Major pollutant changes affected by Covid-19 lockdown in Shanghai

: China repond quickly to explosure of Covid-19. This paper aims to 7 evaluate the impact of lockdown on 5 major airborne pollutant in Shanghai. 8 Chongming, Xuhui and Jinshan areas of Shanghai were studied and compared. 9 Overall, the Covid-19 lockdown has changed pollutant concentration in both long and 10 short terms. All the five concentrations except for O 3 increased. SO 2 had significant 11 correlations with all other pollutants. Ozone on eath surface are mainly from green 12 plants and can be reduced by both NO 2 and some organic matters. PM 2.5 nd PM 10 are 13 more external than local procuced. NO 2 , SO 2 and PM matters sharply reduced in 14 Jinshan and Xuhui during dued to the limited usage of fossil fuel. All these activies 15 could both meet humen’s neccesary needs and keep a better air quality. Proper and 16 systematic measures should be taken to in keeping better air quality and economic 17 growth in the whole world. 18

1 st January to 9 th February, 2020 [10] . Data of the same pollutant belonging to different functional areas were compared to make a comprehensive study. Besides, the corresponding average data from years 2018 and 2019 were used as a baseline to make a comparison.

Major pollutants in different areas of Shanghai during Covid-19 in year 2020.
Althouth concentrations of different pollutants had diffferernt maximum and minimum values at differernt parts in Shanghai, their changing trends were generally the same (Fig. 1). Four of the five pollutants had decreasing contrations while O3 had increasing concentrations, which showed maximum value at Chongming with large part of green plants, followed by Xuhui with high population density and Jinshan with heavy industry. This was in line with the oxidation effect of O3 to VOCs and NOx [10,21,26] . So, very significant negtive correlations (p < 0.001) were shown between NO2 and O3 (Fig. 2). NO2 decreased immediately after the lockdown as this mainly produced by cars with maximum values in Xuhui, where has the most population density and cars. Correlations between PM and O3 were also negtive because some VOCs and NOx attached to PM [21,26,27] . Due to Spring Festival celebrating by familly barbecues, little peeks appeared after about seven days during the late time of the Chinese Spring Festival (Fig. 1). The celebration increased NOx and VOCs, which increased the concentration of O3 [5,21,26,27] . The siginificant (p < 0.05) positive correlation between PM2.5 and O3 before lockdown shows that increase of PM2.5 attaching with NOx and VOCs also increased the concentration of O3.

Comparison of major pollutants in different functional areas.
In Chongming (Fig.   1), the concentration of both NO2 and SO2 decreased by 51%, which was less than half of their original level during the block period. Comparatively, the concentration of PM2.5 only decreased 20% of concentration before the blockade. The average concentration of O3 almost kept smoothly with values of 79.4 and 79.1 μg/m³, respectively, before and after the lockdown period. In Xuhui, the concentration of NO2 and SO2 decreased by 24% and 28%, respectively; and of PM10 and PM2.5 decreased by 28% and 14%, respectively. However, concentration of O3 increased 9% of that in the blockade period. Comparatively, the concentration of PM2.5 only decreased 20% of concentration before the blockade. In Jinshan, NO2, SO2 and PM10 relatively decreased by 29%, 37% and 39% compared with those before the lockdown period. This is in accordance with that this area has the most factories. Like Xuhui, concentration of O3 also increased.
No significant effects on both PM2.5 and O3 were shown by the blockade in all the three parts of Shanghai (Fig. 1). Whereas, very significant effects (p < 0.001) on NO2 and SO2 were shown. Weak (p < 0.05), significant (p < 0.01) and very significant (p < 0.001) effects were respectively shown on PM10 in Chongming, Xuhui and Jinshan parts. This is in line with the control of the two main resouces of PM10, i.e., fossil combustions and vehicle traffics. Significant decrease of both NO2 and SO2 and other reductants reduced the decrease of O3 in both Xuhui and Jinshan. However, the results of PM2.5 was intresting that although the concentration was also obviously decreased, no significant changes were found before and during the lockdown period in different functional parts of Shanghai. This indicates that local combustion of fossil fuels may not be the main source of PM2.5 [22,26] . Generally, all the pollutants had very significant (p < 0.001) changes before and during the lockdown period. Only O3 increased more than 20% than that before the lockdow. Comparatively, concentration of SO2 decreased to 27%, and those of NO2, PM2.5 and PM10, respectively, decreased to 36%, Comparison of major pollutants in the same days from different years. All pollutants varied dramatically in 2020 compared with the same period in 2018 and 2019, especially during the Covid-19 lockdown period (Fig.3). The average decreasing rate of SO2, NO2, PM10, PM2.5 and O3 were 46%, 10%, 32%, 34% and 3%, respectively, before the lockdown period. Whereas, the corresponding data were 46%, 54%, 38%, 34% and -16.2%, respectively, during the lockdown period. So, the lockdown policy indeed altered the pollutant concentrations. Reducing heavy industry and motor vehicles are effective ways to control most pollutants.

