Aerosol Composition and Source Characteristics Across Coastal and Inland Nigerian Cities: A MERRA-2 Assessment of Carbonaceous, Sulfate, Dust, and Sea-Salt Aerosols

Abstract

Atmospheric aerosols play a critical role in climate regulation, air quality degradation, and radiative forcing, yet comprehensive assessments of aerosol composition across Nigerian urban environments remain limited. This study investigated the composition, seasonal variability, and interrelationships of major aerosol species over Lagos, Abuja, and Kano using MERRA-2 aerosol reanalysis data for the period 2014–2018. Daily aerosol optical depth components, including dust aerosol extinction optical depth (DUEXTTAU), sulfate aerosol extinction optical depth (SUEXTTAU), black carbon aerosol extinction optical depth (BCEXTTAU), organic carbon aerosol extinction optical depth (OCEXTTAU), sea-salt aerosol extinction optical depth (SSEXTTAU), and total aerosol optical depth (TOTEXTTAU), were analyzed. Aerosol composition analysis revealed that dust aerosols dominated Kano, contributing 42.80% of total aerosol loading, compared with 33.35% in Abuja and 27.24% in Lagos. Sulfate aerosol contributions increased progressively from Kano (21.42%) to Abuja (24.95%) and Lagos (27.21%), indicating stronger anthropogenic and coastal influences in southern Nigeria. Lagos also recorded the highest contributions of black carbon (13.61%), organic carbon (22.87%), and sea-salt aerosols (9.07%). Seasonal analysis showed that sulfate aerosols peaked during April–May, reaching approximately 0.154 in Lagos, 0.125 in Kano, and 0.109 in Abuja, followed by declines during the rainy season due to enhanced wet deposition. Time-series analysis further revealed a gradual increase in sulfate loading across all cities between 2014 and 2018, reflecting increasing urbanization and anthropogenic activities. Correlation analysis showed strong relationships between sulfate aerosols and total aerosol loading (r = 0.88), black carbon (r = 0.86), organic carbon (r = 0.86), and sea-salt aerosols (r = 0.86), whereas dust aerosols exhibited weaker correlations (r = 0.40–0.41) with other aerosol species, indicating a distinct source regime. The results demonstrate clear spatial contrasts in aerosol composition between coastal and inland Nigeria and highlight the combined influence of natural and anthropogenic aerosol sources on regional atmospheric conditions.