On 2026-05-27, the most notable air quality issue in Basel was elevated ozone (O3) levels. The maximum ozone concentration of 153.2 µg/m³ ranked 7th highest out of the last 366 days, indicating a significant peak. This value exceeded the regulatory threshold of 120 µg/m³ for the maximum hourly average, which is permitted only once per year under the Luftreinhalte-Verordnung. The average ozone concentration for the day was 99.6 µg/m³, which is well above the 30-day average of 73.3 µg/m³ and the seasonal average of 64.7 µg/m³, confirming that ozone levels were unusually high.

Nitrogen dioxide (NO2) concentrations remained low, with a maximum of 26.2 µg/m³ and an average of 10.1 µg/m³. These values are well below the regulatory daily average limit of 80 µg/m³ and are consistent with typical levels, ranking 152nd out of 366 days. Fine particulate matter (PM10) reached a maximum of 54.4 µg/m³ and an average of 37.2 µg/m³. While the average did not exceed the daily limit of 50 µg/m³, the maximum concentration ranked 8th highest in the past year, suggesting a notable but brief elevation. The PM10 average was significantly higher than the 30-day average of 11.7 µg/m³ and the seasonal average of 13.5 µg/m³.

Comparing these values to historical data since 2018, the maximum ozone concentration of 153.2 µg/m³ is below the all-time maximum of 206.3 µg/m³ recorded on June 27, 2019, but it remains a high value for the season. The PM10 maximum of 54.4 µg/m³ is also well below the historical peak of 165.1 µg/m³ from August 24, 2023. The NO2 levels were unremarkable compared to the historical maximum of 83.2 µg/m³ from February 15, 2019.

Overall, the air quality on 2026-05-27 was not consistent with historical norms due to the notably elevated ozone and PM10 concentrations. The high ozone levels, which triggered an exceedance of the regulatory threshold, are likely linked to strong solar radiation and high temperatures typical of late spring, which promote photochemical ozone formation. The elevated PM10 may be associated with the same meteorological conditions or local sources such as road traffic or construction. While NO2 remained low, the combination of high ozone and PM10 resulted in a considerable air quality rating for the day.