## 4 Results and Analyses
### Potential Source Region Contributions  
The CWT method is adopted to calculate the weighted concentrations of trajectories originating from various potential source regions. The CWT value represents a weighted specific concentration value by averaging sample concentrations that have associated trajectories that cross the grid cell, under the condition of other factors remaining relatively constant. The larger is the CWT value, the greater is the contribution of the grid cell to the pollutant concentration of Weifang City. The results are illustrated in Figure 6. The regions colored in black correspond to the main contributing sources associated with the highest PM\({}_{2.5}\) values, while the green color represents regions with low values.  
In spring, higher WCWT values are mostly concentrated in northeastern Henan and southwestern Shandong, which are important sources of PM\({}_{2.5}\) in Weifang. These areas have daily average PM\({}_{2.5}\) concentration contributions above 80 \(\upmu\)g/m\({}^{3}\). Additionally, other regions in Shandong, western Tianjin, and the central Bohai region influence pollution in Weifang. For these areas, the daily average PM\({}_{2.5}\) concentration contributions are more than 60 \(\upmu\)g/m\({}^{3}\).  
In summer, WCWT values are generally smaller. The highest CWT values covering the map were distributed in coastal areas in the east, such as Zhejiang and Jiangsu, with values of 60-80 \(\upmu\)g/m\({}^{3}\). This is possibly becauseoutheasterly wind dominates in Weifang in summer. Sea-salt aerosols from the south are easily transported to the Weifang region via the monsoon traveling northward.  
Figure 6: Spatial distribution of WCWT values of PM\({}_{2.5}\) in spring, summer, autumn, and winter from March 2015 to February 2016.  
Compared to those in summer, higher WCWT values in autumn are found in areas further south and northeast. These values are mainly observed at the intersection of Anhui, Jiangsu, and Shandong, with daily average PM\({}_{2.5}\) concentration contributions of above 80 \(\mathrm{\SIUnitSymbolMicro g/m^{3}}\). Furthermore, relatively substantial contributions are observed in certain localized areas such as eastern Hebei, southern Beijing, and Tianjin, and the daily average PM\({}_{2.5}\) concentration contributions of these areas exceed 60 \(\mathrm{\SIUnitSymbolMicro g/m^{3}}\).  
The WCWT values in winter are greater than those in other seasons. The greatest contributions to the PM\({}_{2.5}\) concentration in Weifang are throughout Shandong as well as certain nearby areas such as northeastern Henan, northern Jiangsu, and southwestern Hebei. The daily average contributions are more than 80 \(\mathrm{\SIUnitSymbolMicro g/m^{3}}\). This is probably because many pollutants are emitted when coal is burned for heating in the north in winter.  
When CWT results are compared to the WPSCF results, they have basically consistent results for areas contributing to the PM\({}_{2.5}\) concentration in Weifang, which demonstrates the credibility and accuracy of the analysis results. Nevertheless, the potential source regions obtained via the CWT method for spring, summer, and autumn cover larger areas than those simulated by the PSCF method, while the results for winter are similar. These results are consistent with the findings reported by [PERSON] et al. (2018) in Yinchuan [44]. These observations may arise because the CWT method takes into account all the concentrations rather than a subset of high concentrations in the PSCF method. The findings demonstrate that the seasonal distributions of PM\({}_{2.5}\) source areas obtained by these two methods share generally similar characteristics, suggesting the credibility and accuracy of the analysis results.