**1. Introduction**

Since the industrial revolution, the increase in human activities has exacerbated climate change in the earth system. At present, observation results have proved that global

**Citation:** Wei, W.; Zhuang, B.; Lin, H.; Shu, Y.; Wang, T.; Chen, H.; Gao, Y. Analysis of the Interactions between the 200 hPa Jet and Air Pollutants in the Near-Surface Layer over East Asia in Summer. *Atmosphere* **2021**, *12*, 886. https://doi.org/10.3390/ atmos12070886

Academic Editors: Duanyang Liu, Kai Qin and Honglei Wang

Received: 4 June 2021 Accepted: 6 July 2021 Published: 8 July 2021

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climate change has become an unequivocal fact, such as the continuous rise of the global temperature, glacier melting and frequent extreme weather events, which are serious threats to human survival and development [1]. Therefore, the in-depth understanding of climate change is the current hotspot of scientific research, which will provide scientific support for climatic policymaking.

Some studies have pointed out that there is an interaction between climate change and air pollution [2]. Among the influencing factors of climate change, the role of aerosol is the most uncertain [1]. The main component of air pollutants is the atmospheric aerosol. The atmospheric aerosol refers to the particulate matter (PM) suspended in the atmosphere. Aerosol particles can be solid or liquid and can also exist in the mixing form of solid and liquid. In general, the diameters of the atmospheric aerosol particles are several nanometers to tens of microns. The sources of aerosol particles in the atmosphere are different, and different aerosols have different physical, chemical and optical properties, resulting in more complex climatic effects of aerosols. The climatic effects of aerosols can be divided into direct climatic effects, indirect effects and semi-direct effects. Although the three effects are different in their interaction mechanisms, they all essentially lead to the change in the earth climate system by affecting the radiation budget balance of the earth-atmosphere system [3–7]. At the same time, variations in the climate system will cause the variation in relevant meteorological factors, which will have an impact on the distribution of air pollutants [8,9].

The East Asian upper-level jet is a narrow wind belt with a high wind speed above 500 hPa in the East Asia region [10]. Many observational data show that the strongest subtropical westerly wind speed generally exists at 200 hPa [10,11], and the East Asian jet is generally defined as the 200 hPa maximum zonal wind speed zone. The East Asian upper-level jet has significant seasonal variation, and its location and intensity will change accordingly. The jet stream, with its strong shears, plays an important role in forming upper level convergence and divergence. Therefore, it causes variations of the weight of all the air in a column from the ground to the limit of the atmosphere. In other words, The upper level jet stream makes the surface pressure change, which could result in variations in the air flow field at the ground [12]. At present, a large number of studies have shown that the East Asian upper-level jet controls the atmospheric circulation in East Asia and has an extremely important impact on the weather and climate in East Asia [13–15]. Secondary circulations will be generated around the East Asian upper-level jet, leading to the coupling of upper-level and low-level weather systems accompanied by the exchange of the matter, momentum and energy between the upper level and the ground. Therefore, the East Asian upper-level jet, as important weather and climate system in East Asia, may have a certain interaction with the surface pollutants. Studies have shown that surface pollutants have effects on the upper-level jet stream. Song et al. [16] and Chen et al. [17] pointed out that the increase in summer aerosols will cause the southward movement of the upper-level jet stream, which is mainly due to the change in the upperlevel temperature gradient caused by the aerosol forcing. Liu et al. [18] pointed out that, in winter, to the north of 30◦ N, the mid-latitude cooling caused by aerosols leads to the enhancement of the subtropical jet stream and the weakening of the temperate jet stream, which further makes the upper-level jet stream move southward. In other studies, it has been found that the upper-level jet stream can affect the distribution of surface pollutants. Ordóñez et al. [19] found that the location of the North Atlantic jet stream has a greater impact on the concentration distribution of the surface PM10 than on its intensity. Barnes and Fiore [20] have shown that the location of the jet stream in eastern North America in summer is closely related to the surface ozone concentration. Kerr et al. [21] used a model to analyze the position of the upper-level jet stream affecting the transport of the ozone by affecting the surface meridional wind. However, current research mainly focusses on the one-way effect between the upper-level jet stream and surface pollutants and rarely focusses on the interaction between them. In addition, the East Asia region has a wide zonal range, and the distributions of the terrain, coastline and land use are

relatively complex. The region is mainly controlled by the monsoon system, and the seasonal change in the climate is distinctive. In particular, the role of the summer monsoon system is relatively significant. Meanwhile, the population in East Asia accounts for one third of the world's population. On the one hand, the climate change in East Asia has a significant impact on the production and lives of the local people. On the other hand, the climate in East Asia is also strongly affected by human factors related to the rapid economic development of Asian countries. Therefore, the East Asian upper-level jet is one of the main members of the monsoon system that controls the weather and climate in East Asia, and the surface pollutant is an important factor affecting weather and climate changes. It is of great practical significance to study the interaction between them in summer and explore the mechanism.
