## 4 Results and Analyses
### Transport Trajectories in Different Seasons
These types together account for 60.98% of the seasonal total polluted trajectories, suggesting that PM\({}_{2.5}\) is mainly transported from the north and southeast directions to Weifang in spring. 2. Pollution characteristics in summer The average PM\({}_{2.5}\) concentration of summer trajectories is the lowest, only 49.40 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\). In summer, many plants flourish, and the total leaf area of surface vegetation significantly increases, which is conducive to the adsorption of atmospheric particulates. Moreover, precipitation in summer is more concentrated, and the increased precipitation greatly facilitates wet deposition and dilution of atmospheric pollutants. However, under the control of the subtropical high and typhoon, the trajectories are diverse. The trajectory clusters in descending order of average PM\({}_{2.5}\) concentration are: \(4>3>5>2>1\). The airflow trajectories from southeast have the highest value (68.16 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)), followed by those from northwest (62.95 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)). This pattern indicates that external pollutant emissions from southeast and northwest have a substantial effect on PM\({}_{2.5}\) pollution in Weifang. Although Type 4 originated from the Yellow Sea, affected by subtropical auxiliary high pressure and wheat straw burning in south China, pollutants and suspended particles from eastern Zhejiang and southern Shandong are easily carried by the trajectory and accumulate Weifang. Type 1 trajectories originating from the Bohai Sea have the lowest average PM\({}_{2.5}\) concentration (41.50 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) and make up the largest proportion of the total trajectories. These trajectories mainly pass over waters and coastal areas in eastern Shandong. These areas are characterized by strong air masses, cleanliness, and intensive atmospheric wet deposition, which facilitate wet deposition and dilution of pollutants [1; 6; 42]. The average PM\({}_{2.5}\) concentration of polluted trajectories is 94.92 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\). Types 3-5 trajectories account for 20.68%, 43.19%, and 19.63% of the total polluted summer trajectories, respectively. They together account for 83.50% of the seasonal total polluted trajectories, indicating that PM\({}_{2.5}\) is mainly transported in easterly directions to Weifang in summer. 3. Pollution characteristics in autumn Autumn trajectories have an average PM\({}_{2.5}\) concentration of 80.53 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\). The autumn trajectory clusters in descending order of PM\({}_{2.5}\) concentration are: \(1>2\). In particular, Type 2 trajectories are longer and have lower PM\({}_{2.5}\) concentrations. These trajectories originate mostly from northwestern Mongolia, central Inner Mongolia, and northern Hebei and travel over extensive areas at high wind speeds. In addition, they pass over the waters of the Bohai Sea and enter Weifang directly. The air masses are relatively clean, which is conducive to the diffusion of pollutants. In contrast, the PM\({}_{2.5}\) concentrations of Type 1 trajectories are higher, with an average of 94.52 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\). This is because short and slow Type 1 trajectories do not favor the diffusion of pollutants. Furthermore, these airflowtrajectories pass over the northeastern part of Shandong before they reach Weifang and often carry pollutants from Shandong, leading to higher PM\({}_{2.5}\) concentrations. The polluted trajectories have an average PM\({}_{2.5}\) concentration of 120.72 \(\upmu\)g/m\({}^{3}\). Type 1 trajectories account for 52.60% of the total polluted trajectories. It follows that the northeasterly direction is the major PM\({}_{2.5}\) transport direction to Weifang in autumn. (4) Pollution characteristics in winter  
Compared to the other three seasons, the average PM\({}_{2.5}\) concentration of winter trajectories is the highest, reaching 115.54 \(\upmu\)g/m\({}^{3}\).