For ambient particulate matter pollution, two complete, high resolution estimates exist of the concentration of particulate matter smaller than 2·5 µm in aerodynamic diameter (PM2·5) in ambient air: TM5 estimates—based on a nested three-dimensional global atmospheric chemistry transport model—which simulates both particulate matter and ozone at a high spatial resolution;22,23,121 and satellite-based estimates, which are based on satellite observations of aerosol optical depth, a measure of light extinction by aerosols in the total atmospheric column.25 TM5 and satellite-based estimates of PM2·5, measured in µg/m3, were averaged at a 0·1° × 0·1° grid cell resolution (equivalent to roughly 11 km × 11 km at the equator) and linked to available measures of PM2·5 from ground-based monitors. We used a regression model with the average of TM5 and satellite-based estimates as the predictor to estimate ground-based PM2·5 for all grid cells.26 For ozone, we relied solely on the TM5 model.
