An accurate fine-resolution surface of the chemical composition of fine particulate matter (PM 2.5 ) would offer valuable information for epidemiological studies and health impact assessments. We develop geoscience-derived estimates of PM 2.5 composition from a chemical transport model (GEOS-Chem) and satellite observations of aerosol optical depth, and statistically fuse these estimates with ground-based observations using a geographically weighted regression over North America to produce a spatially complete representation of sulfate, nitrate, ammonium, black carbon, organic matter, mineral dust, and seasalt over 2000−2016. Significant long-term agreement is found with cross-validation sites over North America (R 2 = 0.570.96), with the strongest agreement for sulfate (R 2 = 0.96), nitrate (R 2 = 0.90), and ammonium (R 2 = 0.86). We find that North American decreases in population-weighted fine particulate matter (PM 2.5 ) concentrations since 2000 have been most heavily influenced by regional changes in sulfate and organic matter. Regionally, the relative importance of several chemical components are found to change with PM 2.5 concentration, such as higher PM 2.5 concentrations having a larger proportion of nitrate and a smaller proportion of sulfate. This data set offers information for research into the health effects of PM 2.5 chemical components.

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We interpret in situ and satellite observations with a chemical transport model (GEOS-Chem, downscaled to 0.1° × 0.1°) to understand global trends in population-weighted mean chemical composition of fine particulate matter (PM). Trends in observed and simulated population-weighted mean PM composition over 1989-2013 are highly consistent for PM (-2.4 vs -2.4%/yr), secondary inorganic aerosols (-4.3 vs -4.1%/yr), organic aerosols (OA, -3.6 vs -3.0%/yr) and black carbon (-4.3 vs -3.9%/yr) over North America, as well as for sulfate (-4.7 vs -5.8%/yr) over Europe. Simulated trends over 1998-2013 also have overlapping 95% confidence intervals with satellite-derived trends in population-weighted mean PM for 20 of 21 global regions. Over 1989-2013, most (79%) of the simulated increase in global population-weighted mean PM of 0.28 μg myr is explained by significantly (p < 0.05) increasing OA (0.10 μg myr), nitrate (0.05 μg myr), sulfate (0.04 μg myr), and ammonium (0.03 μg myr). These four components predominantly drive trends in population-weighted mean PM over populous regions of South Asia (0.94 μg myr), East Asia (0.66 μg myr), Western Europe (-0.47 μg myr), and North America (-0.32 μg myr). Trends in area-weighted mean and population-weighted mean PM composition differ significantly.

The World Health Organization (WHO) recently released new guidelines for outdoor fine particulate air pollution (PM 2.5 ) recommending an annual average concentration of 5 μg/m 3 . Yet, our understanding of the concentration-response relationship between outdoor PM 2.5 and mortality in this range of near-background concentrations remains incomplete. To address this uncertainty, we conducted a population-based cohort study of 7.1 million adults in one of the world’s lowest exposure environments. Our findings reveal a supralinear concentration-response relationship between outdoor PM 2.5 and mortality at very low (<5 μg/m 3 ) concentrations. Our updated global concentration-response function incorporating this new information suggests an additional 1.5 million deaths globally attributable to outdoor PM 2.5 annually compared to previous estimates. The global health benefits of meeting the new WHO guideline for outdoor PM 2.5 are greater than previously assumed and indicate a need for continued reductions in outdoor air pollution around the world.