Investigation of Atmospheric Aerosol Optical Depth and Its Radiative Forcing Over the Sahel-Savanna Transition Zone in Northern Nigeria

Atmospheric aerosols in the Sahel-savanna transition zone of northern Nigeria arise from multiple sources including Saharan dust transport, biomass burning during agricultural land clearing, and urban combustion emissions, creating a complex aerosol mixture with significant but poorly quantified regional climate forcing. This study investigates aerosol optical depth and associated direct radiative forcing over the transition zone encompassing Katsina, Jigawa, and Bauchi States, North West and North East Nigeria, using a combination of AERONET sun photometer measurements, MODIS satellite-retrieved aerosol optical depth products, and radiative transfer modeling. A Cimel sun photometer was operated at the Katsina Polytechnic campus for 18 months, providing spectral aerosol optical depth measurements at 340, 380, 440, 500, 675, 870, and 1020 nm wavelengths. Aerosol size distribution parameters and single-scattering albedo were retrieved by Almucantar sky radiance inversions. Angstrom exponent analysis distinguished coarse-mode dust-dominated aerosol days from fine-mode pollution-dominated conditions. Direct radiative forcing at the surface and top of atmosphere was computed using the SBDART radiative transfer code for identified aerosol types. Mean aerosol optical depth at 550 nm reached 0.78 during peak harmattan episodes and 0.42 during biomass burning season, both representing substantial solar attenuation. Surface radiative forcing during dust events reached negative 45 W/m2, indicating significant solar radiation reduction relevant to photovoltaic energy generation and crop evapotranspiration. Keywords: aerosol optical depth, radiative forcing, Sahel, harmattan dust, atmospheric physics