*3.1. Characteristics of the East Asian Upper-Level Jet and Surface Pollutants in Summer*

3.1.1. Spatio-Temporal Characteristics of the East Asian Upper-Level Jet in Summer

The monthly average data of 200 hPa zonal wind in summer over East Asia from 2009 to 2018 are selected, and the spatio-temporal decomposition is carried out based on the data. The covariance contribution rates of the first two modes of the EOF (hereafter referred to as EOF1 and EOF2, respectively) decomposition results (Figure 2) are 57.54% and 8.78%, respectively. The spatial distribution of the EOF1 shows that the dividing line of the 200 hPa zonal wind is around 40◦ N, which is the average position of the upper-level jet stream in summer. The variations in the north and the south are opposite, which shows that the EOF1 represents the position variation in the upper-level jet stream. In the time series corresponding to the EOF1, the time coefficients are all negative in June during 2009–2018, while the time coefficients are both positive in July and August in the same years. This indicates that the position of the jet stream in June in this decade is to the south of that in July and August in the same years. The spatial distribution of the EOF2 of the 200 hPa upper-level zonal wind shows that there is a minimum area centered around 40◦ N, which is the average position of the upper-level jet stream in summer. Therefore, the EOF2 represents the intensity variation in the upper-level jet stream.

**Figure 2.** Spatial distributions (**a**,**c**) and the time coefficients (**b**,**d**) of the EOF1 and EOF2 of the 200 hPa zonal wind in summer from 2009 to 2018.

3.1.2. Distribution Characteristics of Surface Pollutants in East Asia in Summer

The average concentrations of the NO2, PM10, O3 (the maximum concentration in 8 h) and PM2.5 in summer from 2013 to 2018 are shown in Figure 3. Combined with the first-level concentration indices of pollutants in the Ambient Air Quality Standards (GB 3095–2012), it can be seen that the overall NO2 concentration in summer in China is within the normal standard range. There is relatively serious PM pollution in the Tarim Basin (37–42 ◦ N, 75–90 ◦ E) and most parts of northern China. The O3 concentration is relatively high in northern China and most areas of Qinghai-Tibet. The O3 pollution in the North China Plain (35–40◦ N, 113–123◦ E) is the most serious. Therefore, the PM10 and PM2.5 in the Tarim Basin and the PM10, O3, and PM2.5 in the North China Plain are taken as the research objects of summer pollutants in this study.

**Figure 3.** Season mean concentrations of air pollutants (**a**) NO2, (**b**) PM10, (**c**) O3 (8 h maximum concentration) and (**d**) PM2.5 in China in summer during 2013–2018 (unit: μg·m<sup>−</sup>3, the slashed area indicates that the pollutant concentration in the area has exceeded the first-level concentration index of the Ambient Air Quality Standards in China).

In summer during 2013–2018, the average concentrations of the PM2.5 and PM10 in the Tarim Basin are 45.19 and 49.08 μg·m−<sup>3</sup> (Table 1), respectively. The interannual variations of the PM concentration increased year by year before 2015 and decreased after 2015. The PM concentration reached the maximum in 2015, and many discrete values in 2018 indicate that severe PM pollution events occurred frequently in that year (Figure 4a,b). In Figure 5a, the days with the PM2.5 exceeding the standard are more than those of the PM10 in summer in the Tarim Basin with a total of 265 and 193 days during 2013 to 2018, respectively. The ratio of PM2.5 to PM10 in this area is high with an average of approximately 0.9 (Figure 7). The ratio of the PM2.5 to PM10 reaches a high value in 2013 and 2014 with the maximum reaching 1, but the ratio declines in subsequent years and maintains at around 0.85 (Figure 6a).

**Table 1.** Average concentration of pollutants in the Tarim Basin and the North China Plain from 2013 to 2018 (unit: μg·m<sup>−</sup>3).


**Figure 4.** Interannual variation in the average pollutant concentrations in the Tarim Basin (**a**,**b**) and the North China Plain (**c**–**e**) (unit: μg·m<sup>−</sup>3, orange lines represent the median, green triangles represent the average value, and hollow dots represent the discrete value).

