Open Data Insight of the Day

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 pattern with higher concentrations of key pollutants compared to the baseline period of spring and fall. At the Binningen station, the average winter concentration of PM10 was 13 µg/m³, which was slightly higher than the baseline average of 12 µg/m³. The average winter concentration of PM2.5 was 12 µg/m³, compared to a baseline average of 8 µg/m³, showing a more pronounced seasonal increase for fine particles. For nitrogen dioxide, the winter average was 21 µg/m³, nearly double the baseline average of 11 µg/m³. As illustrated in Figures 1 and 2, which plot the daily maximum and average values for NO₂, PM10, and PM2.5, a prominent peak in fine particle concentrations occurred at the end of December. This peak, where the maximum PM2.5 concentration reached 89 µg/m³, is likely attributable to emissions from New Year's fireworks.

Examining the yearly data from 2018 to 2025 reveals trends in winter air pollution. For PM10, winter average concentrations have shown variability, ranging from a high of 17 µg/m³ in 2018 to a low of 12 µg/m³ in 2023, with the 2025 average of 13 µg/m³ being near the middle of this range. Winter maximum PM10 concentrations have also fluctuated, with the 2025 maximum of 106 µg/m³ being the highest in the dataset, surpassing the previous high of 94 µg/m³ in 2020. For PM2.5, the winter average in 2025 was 12 µg/m³, which is consistent with levels observed in recent years and remains below the peak of 14 µg/m³ seen in 2018 and 2024. However, the winter maximum for PM2.5 in 2025 was 89 µg/m³, which is the second highest value in the series, only exceeded by the 83 µg/m³ recorded in 2021. The data indicates that while average winter concentrations for particles have not shown a clear increasing trend, the peak pollution events, particularly for PM2.5, can reach very high levels, as seen in 2021, 2024, and 2025.