Planetary Boundary Layer and its Relationship with PM2.5 Concentrations in Almaty, Kazakhstan

Abstract

Air pollution is a severe problem in Almaty (Kazakhstan), especially during the cold half of the year (October-March). Almaty is one of the most polluted cities in Kazakhstan and Central Asia, with average winter PM2.5 (particulate matter with aerodynamic diameter ≤ 2.5 µm) concentrations higher than 100 μg m-3. High pollution in the wintertime in Almaty could be caused by emissions from coal combustion for power and heat generation (at power plants and small-scale heating), which could also be worsened by poor dispersion of air pollutants due to certain atmospheric conditions. Based on one-year radiosonde data, the characteristics of the planetary boundary layer height (PBLH) and its effect on ground-level PM2.5 concentrations in Almaty were analyzed in this study using the bulk Richardson number (Ri) and potential temperature increase (PT) methods. During an annual cycle, the concentrations of PM2.5 were highest in the winter months when the daily concentrations were above 100 μg m-3 for 39 days during this period. The results show a clear negative relationship between the daily average PM2.5 concentrations and PBLH at 12.00 UTС. For instance, high PM2.5 concentrations in winter months (94.0 μg m-3) corresponded to a lower PBLH (393 m), and low PM2.5 concentrations in summer months (9.9 μg m-3) corresponded to a higher PBLH (1970 m). During the cold half of the year, the top 20% of PM2.5 concentrations were associated with a lower PBLH and calm wind conditions (lower average wind speeds within the PBL and a lower ventilation coefficient). The results show that PBLH variations during the year have a significant effect on PM2.5 concentrations; however, further analysis is needed with a more substantial amount of observational data to understand this interaction further and to investigate the role of synoptic processes that lead to a shallow PBLH.