Monthly Global Estimates of Fine Particulate Matter and Their Uncertainty.

Annual global satellite-based estimates of fine particulate matter (PM _2.5 ) are widely relied upon for air-quality assessment. Here, we develop and apply a methodology for monthly estimates and uncertainties during the period 1998-2019, which combines satellite retrievals of aerosol optical depth, chemical transport modeling, and ground-based measurements to allow for the characterization of seasonal and episodic exposure, as well as aid air-quality management. Many densely populated regions have their highest PM _2.5 concentrations in winter, exceeding summertime concentrations by factors of 1.5-3.0 over Eastern Europe, Western Europe, South Asia, and East Asia. In South Asia, in January, regional population-weighted monthly mean PM _2.5 concentrations exceed 90 μg/m ³ , with local concentrations of approximately 200 μg/m ³ for parts of the Indo-Gangetic Plain. In East Asia, monthly mean PM _2.5 concentrations have decreased over the period 2010-2019 by 1.6-2.6 μg/m ³ /year, with decreases beginning 2-3 years earlier in summer than in winter. We find evidence that global-monitored locations tend to be in cleaner regions than global mean PM _2.5 exposure, with large measurement gaps in the Global South. Uncertainty estimates exhibit regional consistency with observed differences between ground-based and satellite-derived PM _2.5 . The evaluation of uncertainty for agglomerated values indicates that hybrid PM _2.5 estimates provide precise regional-scale representation, with residual uncertainty inversely proportional to the sample size.