Trends in Chemical Composition of Global and Regional Population-Weighted Fine Particulate Matter Estimated for 25 Years.

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.