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, the average NO₂ concentration at the Binningen station was 21 µg/m³, compared to a baseline of 10 µg/m³ in spring through fall, while the maximum NO₂ reached 60 µg/m³ versus 52 µg/m³ in the baseline. For PM10, the winter average was 13 µg/m³, slightly above the baseline average of 12 µg/m³, and the winter maximum was 106 µg/m³, far exceeding the baseline maximum of 58 µg/m³. PM2.5 showed a winter average of 12 µg/m³ against a baseline of 8 µg/m³, with a winter maximum of 89 µg/m³ compared to 49 µg/m³ in the baseline. Figures 1 and 2 illustrate these daily maximum and average values for NO₂, PM10, and PM2.5, highlighting the wintertime accumulation. A prominent peak in maximum fine particles occurred at the end of December, likely caused by New Year's fireworks, which sharply elevated PM2.5 and PM10 concentrations for a short period.

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 values ranged from 51 to 106 µg/m³ and showed a notable increase in 2025 compared to the previous two years. Winter average PM2.5 values have remained relatively stable, mostly between 10 and 14 µg/m³, but the winter maximum PM2.5 in 2025 reached 89 µg/m³, the highest since 2021 and well above the baseline maximum of 49 µg/m³. Baseline average PM10 and PM2.5 values have generally declined slightly over the period, from 14 and 10 µg/m³ in 2018 to 12 and 8 µg/m³ in 2025, respectively. Winter average NO₂ has decreased from 26 µg/m³ in 2018 to 21 µg/m³ in 2025, while baseline average NO₂ fell from 16 to 11 µg/m³ over the same period. The data indicate that while average winter pollution levels have not increased dramatically, extreme short-term events such as fireworks and stagnant weather episodes continue to produce high maximum concentrations, particularly for fine particles.