**Figure 5.** Days with pollutant concentrations exceeding the national first-level environmental standard in the Tarim Basin (**a**) and the North China Plain (**b**) in summer from 2013 to 2018.

**Figure 6.** Interannual variation in the ratio of the PM2.5 to PM10 concentration in the Tarim Basin (**a**) and the North China Plain (**b**) in summer from 2013 to 2018 (orange lines represent the median, green triangles represent the mean value and black dots represent the discrete point).

The average concentrations of the three pollutants of the PM2.5, PM10 and O3 in the North China Plain in the summer during 2013–2018 are 45.09, 70.28 and 131.27 μg·m−<sup>3</sup> (Table 1), respectively. The days with concentrations exceeding the standard reach 401, 461 and 488, respectively, and the days with the PM10 exceeding the standard are more than those of the PM2.5 (Figure 5b). Figure 4c–e show that the PM concentrations in the North China Plain show decreasing trends, while the O3 concentration shows an increasing trend. The average ratio of the PM2.5 to PM10 in this area is approximately 0.65, and the ratio shows a decreasing trend (Figures 6 and 7).

**Figure 7.** Spatial distribution of the ratio of the PM2.5 to PM10 in summer from 2013 to 2018.

*3.2. Relationship between the East Asian Upper-Level Jet and Surface Pollutants in Summer* 3.2.1. Preliminary Analysis of the Relationship between the East Asian Upper-Level Jet and Surface Pollutants in Summer

According to the analysis results in Section 3.1.1, it can be concluded that there is an intraseasonal northward shift of the jet stream position in summer during 2013–2018. Therefore, the impact of the East Asian upper-level jet on pollutants in each month in summer is discussed separately. The temporal average of the monthly 200 hPa zonal wind and the pollutant concentration, including the PM10, O3, and PM2.5, in the summer during 2013–2018 are calculated. Figure 8 shows that the average position of the upper-level jet stream in June is around 40◦ N, and the average central wind speed is higher than 39 m·s<sup>−</sup>1. The average positions of the upper-level jet stream in July and August are around 45◦ N. The average wind speeds of the jet stream centers in July and August are approximately 31 and 35 m·s<sup>−</sup>1, respectively. The above results show that the upper-level jet stream has an obvious northward jump in summer, which is consistent with the EOF analysis result. The intensity of the upper-level jet stream in summer is the strongest in June and the weakest in July.

In addition, the pollutants in the North China Plain in June locate near the left side of the entrance region of the upper-level jet stream (Figure 8a,d). Combined with the atmospheric meridional vertical circulation in June (Figure 9a), it can be seen that the North China Plain, locating near 32–40◦ N, is dominated by the descending motion in the left side of the entrance region of the upper-level jet stream between 850 and 300 hPa, while there is a weak ascending motion below 850 hPa, the average vertical velocity in North China Plain in June from Table 2 can also prove this. It indicates that the atmospheric stratification over the North China Plain is relatively stable in June, which is conducive to the pollutant accumulation. The 1000 hPa surface wind during the same period (Figure 8g) shows that the pollutants from southern China are transported to the North China Plain due to the large-scale southerly wind. The low wind speed in the North China Plain is not conducive to the pollutant diffusion in the region. Therefore, the pollutant concentrations are high in the North China Plain in June including PM10, O3 and PM2.5.

**Figure 8.** Variations of the 200 hPa westerly jet stream (unit: m·s<sup>−</sup>1, **a**–**f**, contours), distributions of the monthly mean surface air pollutants (unit: μg·m<sup>−</sup>3, including PM10, O3 and PM2.5) and 1000 hPa wind fields (unit: m·s<sup>−</sup>1, **g**–**i**, vectors) in June, July and August from 2013 to 2018. The PM10, O3, PM2.5 are plotted in shaded in **a**–**c**, **d**–**f**, **g**–**i**, respectively.

**Figure 9.** The height-latitude profile of the meridional-averaged zonal wind (unit: m·s<sup>−</sup>1, contours) and vertical movement (unit: pa·s<sup>−</sup>1, shades: red indicates ascending motion, blue indicates descending motion) in different areas in July and August. (**a**,**c**,**e**) represent the situation in the North China Plain, and (**b**,**d**,**f**) represent the situation in the Tarim Basin.


