**4. Discussion**

The coronavirus first outbreak started in December 2019 and human-to-human transmission was confirmed on 20 January 2020 [14]. Wuhan city was locked from other regions

of the country to stop the spread of the SARS-CoV virus since 23 January 2020. After a few days, the administrative authorities of Guangzhou and Beijing declared the lockdown. The highest level of public health emergency was announced within a week in a few administrative units [13,15]. The anthropogenic emissions from industrial and manufacturing units were closed after the spreading of coronavirus which caused improvement in overall air quality in China [14].

The present study found that there was a substantial decrease in NO2 concentration and consequently improvement in air quality. It is noteworthy that NO2 concentration has tremendously decreased in the cities of Guangzhou, Wuhan, and Chengdu. These cities come under the same category, where NO2 tropospheric column (molecules/cm2) ranges between 1.6e+16 and 3e+16 before the pandemic but after the shutdown none of these cities reported high emissions of NO2. NASA and European Space Agency (ESA) pollution monitoring satellites have also detected a substantial decrease in NO2 over China due to a complete shutdown. The concentration of nitrogen dioxide was reduced as the sources of this pollutant such as motor vehicles, power plants, and industrial facilities were closed for two months [58]. In addition, the lockdown has coincided with the Lunar New Year celebration which also facilitated to decrease the concentration of NO2 in all major cities in China's mainland. Some of the preceding studies have also established that many cities such as Guangzhou, Beijing, Chengdu, Shenzhen, Nanjing, Shanghai, Chongqing, etc., have experienced low concentrations of NO2 and particulate matter in the atmosphere due to regulatory effects in China [76–79].

The study found a substantial decline in CO in all cities except Beijing where it remained static. The maximum declining rate of CO was observed in Lanzhou (40%), Jinan (37%), and Zhengzhou (31%) whereas the minimum declining rate of CO was measured in Anshan (14%), Wuhan (17%), and Shanghai (21%) because of the complete halt of vehicles, wood-burning, and industry. Similar to this line, several studies established that vehicles, wood-burning, and industry are the main sources of CO [80–84]. The reduction in CO in 60 days is a significant indicator for air quality because of its 1–2-month life span in the atmosphere, thereby improved quality of air [85,86].

The particulate pollutants such as PM2.5 and PM10 have also decreased substantially during the lockdown period as the sources of these pollutants were restricted. Several previous studies have documented that the primary sources of PM2.5 and PM10 are automobile emissions, incomplete combustion, wind-blown soil and dust, construction dust, and biomass burning [82,87,88]. The result displays that the maximum declining rate of PM10 and PM2.5 was measured in Guangzhou (56% and 43%), Wuhan (45% and 39%), Jinan (37% and 38%), and Nanjing (37% and 36%). Similarly, some other studies have also found that Beijing, Lanzhou, Tianjin, Nanning, and Chongqing reported lower concentrations of these particulate matters during the regulation effect on certain occasions [89–91]. On the other hand, the concentration of SO2 has also declined in all cities due to strict regulation and the closing of heavy industrial factories such as iron, steel, and cement industries [92–95]. The major sources of SO2 are coal combustion of biomass in coal-fired power plants and industry sector [9,96–98] that were completely locked during the pandemic. Some other studies have exhibited that SO2, combined with volatile organic compounds (VOCs), enhances the formation of new particles, particularly sulphate which is one of the main components of PM2.5 [99–102].

Consistent with previous studies, this study [103–105] also found that due to the decrease in all air pollutants including CO, NO2, PM2.5, PM10, and SO2 the total concentration of O3 has increased in all 15 cities in China during the study period. The maximum increasing rate of O3 was found in Xian (200%), Zhengzhou (155%), Jinan (149%), and Chongqing (145%) whereas the minimum increasing trend was found in Guangzhou (13%), Xiamen (17%), and Anshan (57%). Several reasons have been proposed to explain this complex relation: (a) inverse relation between O3 and its precursors NO2, PM2.5, PM10, and SO2 lower emission of these pollutants results in faster ozone production in nitrogen oxide (NOx) concentrated areas [106–109]; (b) low concentration of NOx also causes less

destruction of ozone [88,110]; (c) decreased level of atmospheric pollutants leads to more clear sunshine that also accentuates more ozone production [108,111]. In addition, a pause of vehicles also resulted in a reduction in CO and NOx including NO and NO2 facilitating an increase in the level of ozone in the atmosphere [81,112–114]. The production of ozone under the influence of anthropogenic activities in the troposphere and involving catalysis by NO2 and NO should be significant [22]. Its precursor compounds NOx and VOC have a wide variety of sources and can exhibit a non-linear effect on ozone production, while its accumulation is strongly influenced by meteorological parameters [115]. Although reductions in atmospheric ozone allow more solar radiation to reach ground level, resulting in higher surface temperatures, a decrease in the downward longwave radiation emitted by CO2, O3, and H20 from a cooler lower stratosphere with less ozone would result in a decrease in surface temperatures [22].

One of the major outcomes of reduced atmospheric pollutants is a substantial improvement in overall air quality except for Beijing. Since most of the industrial production, vehicle movements, and other anthropogenic activities in the cities were closed, the reduced level of NO2, CO, PM2.5, PM10, and SO2 improved the air quality in these cities. Similar to this, several previous studies have found that air quality is positively related to the low concentration of atmospheric pollutants [116–118]. Conversely, Beijing has reported a decline in the overall air quality during the study period despite the closure of manufacturing units. The reason may be the increasing level of PM2.5 and PM10 during the same period. The concentration of PM2.5 has increased manyfold in Beijing because the city experienced a number of severe haze events during the lockdown period. Some of the previous studies [25,31,79] mentioned that complex relationships among changes in relative humidity, near-surface wind speed and direction, planetary boundary layer height, and precipitation have influenced the increasing concentration of PM2.5 in Beijing. Climatologically, Beijing has dry air during the wintertime, but a larger than normal amount of moisture accumulated near the surface during the lockdown period. This has facilitated multiphase reactions for aerosol formation and growth [25]. Moreover, wind conditions also facilitate to formation of haze in Beijing as the mean wind speed declined. In addition, the wind direction changed to southerly which usually carries polluted air from Hebei Province's industrial regions. Moreover, the planetary boundary layer height in northern China also declined during the lockdown and this lower height facilitated stagnant air and subsequently resulted in increasing PM2.5 in Beijing. Further, during the lockdown period, precipitation mainly occurred in southern China and the Northern part did not receive enough rain to wash out the haze that formed over the region [40].
