Fig. 6: Relationship between HCHO VCD in the eastern region and transportation, and source of transportation.

A Relationship between the average HCHO VCD in the eastern region and the dot product of the wind vector and the normal vector at the southern boundary of the eastern region. The average HCHO VCD in the corresponding dot product segment is the red solid line. B Backward trajectory of the southern boundary points in the four seasons of the eastern region from 2013 to 2021. The backward trajectory (−1 day) and proportion of each backward trajectory in a Spring, b Summer, c Autumn, and d Winter. The total proportion of the four backward trajectories in one map is 100%, and the number next to the line is the proportion of the corresponding backward trajectory.