Spatiotemporal Variability of Aerosols over Ghana: Changes Associated with the COVID-19 Period

Acheampong Baafi-Adomako , Abishek Murugesan, MS Shyam Sunder , Isaac Osei

Exposure to fine particulate matter (PM2.5) is a leading environmental health risk in West Africa, yet long-term characterization of aerosol variability in Ghana is constrained by sparse surface monitoring. Here we combine aerosol optical depth (AOD) from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra and Aqua satellites (2010-2022), the daily, 1 km2 Gridded Africa Surface Pollution (GRASP) PM2.5 data set (2010−2021), surface PM2.5 observations (2021), precipitation records, and HYSPLIT backward trajectories to characterize aerosol variability across the coastal, forest, and northern zones of Ghana, with emphasis on changes associated with the COVID-19 period. AOD and PM2.5 peak during the Harmattan dry season (November−February), when more than 86% of air masses arriving at Accra, Kumasi, and Tamale originate over the Sahara and Sahel. Daily PM2.5 exceeded the World Health Organization (WHO) 24-h guideline of 25 μg m−3 on 75-93% of days in 2021, with Harmattan peaks above 400 μg m−3 in Tamale. MODIS AOD was significantly correlated with GRASP PM2.5 at all three cities (R2 = 0.67−0.91, p < 0.001), and AOD decreased exponentially with monthly precipitation (R2 = 0.81). Relative to the 2010-2019 baseline, mean PM2.5 in 2020 decreased by 13.7% in Accra, 8.3% in Kumasi, and 3.6% in Tamale, with larger reductions of 33.6% and 28.4% in Accra and Kumasi during the March 30-April 20 partial lockdown. Our findings demonstrate the value of integrating satellite and gridded surface data sets to support air quality management and geographically targeted interventions in Ghana.