Discussion
Exposure to air pollution may play important role in COVID-19 incidence and deaths [1,4,13,23,24] . Especially NO2 and PM2.5, released from tailpipe [32] , is able to increase the risk of lung infections [12,15,30,31] . During the 5 major pollutants in Shanghai, NO2 and SO2 are environmental indicators directly related to local economic activities [10] . More sustainable industry should be established for desulphurisation and to maintain SO2 at a low concentration [10,14] . NO2 concentration was serious to pollution level in Shanghai [6] . When GDP in Shanghai reached 38,155.32 billion yuan in 2019 (China Statistics Bureau, 2019), NO2 became the only major pollutant that failed to meet the nation standard (GB3095-2012). Besides its role in causing lung disease, NO2 might be an important trigger of mental disorders [33,34] and associated with morbidity and mortality of COVID-19 [35] . Lockdown in 2020 sharply decreased concentration of NO2 by about 45% compared with the relative value before that period. The whole data were about 28% and 46%, respectively, lower than the relative decreasing rates in Delhi and Mumbai [5] ; and about 20% lower in Almaty while [35,36] . The globle decreasing rate were about 9% and 10% respectively, in March and April 2020 [5] . This was much higher than the decreasing rate during the lockdown peiod in Shanghai, but almost the same with that before the lockdown, i.e, from 1 st to 23 rd Jannuary 2020 (Fig.2). Consequently, Shanhai performed well in controlling SO2 compared with other cities in the world.
Comparatively, the decreasing rate of SO2 varied little before and during the lockdown periods in this study, but about 10% higher than those released by traffic in 2020 [5] . Comparatively, the decreasing rate were 27% and 9% higher than those in in Delhi and Mumbai. Consequently, SO2 were mainly from heavy industry and traffic, both of which use fossil as fuel. SO2 was the most decreased major pollutant in year 2020 compared with those from 2018 and 2019. This is different from those of Lima, Madrid, Moscow, Rome, Sao Paulo and Wuhan, where SO2 concentration remained unchanged during lockdown, because power plants and traffic were still operational [36,37] .
Driven by particular meteorological conditions, PM are primary pollutant during winter in Shanghai (Shanghai Municipal Bureau of Ecology and Environment, 2020).
The wind which contains pollutants came across the Yangzi Delta from November onwards seasonally [5] . The concentration of PM2.5 and PM10 result in more dust pollution [38,39] . The average concentrations of these two matters were dropped by 66% and 65% during past 5 years with proper control of gas emissions from factories in Shanghai and nearby (Shanghai Municipal Bureau of Ecology and Environment, 2020). The decrease of PM2.5 during the lockdown period was more than 4 times of the average all over China (72%:17%) [40] . The decrease were about 10% lower than the values caused by traffic in the same periods in Shanghai (20~40% : 24-47%) [5] , but still more than 10% higher than the coresponding values in Delhi and Mumbai of Indian [5] . So, like SO2, emission of PM2.5 is also well controlled in Shanghai compared with other international cities. There were almost no difference between the concentrations from year of 2018-2019 and 2020 in Shanghai with decreasing rate of 34%. This is comaparable with those in northern China (29 ± 22%) [41] and further validates that large amount of PM2.5 in Shanghai is from the northern region [5,22,26,32] .
Ozone is another major pollutant in Shanghai (Shanghai Municipal Bureau of Ecology and Environment, 2020). It also varies with season and meteorology [5,13,16] .
The peak of its concentration occurred in summer due to higher temperature and more sunshine [5] . Although concentration of O3 has been recognised by municipal government in Shanghai in 2016, the complexity in O3 forming made it hard to regulate its emission [5,42] . In 2016, concentration of O3 accounted for 57.8% of primary pollutants in summer and was not improved in following years [10] . However, O3 increased much in 2020 with an opposite changing trend of NO2 because of the complex oxidiation effect of O3 to NOx [5,26,41,42] . The average increasing rate (7%) of O3 in our study were in accordance with that at the non-road side sites [5] . As our data were all from non-road side sites, O3 increased before the lockdowan period exept for that in Chongming with large parts of green plants. This validated the decrease of NO2 (p < 0.001) and VOCs could increase O3 [41] and also indicates a banlance has reached in Chongming. Concentration (16%) during the lockdown period were comparible with those at the roadside sites (17%) in Shanghai [5] ; and increased more than 2.5 times than those in northern China [41,43] . All the five major pollutants and the AIQ varied much more than the averages all over China [44] .
Major pollutants in different functional areas duiring Covid-19. To our knowledge, most work only analyzed the effect of lockdown on some of the airborne pollutant. There are almost no studies to trace pollutant changes in long time series and comparing data between pre-lockdown and post-lockdown [2,9,35,45] . They relate empirical evidences to their observed major pollutants and get results of temporary reductions of these pollutions during the lockdown period [2,9,35] . Although most part of the world still suffers from Covid-19 and its varietas with more than 27 million confirmed cases on 4th June 2021 [1] , few comprehensive analysis are caried out from pre-to post lockdown's impact on pollutions [4,5] . As the first reported Covid-19 cases were in China, which really does well in contralling the virus, it offered an excellent oppotunity to study the air-quality before, during and after the lockdown [6,46] .
Concentrations of these pollutants were signifcantly correlated with wether conditions [46,47] . The average temperature was about 8 to 9° C with relatively high pressure in Shanghai in January. This aggravated air pollution (Meteorological Administration Official Website). Furthermore, sand from Mongolia desert was carried to Shanghai in winter. This increased the pollution of PM10 in the air [10] . If the sand dust superposed the impact of local high pressure, it would spread further from north to south Yangzi Delta area including Shanghai to increase the concentration of PM2.5 there [5,25,47,48] . Rainfall and wind speed also matter [5,48] . The average precipitation is 116.06 mmand 69.25mm in in January and February, rspectively [7] . This indicates there was enough rain over the lockdown period. Rain also absorbs the pollutant and lower down the pollutant concentration [16,22,29] . The average wind speed was 3.75m/s, a low speed for the spread of airborne pollutants in Shanghai [10] . So, although there were enough rainfall, the low wind speed and high pressure still increased airborne pollutants.

