On August 4, 2026, air quality in Basel was notably impacted by elevated ozone levels. The maximum ozone concentration reached 145.1 µg/m³, which exceeded the regulatory threshold of 120 µg/m³. This value ranked as the 23rd highest daily maximum in the past 365 days. In contrast, nitrogen dioxide and particulate matter remained well within typical bounds. The maximum nitrogen dioxide reading was 25.3 µg/m³, far below its 80 µg/m³ daily average limit, and the maximum PM10 concentration was 28.1 µg/m³, comfortably under its 50 µg/m³ daily average threshold. The average concentrations for the day were 89.2 µg/m³ for ozone, 9.3 µg/m³ for nitrogen dioxide, and 18.9 µg/m³ for PM10.

Comparing the day’s measurements to historical data since 2018 reveals a mixed picture. The ozone maximum of 145.1 µg/m³ is elevated but not exceptional; it ranks 132nd out of 2,983 days in the all-time record, placing it within the top five percent of historical daily maxima. The absolute historical maximum for ozone was 206.3 µg/m³, recorded on June 27, 2019. The day’s average ozone of 89.2 µg/m³ was slightly below the 30-day average of 98.0 µg/m³ and nearly identical to the seasonal average of 88.3 µg/m³, suggesting that while the peak was high, overall ozone levels were typical for the time of year. Nitrogen dioxide values were unremarkable, with the maximum ranking 1,596th historically and the average of 9.3 µg/m³ being only marginally above the seasonal average of 6.9 µg/m³. PM10 levels were similarly ordinary, with the maximum ranking 549th all-time and the average of 18.9 µg/m³ slightly above the seasonal average of 14.1 µg/m³. The absolute historical maximum for PM10 was 165.1 µg/m³ on August 24, 2023.

Overall, the air quality on August 4, 2026, was characterized by a significant but not unprecedented ozone peak that triggered an exceedance of the regulatory threshold. This elevated ozone was the primary factor distinguishing the day from typical conditions, while nitrogen dioxide and particulate matter remained consistent with historical norms. The high ozone maximum likely reflects intense photochemical activity driven by strong sunlight and warm temperatures, a common summertime phenomenon.