3.3.1. Relationship between the Summer Jet Stream and Surface Meteorological Variables

According to the analyses in Section 3.1.1, it can be concluded that the EOF1 of the 200 hPa zonal wind in the summer from 2013 to 2018 represents the position variation in the summer jet stream. The correlation analysis between the time series of the EOF1 and the surface meteorological variables in the corresponding period can be regarded as the correlation analysis between the position of the upper-level jet stream and the surface meteorological variables in summer. Figure 12 shows that, in the North China Plain, the position of the East Asian upper-level jet in summer has significant positive correlations with the surface humidity and temperature and negative correlations with the surface meridional and zonal wind. In the Tarim Basin, the position variation in the East Asian upper-level jet in summer is significantly positively correlated with the humidity and temperature and negatively correlated with the surface zonal wind. However, the position variation in the East Asian upper-level jet is positively correlated with the surface meridional wind in a small region in the western part of the Tarim Basin, and there is a negative correlation between them in the eastern part of the Tarim Basin.

Combined with the spatio-temporal distribution of the first mode of the 200 hPa zonal wind, it can be said that when the position of the East Asian upper-level jet is more northward, the surface humidity and temperature in North China Plain are higher, and the surface meridional wind and zonal wind are weaker. The surface humidity and temperature in the Tarim Basin are higher, and the surface zonal wind is weaker. The surface meridional wind in the west part of the Tarim Basin is stronger, and weaker in the east part, and vice versa.

The EOF2 of the 200 hPa zonal wind in summer from 2013 to 2018 represents the intensity variation in the summer jet stream. The correlation analyses between the time series of the EOF2 and the surface meteorological variables in the corresponding period represent the relationship between the intensity of summer upper-level jet stream and the surface meteorological variables. In Figure 13, the intensity of the East Asian jet stream is proportional to the surface temperature in the North China Plain, while it is insignificantly related to the surface humidity, meridional wind and zonal wind. In the Tarim Basin, the intensity of the East Asian upper-level jet has a significantly negative correlation with the surface humidity in the northern region, a significantly positive correlation with the surface temperature in the whole area and a significantly negative correlation with the surface zonal wind in the eastern region, but its relationship with the surface meridional wind is insignificant.

**Figure 12.** The correlation coefficients between the time series of the EOF1 of the 200 hPa zonal wind and surface meteorological variables of the (**a**) humidity, (**b**) temperature, (**c**) surface zonal wind and (**d**) surface meridional wind in summer during 2013–2018. The slashes indicate that the results passed the 95% Monte Carlo correlation test.

**Figure 13.** The correlation coefficients between the time series of the EOF2 of the 200 hPa zonal wind and surface meteorological variables of the (**a**) humidity, (**b**) temperature, (**c**) surface zonal wind and (**d**) surface meridional wind in summer during 2013–2018. The slashes indicate that the results passed the 95% Monte Carlo correlation test.

According to the spatio-temporal distribution of the second mode of the 200 hPa zonal wind, when the intensity of the East Asian upper-level jet is weaker, the surface temperature in the North China Plain is higher, the surface humidity in the northern Tarim Basin is lower, the surface temperature in the region is higher and the surface zonal wind in the eastern part of the basin is weaker, and vice versa.

3.3.2. Relationship between Pollutants and Surface Meteorological Variables in Summer

The correlation coefficients of the surface pollutants including PM10, O3 and PM2.5 with the daily average data of surface meteorological variables in summer from 2013 to 2018 are shown in Figure 14. The PM10, O3, and PM2.5 in the North China Plain are negatively correlated with the surface humidity and are significantly positively correlated with the surface temperature, zonal wind and meridional wind. However, the significant regions of the correlations between different pollutants and meteorological variables are different. The PM2.5 has a significantly negative correlation with the humidity only in the southern part of the North China Plain. The PMs maintain significant positive correlations with the surface temperature only in the northern and southeastern parts of the North China Plain. In addition, these three pollutants are significantly negatively correlated with the surface zonal wind in different areas in the east parts of the North China Plain. That is, when the pollutant concentrations in the North China Plain are higher (lower), the surface humidity in the certain region is lower (higher) correspondingly, the temperature is higher (lower), and the zonal wind and the meridional wind are stronger (weaker).

**Figure 14.** Correlation coefficients between surface pollutants and corresponding surface meteorological variables in summer from 2013 to 2018. (**a**) PM10 and humidity, (**d**) PM10 and temperature, (**g**) PM10 and zonal wind and (**j**) PM10 and meridional wind. (**b**) O3 and humidity, (**e**) O3 and temperature, (**h**) O3 and zonal wind and (**k**) O3 and meridional wind. (**c**) PM2.5 and humidity, (**f**) PM2.5 and temperature, (**i**) PM2.5 and zonal wind and (**l**) PM2.5 and meridional wind. The slashes indicate that the results passed the 95% Monte Carlo correlation test.

Both the PM10 and PM2.5 in the Tarim Basin have good heterocorrelations with the surface humidity and zonal wind, and the PMs are significantly positively correlated with the surface temperature only in the west part of the Tarim Basin and have significant negative correlations with the surface meridional wind in the south part of the Tarim Basin. That is, when the concentrations of the PM10 and PM2.5 in the Tarim Basin are higher (lower), the surface humidity in the region is lower (higher) and the zonal wind is weaker (stronger). The surface temperature in the west part of the region increases (decreases) and the surface meridional wind in the south part of the region weakens (strengthens).

