Season-Specific Spatial Organization of Global Aerosol Optical Depth from 25 Years of MODIS Observations Revealed by Nonnegative Matrix Factorization

This study presents a season-specific non-negative matrix factorization (NMF) framework for investigating the long-term spatiotemporal variability of global aerosol optical depth (AOD) at 550 nm using monthly MODIS Collection 6.1 observations (MOD08_M3) spanning 2000–2025. To maximize spatial completeness in the long-term dataset, NMF is performed independently for each calendar month, followed by a season-specific mode-matching strategy that establishes consistent correspondence among the extracted spatial organizations within each climatological season. The proposed framework represents the global AOD field using a limited number of dominant spatial organizations and their corresponding temporal coefficients, revealing recurrent AOD patterns spatially associated with major continental aerosol regions, trans-Atlantic Saharan dust transport, biomass-burning regions over tropical Africa and South America, and large-scale continental background variability. The extracted spatial organizations remain highly coherent within individual seasons, while their temporal coefficients exhibit pronounced interannual variability. Independent assessment using the MODIS Deep Blue Ångström exponent provides complementary particle-size information that supports the physical interpretation of these AOD spatial organizations. These results demonstrate that global AOD variability can be characterized by the seasonal redistribution and varying expression of recurrent spatial organizations rather than by fundamental changes in their geographical structures. This framework provides a compact, physically interpretable, and low-dimensional representation of global aerosol spatial and temporal variability and establishes a general strategy for investigating long-term satellite observations of atmospheric and other Earth system variables.