3.2.2. PSCF Analysis

As shown in Figure 6, the potential source areas for PM2.5 in Harbin were mainly in Mongolia, Inner Mongolia, and Hebei in 2016, with the addition of Henan in 2020, and the potential source areas in Changchun were mainly in Mongolia, Shandong, and Jiangsu in 2016, with Shandong and eastern Russia in 2020. The potential source areas of PM2.5 in Shenyang were mainly located in Mongolia, Inner Mongolia, and Shanxi as well as the border between Heilongjiang and Jilin and the East China Sea, which will be reduced to Mongolia, Jilin Province and Heilongjiang Province in 2020. The above results show that the northwestern countries and provinces are the main contributors to the external sources of PM2.5 in these three large cities. The reason is that the dust aerosols from Mongolia, Russia, and Inner Mongolia and the anthropogenic particulate emissions from the passing areas arrive in northeastern China along with the northwestern winds, thus contributing to a certain PM2.5 concentration, indicating that the long-distance transmission of particulate matter is noteworthy.

**Figure 6.** Potential source areas of PM2.5 and O3 in the capital cities of northeast China.

The potential source areas of O3 in Harbin in 2016 were mainly distributed in Shandong Province, Jiangsu Province, and the Yellow Sea and Bohai Sea, and remain unchanged in 2020. The potential source areas of O3 in Changchun in 2016 were mainly distributed in the same way as Harbin in 2016, and some areas in Anhui Province were added in 2020. The potential source areas of O3 in Shenyang in 2016 were mainly distributed in Anhui, Jiangsu, and the Yellow Sea and Bohai Sea, and remain unchanged in 2020. The distribution of potential source areas for O3 in Shenyang in 2016 was mainly in Anhui, Jiangsu, and the Yellow Sea and Bohai Sea, and remained the same in 2020. In summary, the potential source areas of O3 in these three cities were generally concentrated in Shandong, Anhui, Jiangsu, and the Yellow Sea and Bohai Sea, among which the WPSCF values were generally higher in the Yangtze River Delta region, which emits NOx and VOC plus photochemical pollution due to the developed shipbuilding industry, petrochemical industry, and industry, which play an obvious role in promoting the generation of O3 and will further influence the atmospheric oxidation through regional transmission. This affects the atmospheric oxidation in the northeast, resulting in high WPSCF values in these areas. The O3 generated from the refraction of sunlight by marine droplets in the Yellow Sea and Bohai Sea reaches

the northeast region through regional transport, which is also the reason for the high WPSCF values in the Yellow Sea and Bohai Sea region.

## 3.2.3. CWT Analysis

The CWT distribution characteristics of PM2.5 and O3 in the capital cities of northeast China in 2016–2020 are shown in Figure 7. For PM2.5, in the results for Harbin, the high contributing regions are mainly concentrated in Mongolia, Russia, and Inner Mongolia in 2016 and 2020. In Changchun's results, the high contributing regions were concentrated in Heilongjiang, Inner Mongolia, Shandong, and Hebei Provinces in 2016, and changed to Russia, Mongolia, Jilin, and Shandong Provinces in 2020. For Shenyang, the high contributing regions were concentrated in Russia, Mongolia, Inner Mongolia, Shanxi, Hebei, and northeast China in 2016 and reduced to Shanxi and Hebei in 2020. For O3, in the results for Harbin, the high contributing regions in 2016 were mainly in Shandong, Jiangsu, and the Yellow and Bohai Seas, with the addition of Liaoning in 2020. For Changchun, the high contributing regions were mainly located in Hebei, Shandong, and the Yellow and Bohai Seas in 2016, with the addition of Jiangsu and Liaoning in 2020. In the results for Shenyang, the high contributing areas were mainly located in Shandong, Jiangsu, Anhui, and the Yellow Sea and Bohai Sea in 2016, and remain unchanged in 2020.

**Figure 7.** Results of CWT analysis of PM2.5 and O3 in the capital cities of northeast China.

In summary, it can be concluded that the PM2.5 concentrations contributed by neighboring cities are essentially the contribution of local emissions in northeast China, while long-range transport also contributes a high PM2.5 concentration. Compared with O3, the concentration contribution of PM2.5 through regional transport comes more from the northwest, which is consistent with the cluster analysis results. In contrast, O3 is more influenced by air masses transmitted from the south, which also confirms that O3 pollution south of northeastern China is serious and even affects the air quality in the north, as pointed out by other studies [34].
