On August 16, 2025, the air quality in Basel was notably impacted by elevated ozone (O₃) levels. The maximum concentration of ozone reached 152.6 µg/m³, which is significantly above the regulatory threshold of 120 µg/m³ for maximum hourly averages. This concentration ranks 8th in terms of the highest values recorded over the past 365 days. The average ozone concentration for the day was 99.6 µg/m³, which is considerably higher than the recent 30-day average of 69.8 µg/m³ and the seasonal average of 75.7 µg/m³. Historically, the maximum recorded ozone concentration since 2018 was 206.3 µg/m³ on June 27, 2019, indicating that while the current levels are high, they are not unprecedented.

Nitrogen dioxide (NO₂) levels were relatively low, with a maximum concentration of 20.8 µg/m³ and an average of 11.4 µg/m³, both well below the regulatory daily average threshold of 80 µg/m³. These values are consistent with the recent 30-day average of 7.4 µg/m³ and the seasonal average, suggesting typical conditions for this pollutant.

Particulate matter (PM₁₀) concentrations were also within acceptable limits, with a maximum of 34.3 µg/m³ and an average of 29.4 µg/m³, both below the regulatory threshold of 50 µg/m³. These values are higher than the recent 30-day average of 11.7 µg/m³ and the seasonal average of 12.3 µg/m³, indicating a slight increase but still within a normal range.

Overall, the air quality on August 16, 2025, was characterized by unusually high ozone levels, which exceeded regulatory standards and were significantly above typical recent averages. This spike in ozone could be attributed to specific meteorological conditions or increased emissions, warranting further investigation. Other pollutants, such as NO₂ and PM₁₀, remained within normal historical ranges and did not show any unusual characteristics.