On 2026-05-29, the most notable air quality issue in Basel was elevated ozone (O3) concentrations. The maximum ozone level reached 136.7 µg/m³, which exceeded the regulatory threshold of 120.0 µg/m³ set by the Swiss Clean Air Ordinance. This value ranked 22nd highest among the last 365 days, placing it within the top 10 percent of recent measurements. The average ozone concentration for the day was 98.3 µg/m³, which is well above the seasonal average of 65.5 µg/m³ and the 30-day average of 74.7 µg/m³, indicating a significant spike compared to typical conditions for this time of year.

Nitrogen dioxide (NO2) and fine particulate matter (PM10) remained at low levels. The maximum NO2 concentration was 25.8 µg/m³, and the daily average was 8.4 µg/m³, both far below the regulatory limit of 80.0 µg/m³. The maximum PM10 concentration was 23.6 µg/m³, with an average of 16.9 µg/m³, also well under the 50.0 µg/m³ threshold. Neither pollutant showed any exceedance.

When compared to historical data since 2018, the day's ozone maximum of 136.7 µg/m³ is moderately high but not extreme. The all-time maximum ozone concentration recorded in Basel was 206.3 µg/m³ on June 27, 2019. The current value is therefore about two-thirds of that historical peak. The NO2 and PM10 levels were unremarkable and consistent with typical low-concentration days.

Overall, the air quality on 2026-05-29 was characterized by a notable but not unprecedented ozone episode. The elevated ozone is likely due to strong solar radiation and high temperatures, which promote photochemical ozone formation. While the exceedance of the hourly ozone limit is a concern, the levels were not extreme by historical standards. The low concentrations of NO2 and PM10 suggest that local traffic and combustion sources were not major contributors that day.