On 2026-08-16, air quality in Basel was dominated by elevated ozone and, to a lesser extent, particulate matter. The maximum ozone concentration reached 142.1 µg/m³, exceeding the regulatory threshold of 120 µg/m³, and the daily average was 114.4 µg/m³. This ozone exceedance was the primary reason the day was flagged. The maximum ozone value ranked 28th among the last 365 days, which is not exceptionally high but still above the threshold. Nitrogen dioxide remained very low, with a maximum of only 9.9 µg/m³ and an average of 5.0 µg/m³, far below the daily average limit of 80 µg/m³. PM10 had a maximum of 51.5 µg/m³, which is just above the daily average threshold of 50 µg/m³, but the daily average of 16.9 µg/m³ was well below the limit, so no PM10 exceedance was recorded.

Compared with the last 30 days, the ozone average of 114.4 µg/m³ was above the 30-day average of 100.3 µg/m³, and the maximum of 142.1 µg/m³ was below the 30-day maximum of 168.0 µg/m³. For the season, the ozone average was also above the seasonal average of 91.1 µg/m³. PM10 showed a more striking pattern: the maximum of 51.5 µg/m³ on 2026-08-16 ranked 14th among the last 365 days, indicating a relatively high daily peak. However, this was dwarfed by the all-time maximum PM10 value of 580.2 µg/m³ recorded on 2026-08-15, the previous day. That extreme value ranked 58th among all 2,995 days of data, but it was the highest PM10 concentration ever recorded in the dataset, and it also appeared as the maximum in both the 30-day and seasonal statistics. The PM10 maximum on 2026-08-16 was therefore much lower than the previous day’s extreme event, but still high enough to rank in the top 4 percent of the past year.

Historical comparison since 2018 shows that ozone on 2026-08-16 was within the upper part of the typical summer range. The all-time maximum ozone of 206.3 µg/m³ occurred on 2019-06-27, and the current maximum of 142.1 µg/m³ ranked 157th out of 2,995 days, placing it in the top 5 percent of all daily maxima. Nitrogen dioxide was unusually low, with its maximum ranking 329th among the last 365 days and 2,756th all-time, indicating a very clean day for NO2. PM10 was the most anomalous pollutant in the immediate context: the previous day’s maximum of 580.2 µg/m³ was the highest ever recorded, and while 2026-08-16 itself was less extreme, its maximum of 51.5 µg/m³ still ranked 14th in the past year, suggesting a lingering episode of elevated particulate matter.

Overall, air quality on 2026-08-16 was not consistent with typical historical norms. Ozone was elevated enough to trigger an exceedance, and PM10 remained unusually high following the record-breaking event on the previous day. The combination of high ozone and residual particulate matter points to a period of stagnant, warm air with limited dispersion, likely influenced by the same meteorological conditions that produced the extreme PM10 spike on 2026-08-15. Nitrogen dioxide, by contrast, was exceptionally low, which is consistent with reduced traffic emissions or strong vertical mixing that preferentially removed ground-level NO2 while allowing ozone to accumulate. The day should be considered a notable air quality event, primarily due to ozone exceedance and the ongoing PM10 anomaly.