Air pollution typically increases during winter due to temperature inversions, low wind speeds, and higher emissions from heating and traffic, which trap pollutants close to the ground. This leads to elevated concentrations of particulate matter, especially PM2.5, which rises more strongly than PM10 and remains longer in the air. Other pollutants such as nitrogen dioxide (NO₂) also tend to accumulate, while ozone (O₃) levels are usually lower due to reduced sunlight. These conditions often result in multi-day pollution episodes during cold, stagnant weather. Elevated levels of fine particles, particularly PM2.5, pose significant health risks, contributing to respiratory and cardiovascular diseases and increasing overall mortality. This insight summarizes the smog conditions during the past winter and compares it to historic data since 2018. For more information on smog, check out the data source reference or wikipedia.

During the winter of 2025, air quality in Basel showed the expected seasonal increase in pollutants compared to the spring-to-fall baseline. At the Binningen station, the average NO₂ concentration rose from 11 µg/m³ in the baseline period to 21 µg/m³ in winter, while the maximum NO₂ increased from 52 to 60 µg/m³. Average PM10 increased only slightly from 12 to 13 µg/m³, but the maximum PM10 nearly doubled from 58 to 106 µg/m³. The most striking feature was the prominent peak in maximum fine particles at the end of December, which is likely caused by New Year's fireworks. Figures 1 and 2 show the daily maximum and average values for NO₂, PM10, and PM2.5, illustrating these wintertime elevations and the sharp late-December spike in PM2.5.

Looking at the yearly data from 2018 to 2025, winter average PM10 values have fluctuated between 12 and 17 µg/m³, with no clear long-term trend, while winter maximum PM10 has varied widely from 51 to 106 µg/m³. The highest winter maximum PM10 occurred in 2025, matching the 2020 value of 94 µg/m³ and exceeding earlier years. Winter average PM2.5 has remained relatively stable between 10 and 14 µg/m³, but winter maximum PM2.5 reached 89 µg/m³ in 2025, the highest in the record and well above the 2024 value of 73 µg/m³. Baseline average PM2.5 has declined slightly from 10 µg/m³ in 2018 to 8 µg/m³ in 2025, while baseline maximum PM2.5 has dropped from 62 to 49 µg/m³ over the same period. Winter average NO₂ has decreased from 26 µg/m³ in 2018 to 21 µg/m³ in 2025, and winter maximum NO₂ has fallen from 83 to 60 µg/m³, indicating an improving trend for nitrogen dioxide. Overall, while average winter particle levels have remained fairly steady, the maximum PM2.5 and PM10 values in winter 2025 were notably high, driven in part by the New Year's fireworks episode.