Because the air quality model simulations we perform here require a substantial amount of expertise, time and computational resources, we investigate whether patterns exist in the underlying results that could potentially allow extrapolation beyond the 96 cities we studied. To do so, we analyze the relationship between the fraction of PM2.5 originating from within city sources and various city characteristics such as city population, gross domestic product (GDP), city area, and "population buffer fraction". We define population buffer fraction as the city population divided by the total population within a radius of 200-km from the city centroid. The goal of this supplementary analysis is to explore whether there might exist a straightforward method to reproduce the results shown here without extensive air quality modeling.