Global Estimates and Long-Term Trends of Fine Particulate Matter Concentrations (1998-2018).

Exposure to outdoor fine particulate matter (PM2.5) is a leading risk factor for mortality. We develop global estimates of annual PM2.5 concentrations and trends for 1998-2018 using advances in satellite observations, chemical transport modeling, and ground-based monitoring. Aerosol optical depth (AOD) from advanced satellite products including finer resolution, increased global coverage, and improved long-term stability, are combined and related to surface PM2.5 concentrations using geophysical relationships between surface PM2.5 and AOD simulated by the GEOS-Chem chemical transport model with updated algorithms. The resultant annual mean geophysical PM2.5 estimates are highly consistent with globally distributed ground monitors (R2=0.81; slope=0.90). Geographically weighted regression is applied to the geophysical PM2.5 estimates to predict and account for the residual bias with PM2.5 monitors, yielding even higher cross validated agreement (R2=0.90-0.92; slope=0.90-0.97) with ground monitors, and improved agreement compared to all earlier estimates. The consistent long-term satellite AOD and simulation enable trend assessment over a 21 year period, identifying significant trends for eastern North America (-0.28±0.03 μg/m3/yr), Europe (-0.15±0.03 μg/m3/yr), India (1.13±0.15 μg/m3/yr), and globally (0.04±0.02 μg/m3/yr). The positive trend (2.44±0.44 μg/m3/yr) for India over 2005-2013 and the negative trend (-3.37±0.38 μg/m3/yr) for China over 2011-2018 are remarkable, with implications for the health of billions of people.