The first point is that we did not find any strong patterns among cities that could predict which emission sources contribute most to a city's PM2.5 concentrations, or how much of a city's ambient PM2.5 concentration originates from emissions within the city boundary (Figure ). To some extent, this contradicts previous findings by ; this discrepancy may be explained by the more detailed model of the atmosphere used here. The implication of this point is that it is necessary to perform atmospheric modeling in a given city to get a realistic estimate of the sources of that city's pollution-there don't seem to be any shortcuts. Since it may not be practical to carry out urban-level air quality simulations in a large number of cities using a comprehensive model like GEOS-Chem, this underscores the utility of reduced-complexity models such as InMAP.