As described in Section 2.4, we tested correlations between the fraction of PM2.5 caused by within-city emissions and city characteristics such as city size and urban GDP (Figure ). The results suggest low correlation between fraction of within-city emitted PM2.5 and city population, GDP and city area for total, primary and secondary PM2.5. However, there are positive correlations between the fraction of within-city emitted PM2.5 and population buffer fraction for total and secondary PM2.5.