Emerging threats in Сentral Asia: Comparative characterization of organic and elemental carbon in ambient PM2.5 in urban cities of Kazakhstan.

This study (June 2022-July 2023) investigates the atmospheric concentrations of carbonaceous species, including organic carbon (OC) and elemental carbon (EC), in PM_2.5 in two major cities in Kazakhstan. Samples were collected from two sites in Almaty (Seifullin and KazNU) and one in Astana. The results showed that Almaty had significantly higher annual average concentrations of OC (10.8 and 10.5 μg/m³) and EC (1.68 and 1.87 μg/m³) compared to Astana (OC: 7.1 μg/m³, EC: 0.61 μg/m³). Both cities exhibited pronounced seasonal variations, with significantly elevated concentrations (1.5-3.4 times for OC, 2.1-4.8 times for EC) during the heating season compared to the non-heating season. This indicates a significant influence of coal and biomass combustion for heating on carbonaceous aerosol concentrations. Both cities' OC/EC ratios varied widely (2.6-39.4), showing strong positive correlations (0.61-0.94) across all seasons except summer, suggesting a common primary emission source. Primary organic carbon dominated OC levels in winter (71-74%), whereas secondary organic carbon contributed significantly to OC concentrations in summer (43-50%). Higher OC-EC concentrations correlated with lower atmospheric visibility values. The OC-EC contributions to the total light extinction coefficient were estimated to be 15.3-15.9% for Almaty and 12.0% for Astana stations.