**Table 2.** The average value of the meteorological elements and pollutant concentrations in the North China Plain (NCP) and Tarim Basin (TB) in June, July and August.

\* The vertical velocity is the average value of the vertical velocity below the 200 hPa level.

Combined with Table 2, in July and August, the North China Plain is located near the right side of the exit region of the upper-level jet stream over the Sea of Japan. It is dominated by the ascending motion caused by the upper-level jet stream, which makes pollutants, including PM10, O3 and PM2.5, diffuse in the vertical direction to a certain extent in the North China Plain, and the surface concentration is lower than that in June. However, the surface wind speed in the North China Plain is relatively low, and the horizontal diffusion of pollutants is relatively hard. Therefore, the pollution in the North China Plain in July and August is still serious.

The Tarim Basin has a special topography. Except for the Hexi Corridor to the east, the north, west and south sides are all surrounded by high mountains with an average altitude of more than 5000 m [26,27]. Throughout the summer, the Tarim Basin locates at the right of the entrance region of the upper-level jet stream dominated by the ascending motion caused by the upper-level jet stream (Table 2). However, Figure 9b–e indicate that the ascending motion above 700 hPa is very weak, and the air vertical movement is not enough to carry the surface PM10 and PM2.5 away from the basin. Meanwhile, the Tarim Basin is dominated by the easterly wind, and the surface wind speed is relatively low in summer. The horizontal diffusion of pollutants is hindered by the surrounding mountains (Figure 8g–i). Therefore, the pollutant concentrations including PM2.5 and PM10 are high in summer in the Tarim Basin.

The above analyses show that there is a connection between the summer jet stream and surface pollutants.

3.2.2. Relationship between the Surface Pollutants and the Position and Intensity of the East Asian Upper-Level Jet in Summer

The SVD method is used to further analyze the relationship between the East Asian upper-level jet and surface pollutants in summer. The sum of the cumulative covariance contribution of the first two modes of the SVD (hereafter referred to as SVD1 and SVD2, respectively) of the surface O3 concentration and the 200 hPa zonal wind in summer is 88.46%. The sum of the square of the explained total covariance of the SVD1 is 81.35%, and the correlation coefficient of the time series of the left and right fields is 0.96, showing the synchronized variation in the two fields. In Figure 10e,f, when the time coefficients of the left and right fields are both positive, there are positive anomalies of the O3 concentration in the North China Plain in the left field. The dividing line in the right field is about 40◦N, which is the average position of the East Asian upper-level jet axis in summer, and the north and south regions of the 200 hPa zonal wind show positive anomalies and negative anomalies, respectively. That is, the position of the upper-level jet stream is more southward when the surface O3 concentration is higher in the North China Plain, and vice versa. Moreover, the spatial distribution of the right field heterogeneous correlation of this mode is similar to that of the EOF1 of the 200 hPa zonal wind. Therefore, the SVD1 of the

surface O3 concentration and 200 hPa zonal wind represents the relationship between the surface O3 and the position of the East Asian upper-level jet.

**Figure 10.** The (**a**,**c**,**e**) left and (**b**,**d**,**f**) right heterogeneous correlation diagrams of the SVD1 of the surface pollutants including the PM10, O3 and PM2.5 (the left field) and the 200 hPa zonal wind field (the right field) in summer from 2013 to 2018. The slashes indicate that the results passed the 95% Monte Carlo correlation test.

For the SVD2 of the O3 concentration and 200 hPa zonal wind in summer, the sum of the square of the explained total covariance is 7.31%, and the correlation coefficient of the time series of the left and right fields is 0.94, showing the synchronized variation relationship. The spatial distribution of the left field heterogeneous correlation is similar to that of the EOF2 of the 200 hPa zonal wind. Therefore, the left and right fields heterogeneous correlation of the SVD2 represents the relationship between the surface O3 and the intensity of the East Asian upper-level jet. However, their relationship is not significant in the North China Plain (Figure 11e,f).

