An episode-based assessment for the adverse effects of air mass trajectories on PM_2.5 levels in Astana and Almaty, Kazakhstan

PM_2.5 pollution is a severe problem in most cities in Kazakhstan and is a leading cause of premature deaths globally. This study examines the temporal variation of PM_2.5 levels and the effect of air mass transportation dynamics on episodic air pollution in Kazakhstan's two largest cities. In 2021 annual PM_2.5 concentrations in Astana (22.5 μg m⁻³) and Almaty (35.3 μg m⁻³) exceeded the annual WHO limit by 4.5 and 7.1 times, respectively. The daily limits surpassed 151 days in Astana and 217 days in Almaty. PM_2.5 levels were highest in winter averaging 35.3 μg m⁻³ (Astana) and 76.0 μg m⁻³ (Almaty) over the heating season, with coal combustion as a main contributor. The HYSPLIT model was used to assess the impact of air mass trajectories on pollution episodes and clean air events. Cluster and concentration-weighted trajectory analyses showed that air pollution episodes in both cities were associated with the anticyclone-blocking effect. While both cities were exposed to the influence of long-range transported pollutants, Astana was more frequently impacted by distant sources. Given the transboundary nature of air pollution in both cities and the complex relationship between meteorology and emission sources, a regional integrated air quality improvement strategy is needed.