Concentration estimates for each city (Figure ) demonstrate substantial variability among urban areas, in terms of concentrations as well as the proportion that is primary vs secondary particulate matter. The highest levels of total PM2.5 as predicted here are mainly in Asian cities. As described in Section 3.3, InMAP and other mechanistic model predictions of total PM2.5 concentrations in global cities are often substantially different from measured concentrations. Some studies (for example ) calibrate their mechanistic model predictions to measurement and remote sensing data, but we do not do that here to provide a more accurate sense of the level of uncertainty surrounding our predictions.