*3.2. Backward Trajectory-Based PSCF and CWT Analysis*

#### 3.2.1. Backward Trajectory Clustering Analysis

As shown in Figure 5, Changchun, Harbin, and Shenyang are all dominated by the northwest direction trajectory, followed by the southwest direction, and the trajectory with the longest transmission distance also tends to appear in the northwest direction. For Changchun, the proportion of northwest trajectories was 77.43% in 2016, while it decreased to 64.33% in 2020, while the proportion of southwest trajectories increased from 14.32% in 2016 to 27.94% in 2020. For Harbin, the northwest trajectory accounted for 64.34% in 2016, while it decreased to 56.41% in 2020, and the southwest trajectory increased from 19.73% in 2016 to 23.11% in 2020. For Shenyang, the percentage of northwest trajectory was 65.91% in 2016, while it dropped to 60.33% in 2020, and the percentage of southwest trajectory increased from 22.79% in 2016 to 26.09% in 2020.

**Figure 5.** Backward trajectory clustering results for the northeast provincial capitals, 2016 and 2020 (trajectory simulation time interval was 1 h, simulation time was 72 h).

Overall, the city with the most northwest-oriented trajectory was Changchun, which is located in the central part of the northeast region, with the Changbai Mountains in the west, so the air masses transmitted from Russia and Japan in the east are small, while too many mountains do not block Harbin and Shenyang, so the air masses transmitted in the east share part of the percentage. The city with the most southwestern trajectories was Shenyang, which is due to the different altitudes of the three cities. Shenyang, which has the lowest latitude, was most strongly influenced by the southward Bohai anticyclone, so the total percentage of trajectories was the highest, while Harbin, which has the highest latitude, was the least influenced by the Bohai anticyclone, so the percentage of trajectories was the lowest. It can also be found that compared with 2016, the trajectory share of the southwest direction in all three cities increased in 2020, while the trajectory share of the northwest direction decreased, which was related to the annual meteorological conditions in the year.
