An episode-based assessment for the adverse effects of air mass trajectories on PM2.5 levels in Astana and Almaty, Kazakhstan

PM2.5 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 PM2.5 levels and the effect of air mass transportation dynamics on episodic air pollution in Kazakhstan's two largest cities. In 2021 annual PM2.5 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. PM2.5 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. [Display omitted] •PM2.5 levels in 2021 exceeded WHO limits for 184 and 113 days in Almaty and Astana.•In Almaty and Astana, winter PM2.5 levels were 7 and 3 times higher than in summer.•Anticyclone blocking effects of the Siberian High generated stagnant air conditions.•92% of the air masses during pollution events in Almaty came through Kyrgyzstan.•In Astana, pollution events were affected primarily by locally formed air masses.