On 2026-05-25, the air quality in Basel was notably impacted by elevated levels of fine particulate matter (PM10). The maximum PM10 concentration recorded that day ranked 4th highest out of the past 366 days, indicating a significant short-term increase. The maximum PM10 concentration reached 84.1 µg/m³, while the average concentration was 38.9 µg/m³. Although the average PM10 concentration did not exceed the regulatory threshold of 50 µg/m³, the maximum value was substantially above typical levels.

Ozone (O3) levels were also elevated, with a maximum concentration of 141.1 µg/m³ and an average of 93.3 µg/m³. The maximum ozone value exceeded the regulatory threshold of 120 µg/m³, which is permitted only once per year under Swiss regulations. The maximum ozone concentration ranked 16th highest in the past year, placing it within the upper range of recent measurements but not at an extreme level.

Nitrogen dioxide (NO2) concentrations remained low, with a maximum of 10.9 µg/m³ and an average of 6.4 µg/m³. These values were well below the regulatory daily average limit of 80 µg/m³ and were consistent with typical low levels observed in Basel.

Comparing current measurements with historical data since 2018, the maximum PM10 concentration of 84.1 µg/m³ was unusually high. This value ranks 15th highest out of all 2,916 days in the dataset, placing it among the top 1% of all recorded PM10 maxima. The all-time maximum PM10 concentration was 165.1 µg/m³, recorded on 2023-08-24. The maximum ozone concentration of 141.1 µg/m³ was within the normal historical range, ranking 140th out of all days, and well below the all-time maximum of 206.3 µg/m³ measured on 2019-06-27. Nitrogen dioxide levels were unremarkable compared to historical norms.

Overall, the air quality on 2026-05-25 showed unusual characteristics due to the notably elevated PM10 concentrations. The high PM10 levels, combined with an ozone exceedance, suggest that this day experienced conditions conducive to particulate matter accumulation, possibly due to local sources such as construction, road traffic, or meteorological conditions like temperature inversions or dry weather. The elevated PM10 is the most significant deviation from historical norms and warrants attention regarding its potential sources and health implications.