedance. The elevated ozone is typical for warm, sunny periods when photochemical reactions are enhanced. While the ozone level breached the regulatory threshold, it was not an extreme event compared to the broader historical record.