Metropolitan-level policy can play an important role in urban air quality ), but policymakers in many global cities have not previously had access to detailed information regarding where their ambient PM2.5 pollution comes from. Common initial questions from policy makers, in considering how to address air pollution, include (1) Source apportionment: which sources (electricity generation, transportation, industry, etc.) contribute substantially to ambient pollution levels? and (2) Local influence and authority: how much pollution is generated within the city versus transported in from outside? Previous globally-scoped studies have provided this information at national or subcontinental resolution (e.g., , which is of limited use for decision-making at the urban level, or have provided information on the sources contributing to pollution in cities based on a relatively low-resolution emissions inventory but no information regarding whether those sources are located within or external to the city boundaries .