Spatiotemporal Variability of Black Carbon Aerosol Optical Depth and its Contribution to Total Aerosol Loading in Lagos and Kano, Nigeria

Black carbon (BC) is an important atmospheric aerosol that influences climate, air quality, and human health through its strong absorption of solar radiation. This study investigated the spatiotemporal variability of Black Carbon Aerosol Optical Depth (BCEXTTAU) and its contribution to total aerosol loading (TOTEXTTAU) over Lagos and Kano, Nigeria, during 2014–2018 using MERRA-2 reanalysis data. Annual trends, seasonal variability, Black Carbon Contribution Ratio (BCR), annual growth rates, and the Urban Black Carbon Enhancement Factor (UBEF) were evaluated. Results showed a steady increase in BCEXTTAU in both cities, rising from 0.0398 to 0.0554 in Kano and from 0.0569 to 0.0764 in Lagos between 2014 and 2018. Lagos consistently recorded higher BCEXTTAU values, reflecting stronger anthropogenic emissions. Seasonal analysis revealed maximum BCEXTTAU during April–May and minimum values during September–October due to enhanced wet scavenging during the rainy season. The BCR remained relatively stable, averaging approximately 10.7–10.8% in Kano and 13.5–13.7% in Lagos, indicating a greater contribution of combustion-derived aerosols in Lagos. UBEF values greater than unity further confirmed persistent black carbon enhancement in Lagos relative to Kano. The results demonstrate increasing black carbon loading in Nigerian urban environments and highlight the importance of emission-control strategies for improving air quality and mitigating climate impacts.