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(o) X 0.1(o)) to understand global trends in population-weighted mean chemical composition of fine particulate matter (PM2.5). Trends in observed and simulated population-weighted mean PM2.5 composition over 1989-2013 are highly consistent for PM2.5 (-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 PM2.5 for 20 of 21 global regions. Over 1989-2013, most (79%) of the simulated increase in global population-weighted mean PM2.5 of 0.28 mu g m(-3)yr(-1) is explained by significantly (p < 0.05) increasing OA (0.10 mu g m(-3)yr(-1)), nitrate (0.05 mu g m(-3)yr(-1)), sulfate (0.04 mu g m(-3)yr(-1)), and ammonium (0.03 mu g m(-3)yr(-1)). These four components predominantly drive trends in population-weighted mean PM2.5 over populous regions of South Asia (0.94 mu g m(-3)yr(-1)), East Asia (0.66 mu g m(-3)yr(-1)), Western Europe (-0.47 mu g m(-3)yr(-1)), and North America (-0.32 mu g m(-3)yr(-1)). Trends in area-weighted mean and population-weighted mean PM2.5 composition differ significantly.