On 2026-08-27, air quality in Basel was dominated by elevated ozone, while nitrogen dioxide and particulate matter remained unremarkable. The maximum ozone concentration reached 137.4 µg/m³, exceeding the regulatory threshold of 120 µg/m³, and the daily average was 70.4 µg/m³. This exceedance triggered a “considerable” air quality rating. In contrast, nitrogen dioxide peaked at only 18.2 µg/m³ with an average of 12.3 µg/m³, far below its daily average limit of 80 µg/m³. PM10 reached a maximum of 37.0 µg/m³ and averaged 20.1 µg/m³, also below its daily average limit of 50 µg/m³. Among the past 365 days, the ozone maximum ranked 35th, PM10 ranked 26th, and nitrogen dioxide ranked 237th, indicating that only ozone was moderately elevated relative to recent conditions.

Compared with the last 30 days, the ozone maximum of 137.4 µg/m³ was below the 30-day maximum of 168.0 µg/m³, and the daily average of 70.4 µg/m³ was also below the 30-day average of 92.7 µg/m³. For the current season, the seasonal average ozone was 88.4 µg/m³ and the seasonal maximum was 175.8 µg/m³, so the day’s values were lower than typical seasonal peaks. Nitrogen dioxide and PM10 were similarly unexceptional: the NO2 maximum of 18.2 µg/m³ was below the 30-day maximum of 30.9 µg/m³ and the seasonal maximum of 31.4 µg/m³, while PM10’s maximum of 37.0 µg/m³ was far below the 30-day and seasonal maximum of 580.2 µg/m³, a value recorded on 2026-08-15. Since 2018, the all-time maximum ozone was 206.3 µg/m³ on 2019-06-27, and the all-time maximum NO2 was 83.2 µg/m³ on 2019-02-15. The day’s ozone maximum ranked 196th out of 3006 all-time daily maxima, placing it in the upper 7 percent historically, but not extreme.

Overall, the air quality on 2026-08-27 was typical for a summer day with moderate photochemical activity. The only notable feature was the ozone exceedance of the 120 µg/m³ hourly threshold, which is common during warm, sunny conditions when precursor pollutants react in the atmosphere. Nitrogen dioxide and PM10 were well within normal historical ranges and posed no concern. The high PM10 maximum recorded earlier in the season on 2026-08-15 did not recur, and the day’s PM10 values were unremarkable. In summary, the air quality was consistent with seasonal norms, with ozone as the sole pollutant of mild concern.