## 5 Discussion and Conclusions  
Based on PM\({}_{2.5}\) monitoring data from 2015 to 2016 from 38 air quality monitoring stations, this paper describes an in-depth study of the seasonal variations in internal and external potential sources in Weifang by using different trajectory analysis methods. 1. Seasonal differences in the contributions of potential source regions of PM\({}_{2.5}\) in Weifang. In winter, spring, and autumn, airflows are mostly from the northwesterly and northerly directions and significantly influence the PM\({}_{2.5}\) concentration in Weifang. However, in summer, airflow trajectories are scattered, and warm and humid airflows from the ocean in the southeastern direction dominate. In winter, spring, and autumn, Weifang is more greatly affected by pollutant transport from nearby inland cities in Shandong and Henan. These transport pathways are short in general, and the wind speeds are low, leading to accumulation of the carried pollutants in Weifang. In contrast, in summer, Weifang is less influenced by pollutants from the surrounding areas, and the potential source regions are mainly located in coastal areas in the east, such as Jiangsu, the Bohai Sea, and the Yellow Sea. It should be noted that the region covering central Inner Mongolia and southern Liaoning is also a potential source area for Weifang. 2. Policy implications for PM\({}_{2.5}\) pollution in Weifang. The results indicate that, in formulating relevant pollution control and prevention measures, the government should focus on the control of pollutant sources and take the migration of regional pollution caused by these sources into account. For example, according to the transport patterns of pollutants in Weifang, the surrounding pollutant source regions can be divided and classified (such as key control zones). This finding suggests that different control and management policies can be implemented, and region-specific pollution control and prevention measures can be formulated. Because Weifang is more significantly affected by short-distance pollutant transport from the surrounding cities and provinces as well as inland areas in Shandong Province, the government should pay more attention to short-distance transport from these regions. For instance, more intensive greening measures can be introduced to reduce the short-distance transport of PM\({}_{2.5}\). Furthermore, the interactions between the city and its surrounding areas should be considered, and joint control and cooperation between different regions should be enhanced. Evaluation of the analysis results. The results demonstrate that the seasonal distributions of PM\({}_{2.5}\) source areas obtained by two methods (PSCF and CWT) share generally similar characteristics, demonstrating the credibility and accuracy of the analysis results. Both methods can effectively reflect the potential source areas of pollution in the region. However, there is a limitation in validating the modeling results with the real transport values because emissions and depositions of air pollutants along the trajectories cannot be captured easily. Nevertheless, our results are able to locate the source direction and areas and identify source contributions to a certain extent. To support this point, we analyzed the notice issued by Shandong Provincial Environmental Protection Office, which indicates that the primary source of external dust transport from Inner Mongolia is the main reason that leads to the rapid increase in pollutants in Weifang City in April [45]. Thus, some references indicated that in northern China (such as Weifang), the main pollution sources come from anthropogenic emissions related to coal burning and their transportation [46]. These cases both support our findings regarding spring and winter. In addition, PSCF and CWT are appropriate tools that help identify source contributions to the concentration variations at the destination, assuming that other elements/factors remain the same, such as the period of the kite festival (in April every year).