Validation of Multiple Satellite Aerosol Optical Depth (AOD) Retrievals Using Ground-Based AERONET AOD Data over West Africa

Aerosol Optical Depth (AOD) is an essential parameter for understanding atmospheric aerosol distribution and its impact on climate and air quality. Satellite-based AOD retrievals play a crucial role in large-scale studies, but their accuracy necessitates validation against ground-based measurements. This study focuses on validating AOD data products from multiple satellite sensors (MODIS TERRA, MODIS AQUA, MISR, OMI and MERRA-2) using high-quality ground-based Aerosol Robotic Network (AERONET) AOD data within the West African region. The study aims to assess the performance of different satellite sensors in capturing the spatiotemporal variability of aerosol loading over West Africa for the period 2000–2022. Six AERONET stations (Banizoumbou, Cinzana, Ilorin, Dakar, Capoverde, and Koforidua) are considered within three sub-regions of West Africa (Sahel, Savannah, and Guinea Coast). Rigorous validation techniques, including intercomparison analysis and statistical metrics, are employed to evaluate the agreement between satellite-derived AOD and AERONET measurements. Preliminary results indicate that satellite retrievals generally capture the broad-scale patterns of aerosol distribution in West Africa. However, this study reveals significant discrepancies in some regions, emphasizing the need for improved satellite algorithms, especially in areas with complex aerosol properties. Furthermore, the analysis identifies the best-performing satellite sensors at each AERONET station when employing either daily or monthly data. Daily analysis revealed MODIS AQUA had the best agreement at Banizoumbou, Cinzana Capoverde and Koforidua, while MISR and MODIS TERRA performed best at Dakar and Ilorin, respectively. On the other hand, the monthly analysis revealed MODIS TERRA performs best at Banizoumbou, Dakar, and Capoverde stations, while MERRA performs best at Cinzana and Ilorin stations. MISR shows relatively lower performance compared to MODIS TERRA and MERRA at Banizoumbou and Koforidua stations. But surprisingly, MODIS AQUA wasn’t the best performer at any of the stations based on monthly analysis. These findings highlight the importance of enhancing satellite algorithms to improve the accuracy of aerosol retrievals and the importance of taking caution when selecting a type of satellite data product and the temporal resolution to use for climate studies, air quality monitoring, and environmental management in regions with intricate aerosol characteristics.

Keywords: Aerosol Optical Depth (AOD), MODIS TERRA, MODIS AQUA, MISR, OMI, MERRA-2, Aerosol Robotic Network (AERONET), West Africa.