On 2026-08-12, the air quality in Basel was dominated by elevated ozone concentrations. The maximum ozone level reached 168.0 µg/m³, which exceeded the regulatory threshold of 120.0 µg/m³ and triggered an exceedance flag. This value ranked as the 6th highest daily maximum ozone concentration recorded in the past 365 days, indicating a notably high reading. The daily average ozone concentration was 113.3 µg/m³, which is above the 30-day average of 99.4 µg/m³ and the seasonal average of 89.6 µg/m³, though it remained below the absolute historical maximum of 206.3 µg/m³ recorded on 2019-06-27.

Nitrogen dioxide and particulate matter (PM10) remained unremarkable. The maximum NO₂ concentration was 18.9 µg/m³, well below the 80.0 µg/m³ daily average threshold and far from the historical maximum of 83.2 µg/m³ observed on 2019-02-15. Its rank among the last 365 days was 226, placing it in the lower half of recent readings. The daily average NO₂ was 9.8 µg/m³, slightly above the 30-day average of 7.7 µg/m³ and the seasonal average of 7.0 µg/m³ but still within a normal range. PM10 reached a maximum of 25.2 µg/m³ with a daily average of 19.4 µg/m³, both below the 50.0 µg/m³ daily average threshold. The PM10 maximum ranked 90th out of 362 days in the past year, indicating a moderately typical value, and it remained far below the historical maximum of 165.1 µg/m³ from 2023-08-24.

Compared with historical data since 2018, the ozone maximum on this day was unusually high, ranking 25th out of 2,991 total days, which places it within the top 1% of all recorded daily maxima. The NO₂ maximum was unexceptional, ranking 2,070th out of 2,991 days, and the PM10 maximum ranked 731st, suggesting a slightly above-median but not extreme value. The overall air quality rating was classified as considerable, driven entirely by ozone.

In summary, the air quality on 2026-08-12 was characterized by a significant ozone episode, with concentrations exceeding the regulatory threshold and reaching a level rarely seen in the past year. This pattern is consistent with warm, sunny summer conditions that promote photochemical ozone formation. Nitrogen dioxide and PM10 remained within typical ranges and did not contribute to the elevated air quality index. The day therefore exhibited an unusual characteristic specifically for ozone, while other pollutants were consistent with historical norms.