Aerosol optical depths over four countries: 40-year spatial-temporal characteristics and recent trends

Spatio-temporal analysis of Aerosol Optical Depth (AOD) plays a critical role in modulating climate, air quality, and ecosystem health. This study systematically examines the 40-year spatial-temporal characteristics and recent trends of AOD over four main continents represented by China, Germany, America, and South Africa from 1985 to 2024, using the Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) and the Moderate Resolution Imaging Spectroradiometer Deep Blue (MODIS DB). The long-term annual mean AODs were found to be 0.27, 0.18, 0.13, and 0.10 for China, Germany, America, and South Africa, respectively, with peak values observed in 1992 across all regions likely linked to volcanic activities. During recent period from 2019 to 2024, the seasonality of AOD was characterized by minima in autumn over South Africa, moderate levels in summer over America and Germany, and maxima in spring over China. Meanwhile, recent monthly variations of AOD show diverse patterns, with AOD peaks in March, July, June, and September for China, America, Germany and South Africa, respectively. Furthermore, recent trends showed declining AOD rates of −3.1 × 10⁻³ and −1.0 × 10⁻² per year over China and America, respectively, likely attributable to stringent environmental regulations and clean energy transitions. In contrast, increasing trends of AOD were observed in South Africa (1.3 × 10⁻³ per year) and Germany (1.1 × 10⁻³ per year), probably suggesting more aerosol emissions. Spatially, recent reductions of AOD derived using the Mann-Kendall test were concentrated in densely populated and industrialized areas of China and parts of America, while increases were widespread across Germany and South Africa. The research underscores the need for sustained monitoring and targeted mitigation strategies to control aerosol emissions, and is benefit for the studies of environment and climate change.