Improvement in wintertime air quality in urban and suburban areas of Warsaw within 2016–2025

Winter air pollution in Warsaw, Poland, was analyzed from 2016 to 2025 using hourly data from nine CIEP monitoring stations, meteorological observations from Okecie IMGW station, boundary layer heights from ERA5, and HYSPLIT backward trajectories. The study focused on PM 10 , PM 2.5 , NO 2 , and O 3 , with emphasis on black smog episodes. Air quality improved substantially over the decade: the frequency of good or very good (according to Polish air quality index thresholds) PM 2.5 conditions increased from 10 to 12% (2016–2018) to 30–32% (2023–2025), while for PM 10 from 16 to 24% to 43–45%. NO 2 also declined, with very good conditions increasing from 66% (urban) and 88% (suburban) to 84% and 95%, respectively. Smog episodes occurred under low temperatures (− 3.3 °C on average), weak winds (≤ 5 m/s), strong inversions, and high humidity, with air masses from local, northern, or western origins. PM x concentrations decreased with higher temperature (about 4–8 fold from the lowest temperature bin − 21 to − 16 °C to the 9–14 °C bin), stronger winds (4–6 fold from 0 to 1 to 8–12 m/s), and deeper boundary layers (7–11 fold from 0 to 100 to 1500–2500 m), whereas O 3 increased under deeper layers and cleaner air masses. Comparisons under similar meteorological conditions confirmed a genuine downward trend in PM 2.5 , reflecting emission reductions from residential heating, stricter policies, and better alignment with the EU standards. These results demonstrate that winter air quality in Warsaw has improved due to both emission control and favorable atmospheric conditions, with regional transport modulating urban pollution.