The summer months of 2013–2018 could be divided into the southerly jet month and the northerly jet month, as well as the stronger and weaker jet months according to the time series of the first and second modes of the 200 hPa zonal wind in summer of 2013–2018. In the light of the four classification results, the pollutant concentrations in different jet months in both the North China Plain and Tarim Basin are calculated as shown in Table 3. It can be seen from Table 3 that the concentrations of PM2.5, PM10 and O3 in the North China Plain can reach 48.09, 75.37, and 141.70 μg·m<sup>−</sup>3, respectively, when the East Asian jet shifts southward. These loadings are much higher than their seasonal means in summer of 2013–2018. The average concentration of PM2.5, PM10 and O3 in the North China Plain can reach 41.54, 64.5 and 125.01μg·m<sup>−</sup>3, respectively, when the East Asian jet shifts northward, which is lower than their seasonal means in summer of 2013–2018.

**Table 3.** The average concentration of the air pollutants in North China Plain (NCP) and the Tarim Basin (TB) in different East Asian jet periods in summer from 2013–2018. (units:μg·m<sup>−</sup>3).


The concentrations of PM2.5 PM10 in the Tarim Basin can reach 36.30 and 38.88 μg·m<sup>−</sup>3, respectively, when the intensity of the East Asian jet is relatively stronger. These loadings are lower than their seasonal means in summer of 2013–2018. The concentrations of PM2.5 and PM10 in the Tarim Basin can reach 53.24 and 59.04 μg·m<sup>−</sup>3, respectively, when the intensity of the East Asian jet is weaker, which are higher than their seasonal means in summer of 2013–2018.

Combined with the analyses in Section 3.2.2, it can be concluded that the position of the upper-level jet stream in summer may be related to the PM10, O3 and PM2.5 due to the effects of the surface humidity and the meridional and zonal wind in the corresponding region of North China Plain. When the position of the upper-level jet stream in summer is more northward, the surface humidity is higher, and the meridional and zonal wind is stronger. At this time, the concentrations of three pollutants in North China are all lower, and vice versa. The intensity of the East Asian upper-level jet in summer may have correlations with the PM10 and PM2.5 due to the interaction with the surface humidity in the northern part of the Tarim Basin, the surface temperature in the western part, and the surface zonal wind in the eastern part. When the intensity of the East Asian upper-level jet is weaker, the humidity in the northern part of the region is lower, the temperature in the western part is higher, and the zonal wind in the eastern part is weaker. At this time, the concentrations of surface PMs are higher, and vice versa.

#### **4. Conclusions and Discussion**

Based on the NCEP/NCAR daily wind and vertical velocity data, as well as the surface pollutants and meteorological variables data derived from the Science Data Bank, statistical analysis methods were used to study the relationships between the East Asian

upper-level jet and the high concentration areas of near-surface air pollutants in summer in this study, and the interaction mechanisms between them are preliminarily discussed. The conclusions are as follows.


Chen et al. [17] used CESM and indicated that the regional anthropogenic aerosol caused the 200 hPa jet stream to weaken and shift southward over East Asia in summer. which is in agreement with our results, despite the different kind of aerosol. Wang et al. [28]

found that the sand-dust weather often occurred in Taklimakan Desert in spring and summer. The dust particle also had an influence on the summer atmospheric boundary layer structure in Taklimakan Desert. This result can imply that the upper level jet stream has a connection with surface pollutants in Tarim Basin to some extent. Results here also show some connections between the jet and surface air pollutants in summer. Kerr et al. [21] used the global model to study the influence of the upper-level jet stream position on the surface zonal wind and meridional wind in the mid-latitude region of the northern hemisphere in summer. Their results showed that the influence of the upper-level jet stream position on the surface zonal wind mainly occurred over the sea, while its impact on the surface meridional wind occurred over both the sea and the land. Their finding is slightly different from the conclusion of this paper. The possible reason might be that the range of the study area is different. Further investigations are needed based on the regional numerical models to identify the difference.

In this study, the interactions between meteorological variables and pollutants in the vicinity of pollutant regions are not considered when analyzing the relationships between the concentrations of near-surface air pollutants and meteorological variables. In addition, the research conclusions are all obtained based on statistical methods. The rules revealed in the conclusions and the complex interaction mechanisms between the East Asian upperlevel jet and surface pollutants require further verification and exploration based on the numerical models.

**Author Contributions:** Conceptualization, W.W., B.Z. and Y.S.; software, W.W., H.L. and Y.G.; validation, W.W., B.Z. and T.W.; formal analysis, W.W. and H.C.; writing—original draft preparation, W.W.; writing—review and editing, B.Z.; supervision, B.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key R&D Program of China, the National Natural Science Foundation of China and the Fundamental Research Funds for the Central Universities (2019YFA0606803, 42075099, 0207-14380169, 41675143, 42077192, 41621005).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent statement:** Not applicable.

**Data Availability Statement:** The high-resolution air pollution and the surface meteorological variables reanalysis dataset in China during 2013–2018 and are available here: [https://www.scidb. cn/en/detail?dataSetId=696756084735475712&dataSetType=personal] (accessed on 2 June 2021). The NCEP/NCAR reanalysis data can be found here: [https://psl.noaa.gov/data/gridded/data.ncep. reanalysis.html] (accessed on 2 June 2021).

**Acknowledgments:** We thank Nanjing Hurricane Translation for reviewing the English language quality of this paper. This work was supported by the National Key R&D Program of China, the National Natural Science Foundation of China and the Fundamental Research Funds for the Central Universities (2019YFA0606803, 42075099, 0207-14380169, 41675143, 42077192, 41621005).

**Conflicts of Interest:** The authors declare that they have no competing interests.
