Two Decades of Aerosol Optical Depth evolution from CAMS Reanalysis

Abstract. Aerosol optical depth (AOD) is a key indicator for evaluating global climate change, and its long-term trends provide critical insights into the evolving climatic impact of atmospheric aerosols. In this study, we analyse the long-term (2003–2024) AOD record from the fourth-generation ECMWF Atmospheric Composition Reanalysis (EAC4) to investigate global and regional variability and long-term trends over the past two decades. EAC4 is evaluated against independent AERONET observations from 178 stations selected using strict data-availability criteria. The impact of satellite assimilation is explicitly quantified through comparison with a free-running control simulation (CTRL). EAC4 reproduces observed global AOD variability with high skill (R = 0.84, RMSE = 0.12, IOA = 0.90; >1.6 million collocations) and shows strong agreement with AERONET-derived trends at stations exhibiting statistically significant changes (R = 0.89), correctly capturing the trend sign at 96.7 % of sites. Negative trends are generally well represented, while positive trends are more frequently underestimated. Trend analyses across 18 regions of interest reveal significant AOD declines over eastern China, North Africa, Central Europe, and the eastern United States, alongside persistent increases over South Asia and the Middle East. Decadal analyses identify pronounced transitions, including a reversal from increasing to decreasing trends over eastern China and shifts in several dust- and biomass-burning-dominated regions. Sliding-window trend diagnostics further highlight temporal variability and regime changes. By integrating long-term reanalysis data, independent validation, and component-resolved diagnostics, this study provides a consistent assessment of recent global AOD evolution.