2.3.1. PSCF Analysis

In this study, PSCF analysis was used to locate the pollution sources. The weighting factor Wij used in this study reduced the uncertainty of the PSCF results, called WPSCF, and analyzed the potential source contribution areas of PM2.5 and O3 in the capital cities of northeast China for the years 2016–2020 overall and for single years 2016 and 2020 [21]. In order to calculate the PSCF, the area through which the trajectory passes should first be gridded, and the grid resolution was set to 0.5 × 0.5 in this study [22,23].

For the PSCF analysis of trajectories, we combined the pollution of PM2.5 and O3 in northeast China to mark trajectories with O3 concentrations more significant than the 75th percentile and PM2.5 concentrations greater than 75 μg/m<sup>3</sup> as exceedance trajectories, where the PM2.5 limit value was the limit value of the secondary standard of the National

Ambient Air Quality Standard (NAAQS) set by the Ministry of Ecology and Environment of China. A weighting factor Wij was introduced. Its value will depend on the relationship between the sum of the transmission time of all trajectories in a particular grid and the average residence time of each grid. In this study, Wij is expressed by Equation (4):

$$\mathbf{W}\_{\text{i\bar{j}}} = \begin{cases} 1.0 & \mathbf{n}\_{\text{i\bar{j}}} > 90 \\ 0.8 & 30 < \mathbf{n}\_{\text{i\bar{j}}} \le 90 \\ 0.45 & 20 < \mathbf{n}\_{\text{i\bar{j}}} \le 30 \\ 0.10 & 0 < \mathbf{n}\_{\text{i\bar{j}}} \le 20 \end{cases} \tag{4}$$

## 2.3.2. CWT Analysis

The CWT method can quantify the concentration contribution level of external transport by taking the average value of the concentration of samples corresponding to all trajectories passing through a single grid during the study period [24]. The CWT analysis method can obtain the difference in the pollution level of contaminated trajectories by calculating the weighted degree. In the CWT analysis method, each grid point is assigned a degree of weight. By introducing the same numerical correction from Wij in the PSCF method, the weighted average concentration value (WCWT value) can be used to distinguish the source intensity of potential sources [25,26].

A higher WCWT value in the grid indicates that the air mass passing through the grid results in a high receiving point concentration, and the area corresponding to this grid can be considered as a potential area of high concentration contribution to the external transport of pollutants from the receiving area.
