Assessing black carbon dynamics in Iran: the role of urban growth and land use changes in long-term trends (1980–2023)

Black carbon (BC), the primary light-absorbing aerosol, has significant implications for atmospheric heating and climate change, with far-reaching effects on regional air quality and public health. In Iran, BC concentrations, primarily resulting from combustion processes such as industrial emissions, vehicular exhaust, and biomass burning, constitute a significant air quality challenge, particularly in urban regions with high levels of anthropogenic activity. However, there is a lack of studies on the long-term trends of BC in Iran, particularly regarding the effects of urban growth and land use changes on air quality and human health. This study systematically analyzes trends in BC concentrations from 1980 to 2023, both on a national and regional scales, using high-resolution data from the Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2).  The analysis includes temporal and spatial variations to evaluate the impact of anthropogenic and natural factors on BC levels over this period. A substantial increase in BC concentrations was observed from 1980 to 2023, followed by a decline after 2010. Regional analysis revealed higher BC levels in western Iran, driven by concentrated anthropogenic and industrial activities, compared to the sparsely populated, desert-dominated eastern regions, characterized by arid landscapes. Seasonal variations in BC concentrations were observed nationwide, with peak levels occurring in Tehran and Ahvaz during the winter. Trend analysis across various land use and land cover (LULC) types indicated that urban and agricultural expansion were the primarily drivers of increasing BC concentrations. Positive correlations were observed between the aforementioned factors and aerosol emissions, while water and grassland coverage were associated with reduced emissions in most regions. These findings underscore the necessity of expanding natural land use, such as forest coverage, and promoting sustainable urbanization as strategies to mitigate BC emissions.