In this study, we have produced estimates of PM2.5 concentration, its source composition, and contribution of in-city vs. out-of-city sources-the latter of which was not previously available in many global cities-that can be useful as a screening tool and in many cases may be robust enough to inform policy action to enact more effective strategies for improving public health. We find that although industrial-and energy-related sources are the largest contributors to PM2.5 in a plurality of cities, there is considerable diversity among cities in which source types are most important (Figure ). Because there is considerable uncertainty inherent in the results presented here, in some cases it may be advisable to leverage these results to obtain more precise estimates using a more mechanistically detailed air quality model and locally produced emission inventories before taking policy action, to the extent that resources are available to support the additional analysis.