**1. Introduction**

Air pollution and its quality in cities are the major concerns worldwide and China is no exception. Air pollution has been a serious environmental issue in China since extensive industrial production and other anthropogenic activities exponentially increased the concentration of major atmospheric pollutants that damaged the environmental quality and harmfully impacted human health [1–5]. The World Health Organization (WHO) reiterates that environmental and health risk factors from outdoor air pollution are substantial. In 2016, 91% of the world population was living in places where the WHO air quality guidelines levels were not met [6]. New estimates in 2018 revealed that 9 out of 10 people breathe air containing high levels of pollutants [7]. Over the last four decades,

**Citation:** Rahaman, S.; Jahangir, S.; Chen, R.; Kumar, P. Restricted Anthropogenic Activities and Improved Urban Air Quality in China: Evidence from Real-Time and Remotely Sensed Datasets Using Air Quality Zonal Modeling. *Atmosphere* **2022**, *13*, 961. https://doi.org/ 10.3390/atmos13060961

Academic Editors: Duanyang Liu, Kai Qin and Honglei Wang

Received: 7 May 2022 Accepted: 30 May 2022 Published: 14 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

rapid economic development, industrialization, and urbanization across China has caused serious air pollution problems [8–10]. China's National Western Development Strategies (the 'Go West' movement) by the Chinese government in 1999 also contributed to the spatial expansion of industrialization and urbanization [11]. This has resulted in the GDP reaching around RMB 8270 billion, to which industrial production contributes 34% as the most significant contributing factor (http://www.stats.gov.cn/tjsj/ndsj/, accessed on 18 April 2020). Furthermore, due to rapid industrialization and urbanization air pollution has become the fourth primary health risk factor for all deaths in China after heart attack, dietary risk, and smoking [12]. The recent COVID-19 pandemic at the global and regional level has substantially affected the spatial and temporal characteristics of significant air pollutants, such as NO2, CO, O3, PM10, PM2.5, and SO2 across China [13–15]. In this context, the present study aims to assess the overall air quality from ground-based monitoring and remotely sensed datasets using air quality zonal modeling in 15 major cities of China before and after pandemic lockdown periods. Here, we estimated the changes in pollutant concentration and the spatio-temporal variation characteristics for 60 days before and after the shutting down of the nation. Section 2 discusses data and methods where we mentioned data sources, how we analyzed the data, and what techniques we used. Next are the results in Section 3 where we analyzed the spatio-temporal distribution of NO2 before and after the pandemic; total NO2 load over the area using air quality zonal modeling, and trends of air pollutants in China before and after the pandemic. Section 4 presents the discussion where we critically evaluate the results and link our study with other relevant studies with reasons and evidence.

NO2 is primarily released by anthropogenic emissions, which contain the industrial burning of fossil fuels such as coal, oil, natural gas, vehicle exhaust, biomass burning, and electricity generation [16–18]. Pollutants such as carbon monoxide (CO) and sulphur dioxide (SO2) are mainly released from industrial plants and heating processes due to anthropogenic activities [19]. SO2 is regarded as one of the major air pollutants in cities because of its negative effects on human health and the ecosystem [20]. Particulate Matter (PM10, PM2.5) is a common proxy indicator for air pollution and it affects more people than any other pollutant. The major components of PM are sulfate, nitrates, ammonia, sodium chloride, black carbon, mineral dust, and water [6]. Tropospheric O3 is produced by emissions of nitrogen oxides (NOx = NO + NO2) and volatile organic compounds (VOCs) in the presence of sunlight [21]. The distribution of O3 varies with space and time and is lower in urban polluted areas than elsewhere because it disappears when it reacts with other pollutants [22,23]. Among NOx elements, NO2 is the most important precursor and quencher of O3 through NOx titration particularly during wintertime [24]. The daytime relationship between NO2 and O3 concentration in Northern China demonstrates significant ozone titration. However, this titration effect has been considerably attenuated with increasing PM2.5, which further reduces the incoming solar radiation, during the lockdown period [25]. It is not only harmful to human health but also poses adverse impacts on plants and ecosystems [1,3,26,27].

Various studies have been carried out to investigate the spatial and temporal distribution patterns of NO2, CO, O3. PM10, PM2.5, and SO2 at national, regional, and local levels [2,28–42] along with their driving forces in the recent past [27,43–46]. Numerous studies have shown a strong correlation between level of exposure to atmospheric pollutants and human health [2,4,5]; increased mortality [47–49]; sleeping disorders among elderly [50]; body weight and obesity [51]. To mitigate air pollution, the Chinese government has imposed stringent air quality standards [52] and strengthened emission controls of major atmospheric pollutants [53–57].

The understanding of the spatial distribution of air pollution in China has been recently improved by the application of advanced assessment tools, such as satellite remote sensing. The satellite-retrieved products have many advantages, including global coverage, high spatial-temporal resolution, and historical datasets [41,42,47]. Amid the coronavirus pandemic in China, NASA and European Space Agency (ESA) pollution monitoring satellites

have detected significant decreases in nitrogen dioxide (NO2) over mainland China [13–15]. There is evidence that the change is at least partly related to the economic slowdown following the outbreak of the COVID-19 virus. According to NASA scientists, the reduction in NO2 pollution was first apparent near Wuhan, but eventually broke out across the country [58]. In line with the previous studies of holiday effects on pollution [59–62], the main purpose of this work is to quantify spatial and temporal changes in primary pollutants such as NO2, CO, O3, PM10, PM2.5, and SO2 over China's mainland due to shutdown of anthropogenic activities.
