On July 17, 2026, air quality in Basel was primarily characterized by elevated ozone concentrations. The maximum ozone level of 136.5 µg/m³ exceeded the regulatory threshold of 120 µg/m³, which is the maximum hourly average that may be exceeded only once per year. This exceedance is noted in the data. The average ozone concentration for the day was 105.2 µg/m³.

The maximum ozone concentration on this day ranked 28th highest out of the last 362 days, placing it within the upper range of recent measurements but not among the most extreme. In the historical record since 2018, the absolute maximum ozone concentration was 206.3 µg/m³, recorded on June 27, 2019. The current maximum of 136.5 µg/m³ is well below that historical peak. The average ozone concentration of 105.2 µg/m³ is higher than the seasonal average of 84.4 µg/m³ and the 30-day average of 95.6 µg/m³, indicating that ozone levels were above typical values for this time of year.

Nitrogen dioxide and PM10 concentrations were low. The average NO2 concentration was 5.9 µg/m³, well below the regulatory threshold of 80 µg/m³. The maximum NO2 concentration of 12.9 µg/m³ ranked very low at 300th out of 362 days, indicating unusually clean conditions for this pollutant. The average PM10 concentration was 8.8 µg/m³, also far below the threshold of 50 µg/m³, and its maximum of 14.8 µg/m³ ranked 234th out of 362 days.

Overall, the air quality on July 17, 2026, was not consistent with historical norms. While NO2 and PM10 levels were low and unremarkable, ozone was notably elevated. The exceedance of the ozone threshold is significant and suggests conditions favorable for ozone formation, such as strong sunlight and high temperatures. This pattern is typical for a summer day with photochemical smog. The elevated ozone, combined with low concentrations of other pollutants, points to a day where the primary air quality concern was ground-level ozone.