Assessment of Atmospheric Aerosol Optical Properties Over Ibadan Using Sun Photometry

Atmospheric aerosols in West Africa arise from diverse sources including Saharan dust, biomass burning smoke, and urban-industrial emissions, significantly affecting solar radiation budgets and climate forcing. Ibadan in Oyo State, South West Nigeria, experiences seasonal aerosol loading from harmattan dust in the dry season and from biomass burning during the late dry season. This study assesses aerosol optical properties over Ibadan using a Cimel sun photometer deployed at the University of Ibadan campus and calibrated to AERONET standards. Aerosol optical depth, Angstrom exponent, and single scattering albedo are retrieved from sun and sky radiance measurements at six wavelengths from 340 to 1020 nm over an 18-month period. Cluster analysis of aerosol optical depth and Angstrom exponent pairs identifies three dominant aerosol types, namely mineral dust, biomass burning aerosol, and mixed urban aerosol. Results indicate that aerosol optical depth at 550 nm ranges from a dry season mean of 0.62 to a post-rainy season biomass burning mean of 0.88. The single scattering albedo of biomass burning aerosol (0.82 to 0.87) indicates significant absorptive capacity with potential negative forcing implications. Aerosol direct radiative forcing at the surface reaches minus 18 to minus 32 W per square metre during peak loading periods. Keywords: aerosol optical depth, sun photometry, Ibadan, biomass burning, atmospheric aerosol