The indirect emission flows among various sectors are shown in figure 3. All sectors are marked with different colors. In the inner circle, the trajectories, which have the same color with the import sectors, represent the NOx emission flows from import sectors to export sectors. The width of flows can be used to recognize the flow rates among sectors. The percentage of each flow shown in the outermost arc represents the proportion of inflow or outflow in total emission flows. The middle arc shows the proportion of each inflow in total inflows, and the innermost arc displays the proportion of each outflow in total outflows.