Therefore, there may be a certain relationship between the surface O3 concentration in the North China Plain in summer and the position of the East Asian upper-level jet, but the relationship with the intensity of the upper-level jet stream is not significant.

Since the SVD results of the 200 hPa zonal wind and the surface PM10 and PM2.5 concentrations are similar in summer, the relationship of the 200 hPa zonal wind with the PM10 and that with the PM2.5 are discussed together. For the SVD1 and SVD2 of the 200 hPa zonal wind and the PM10 and PM2.5, the sums of the cumulative covariance contribution are 86.39% and 85.06%, respectively. The sum of squares of the explained total covariance of the SVD1 are 71.13% and 71.76%, respectively. The correlation coefficients of the time series of the left and right fields are 0.96 and 0.84, respectively, showing the synchronized variation relationship. The slashes in Figure 10a–d show that, when the anomalies of the PM10 and PM2.5 concentrations in the North China Plain in the left field are negative, the dividing line of the 200 hPa zonal wind in the right field is about 40◦N, which is the average position of the East Asian upper-level jet axis in summer, and the north and south regions show negative anomalies and positive anomalies, respectively. That is, the position of the East Asian upper-level jet is more northward when the concentrations of the PM10 and

PM2.5 are low in the North China Plain, and vice versa. Moreover, the spatial distribution of the left-field heterogeneous correlation of this mode is similar to that of the EOF1 of the 200 hPa zonal wind. Therefore, the surface PM10 and PM2.5 concentrations are associated with the position of the East Asian upper-level jet.

**Figure 11.** The (**a**,**c**,**e**) left and (**b**,**d**,**f**) right heterogeneous correlation diagrams of the SVD2 of the surface pollutants including the PM10, O3 and PM2.5 (the left field) and the 200 hPa zonal wind field (the right field) in summer from 2013 to 2018. The slashes indicate that the results passed the 95% Monte Carlo correlation test.

For the SVD2 of the 200 hPa zonal wind and the PM10 and PM2.5 surface concentration in summer, the sum of squares of the explained total covariance are 15.26% and 13.30%, respectively. The correlation coefficients of the time series of the left and right fields are 0.91 and 0.83, respectively, presenting the synchronized variation relationship. The slashes in Figure 11a–d show that when the anomalies of the PM10 and PM2.5 concentrations in the Tarim Basin in China are negative, there is a negative anomalous region of the 200 hPa zonal wind centered around 40◦ N, which corresponds to the average position of the East Asian upper-level jet in summer. That is, the intensity of the East Asian jet stream is low (high) when the PM10 and PM2.5 concentrations are high (low) in the Tarim Basin. The spatial distribution of the right field heterogeneous correlation of this mode is similar to that of the EOF2 of the 200 hPa zonal wind. Therefore, the left and right fields heterogeneous correlation of the SVD2 represents the relationship between the surface concentrations of the PM10 and PM2.5 and the intensity of the East Asian upper-level jet.

By comparing the significance of the heterogeneous correlation diagrams of the first and second modes, we found that the anomalous PM10 and PM2.5 concentrations in summer over the North China Plain may have a certain relationship with the position variation in the East Asian upper-level jet, but the relationship with the intensity anomaly of the upperlevel jet stream is not significant. The anomalous surface PM10 and PM2.5 concentrations in the Tarim Basin may have a certain relationship with the intensity anomaly of the East Asian upper-level jet, but the relationship with the position variation in the upper-level jet stream is not significant.

In summary, there is a certain relationship between the movement of the East Asian upper-level jet in summer and the variations of the three pollutants' concentrations, including PM10, PM2.5 and O3, in the North China Plain. When the position of the East Asian upper-level jet is more northward, the concentrations of the PM10, PM2.5 and O3 in North China Plain are significantly lower, and vice versa. There is a connection between the intensity variation in the East Asian upper-level jet in summer and the concentration variations of the PM10 and PM2.5 in the Tarim Basin. When the intensity of the East Asian upper-level jet is relatively high, the concentrations of the PM10 and PM2.5 in the Tarim basin are both low, and vice versa.
