## 4 Results and Analyses
### Potential Source Regions  
To further investigate the sources of atmospheric pollutant transport in Weifang, this paper analyzes the potential source regions of PM\({}_{2.5}\) pollution. First, PM\({}_{2.5}\) concentration data are added to airflow trajectories, and areas covering all airflow trajectories are uniformly divided into 0.5\({}^{\circ}\times\) 0.5\({}^{\circ}\) grids. Then, the weighted potential source contribution function (WPSCF) value of each grid is computed. WPSCF reveals the spatial distribution of PM\({}_{2.5}\) potential sources obtained by combining back-trajectories and measurements of PM\({}_{2.5}\) concentration. A high PSCF value signifies a potential source location. The greater the WPSCF value of a grid is, the higher the contribution levels of potential source regions to PM\({}_{2.5}\) pollution in Weifang, given that other factors remain stable.  
Figure 5 shows the PSCF results for PM\({}_{2.5}\) in Weifang from 2015 to 2016. The colors represent the contribution levels of potential source areas; the black color is associated with high concentrations, while green represents low PM\({}_{2.5}\) concentrations. Distinct seasonal variations are noted in the distribution of the potential source areas of PM\({}_{2.5}\). (1) In spring, high PSCF values are mainly found in north-central Jiangsu and southwestern Shandong. Additionally, areas such as Tianjin, Liaoning and western Jilin have key influences on the sources of pollution in Weifang. (2) In summer, PSCF values are generally smaller, indicating that Weifang is less affected by pollutants from the surrounding areas in summer. Compared to those in spring, the potential sources are shifted eastward and are located in Shandong and the waters of the Bohai Sea and Yellow Sea. This is may because the southeastly  
monsoon carries pollutants emitted from passed areas when it moves northward. Furthermore, stubble burning is the most intensive in some southern regions in summer, creating large fumes that easily travel northward with the monsoon. (3) In autumn, higher PSCF values are increasing and are mainly found in regions such as northern Anhui, northeastern Henan, and southwestern Shandong. In addition, areas such as Hebei, Beijing, Tianjin, eastern Shanxi, and central Inner Mongolia make certain potential contributions. (4) In winter, potential source areas transit and extend northwestward. High values are found in the whole city of Shandong, northern Jiangsu, northeastern Henan, and southern Hebei; in these regions, their situations are very similar to that of Weifang, with the same climate, industrial emission, population density, and winter heating. In addition, those values in eastern Shanxi and western Inner Mongolia increase.  
In summary, in winter, spring, and autumn, Weifang is more substantially affected by nearby inland cities in Jiangsu, Henan, and Shandong, and these areas are potential source regions of pollutants in Weifang. In contrast, in summer, Weifang is less influenced by pollutants from surrounding areas. Moreover, comparisons reveal that the distributions of high PSCF values in different seasons agree well with those of the major trajectory areas shown in Table 1. This finding indicates that the potential source contribution results obtained by the PSCF method in this paper are reasonably reliable.  
Figure 5: Spatial distribution of WPSCF values of PM\({}_{2.5}\) in spring, summer, autumn, and winter from March 2015 to February 2016.