**1. Introduction**

In the past 20 years, China has experienced severe air pollution due to rapid economic development and increasing urbanization [1]. Studies showed that exposure to ambient air pollution has been associated with increased risks of mortality and morbidity worldwide [2,3]. According the Global Burden of Disease (GBD) project, air pollution was responsible for 1.6 million deaths in China and 4.2 million deaths worldwide in 2015 [4]. The continuous and serious air pollution has caused an immense burden for China's medical and economic [5]. In order to cope with serious air quality problems, China has taken a series of measures in recent years [6,7].

In 2005 and 2011, China implemented the installation of desulphurization and selective catalytic reduction (SCR) systems for coal-fired power plants [8]. At the same time, the strategies of upgrading vehicle fuel and prohibiting polluting old vehicles were introduced at the city level [9]. The Ministry of Environmental Protection of China issued the revised "Ambient Air Quality Standards" (CAAQS, GB3095-2012) in February 2012, adding PM2.5 and O3 to CAAQS for the first time [10]. In 10 September 2013, the Chinese government promulgated the Air Pollution Prevention and Control Action Plan. The plan aimed to reduce the number of severely polluted days drastically and improve the national air quality significantly through long-term efforts [11]. Despite these efforts, there were still many cities that have not yet reached the current CAAQS [12]. According to the "2020

**Citation:** Fang, C.; Tan, X.; Zhong, Y.; Wang, J. Research on the Temporal and Spatial Characteristics of Air Pollutants in Sichuan Basin. *Atmosphere* **2021**, *12*, 1504. https:// doi.org/10.3390/atmos12111504

Academic Editors: Duanyang Liu, Kai Qin and Honglei Wang

Received: 15 September 2021 Accepted: 13 November 2021 Published: 15 November 2021

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Reports on the State of Environment of China", there were still 135 cities whose ambient air quality exceeded the standard, accounting for 40.1% of the total number of cities. In the days exceeding the standard, the proportions of PM2.5, O3, and PM10 as the primary pollutants were 51%, 37.1%, and 11.7%, respectively.

Previous studies showed that Beijing-Tianjin-Hebei area (BTH), Yangtze River Delta (YRD), Pearl River Delta (PRD) and Sichuan Basin were the four main regions with severe air pollution in China [13,14]. In Beijing, YRD and PRD, some scholars have carried out a lot of research to understand the basic characteristics, chemical mechanisms, main components and transmission sources of air pollution [15–19]. Since 2000, the air quality in Sichuan Basin has further deteriorated due to increased anthropogenic emissions. However, only a few studies have focused on Sichuan Basin [20,21]. And in the past, related studies on Sichuan Basin were mainly concentrated in the two megacities of Chongqing and Chengdu, and there were few studies on the overall air quality for the whole of large-scale valley terrain [22,23]. The characteristics and source of air pollution for Sichuan Basin in recent years are still unclear [24]. In this study, we analyzed air quality data collected from Sichuan Basin for six years (January 2015 to December 2020) to fill this gap. The main goal is to investigate (1) the temporal and spatial characteristics of the overall air pollution in Sichuan Basin, (2) the industry contribution reflected by the ratio of different pollutants, and (3) a regional-scale air pollution episode that influenced multiple cities in the region. The knowledge gained in this study provides a scientific basis for formulating future emission control policies aimed at reducing severe PM2.5 pollution in this unique watershed
