The largest contributors to both total PM2.5 concentrations (Figure ) and PM2.5 concentrations caused by within-city emissions are most commonly industry, energy transformation and extraction and residential and commercial activities. For example, 49% and 38% of cities have industry ranked the largest for total PM2.5 and PM2.5 concentrations caused by within-city emissions, respectively; 26% of cities have energy transformation and extraction ranked the largest for both total PM2.5 and PM2.5 concentrations caused by within-city emissions; and 9% and 22% of cities have residential and commercial activities ranked the largest total PM2.5 and PM2.5 concentrations caused by within-city emissions, respectively. The two largest sources of PM2.5 caused by out-of-city emissions are the same as total PM2.5 and PM2.5 caused by within-city emissions; however, the third largest source of PM2.5 caused by out-of-city emissions is dust, which is the largest out-of-city source in 21% of cities. Only 9% of cities have surface transportation listed as the largest PM2.5 source caused by within-city emissions. Although industrial combustion/processing and energy transformation/extraction are the top PM2.5 sources in many cities, there is substantial variability in which emission sources contribute the most across all 96 cities. Thus, an important implication of these findings is that one-size-fits-all approaches to air quality management are unlikely to work across urban areas. Instead, management practice likely needs approaches tuned to local context, including which sources dominate for that city.