Vertical Evolution of Global Tropospheric Aerosol Optical Depth During 2006–2021: Contributions From Anthropogenic Influence, Dust Transport, and Wildfire Smoke

Abstract Long‐term changes in aerosol optical depth (AOD) and its vertical structure are important for understanding aerosol climate effects. However, how aerosols from different sources contribute to AOD changes across tropospheric layers remains unclear. Using CALIPSO observations from 2006 to 2021, together with time‐series decomposition, aerosol‐type analysis, and concentration‐weighted trajectory analysis, this study investigates AOD changes in the lower (0–4), middle (4–8), and upper (8–12 km) troposphere, cross‐validated by MODIS and MERRA‐2 data sets. Results show that global AOD changes exhibit pronounced vertical and regional differences. Lower‐tropospheric AOD decreased significantly over densely populated regions such as China and eastern U.S., mainly because of reductions in local anthropogenic‐influenced aerosols. Over West Asia, lower‐ and middle‐tropospheric dust weakened markedly, and the seasonal peak of lower‐tropospheric AOD declined by 41% after 2017, associated with weaker Arabian Peninsula dust emissions and suppressed dust transport under anomalous spring–summer circulation patterns. Across 30°N–60°N, dust and polluted dust declined widely in the middle and upper troposphere. After 2015, the trend components of middle‐ and upper‐tropospheric AOD over the northwestern Pacific decreased by 40% and 27%, respectively, consistent with weakened dust uplift and reduced eastward long‐range transport. Intensified wildfire activity after 2012 altered lower‐tropospheric AOD seasonality over central Russia from bimodal to trimodal and increased the frequency of middle‐ and upper‐tropospheric AOD anomalies over North America and western Europe. These major variations are generally consistent with MODIS and MERRA‐2 observations, highlighting the importance of vertical aerosol structures and transport processes in understanding global aerosol evolution.