Availability of lockdown policy respond to diffusion of Covid-19. As we know,
Covid-19 brings bad effects on not only human health [18,20,29,49] , but also socioeconomic [18,29] . The lockdown policy indirectly offered chances to study and comparison environmental changes in China before, during and after the lockdown time [1] . Policies like lockdowning cities and shutdowning factories could reduce the emission of air pollutants from transport and production [3,16,18,23] . Xuhui, Chongming and Jinshan with various natural landscapes and anthropologic facilities are different functional areas in Shanghai. Compared pollutant changes in the three areas can well discuss how the pollutant concentration was affected by lockdown itself and other determinants [3,16,23,29] . As mentioned earlier, the lockdown included both the restriction on nonessential travel and the interruption of manufacturing activities [3,26,47] . Given that the relatively shorter lockdown in Shanghai during the Spring Festival, i.e., 24 th Jan to 9 th Feb did not involve much industrial production, Table 1 only shows the traffic control in Shanghai as background applicable to all 3 sites. The results is in accordance with the ones in Fig. 1, i.e, only 2 or 3 little peeks were found for SO2, NO2 and PM10, the main resouce of which are fuel combustion [9,25,38] . The general decrease during this period is in accordance with the "holiday effect" [50] . The little pollutant concentration peaks were mainly from cooking and traffic. Whereas, low concentrantions were found to both O3 and PM2.5 As mentioned above ( Fig. 2 and Fig. 3), O3 reduced because of increase of NO2 and organic reduction pruduced by human activities [42,50] . The decreasing rate before lockdown (3%) was almost the same with the value during the lockdown period in northern China [41] . Whereas the source of PM2.5 is external and has less correlation with the local human activities in Shanghai [25] . Only SO2 has significant correlations with all other pollutants, that is because it is mainly from fuel combustion, and can attach to PM2.5 and PM10 [22] .
The globle air quality had a dramatic promotion throughout the pandemic of COVID-19 [44,46] because the lockdown supplied a relatively non-industrial period [36,40,41,[44][45][46][47]50] . Isolation is an effective way to block the spread of COVID-19 [36,40,41,[44][45][46][47]50] . However, this means that people have to be blocked at home or somewhere else. They are unable to go out for work, and plants have to stop or reduce their production. This results in lack of living material in some place and could increase human's mental problems such as anxiety and depression [20,49] , which would increase the instability of society [20] . The progress of social economy is largely dependent on development of some heavy industries, which discharge greenhouse gas and accelerate global warming [2] .

Implication on Environment Policies taken by China. The lockdown peirod in
China during the COVID-19 just rightly provided an effective model that could be pushed throughout the whole world. Concentration of NO2 in oceans was 3 to 4 times lower than those in total land without the Antarctic [41] , where has a huge ozone hole and with low NO2 concentration [51] . The data of Shanghai highlights efforts of keeping pollutant concentration low for years when its growth trajectory takes sustainability into account [17,22] . Although the improvement of air quality is good within the lockdown period, it is unlikely to be maintained for long if the economy is still supposed to grow at a high speed. Necessary policies should be made to maintain or even improve the post-lockdown air quality [45] . Wiser policies rather than forced reducing industry and transport should be taken into consideration [23] . Green energy sources such as wind, solar and photovoltaic power generation are good alternatives to traditional fossil consumptions, which is high in pollution. Besides, China respond to the concept and task of "carbon neutralization" and "carbon peak" at the first time in order to better safeguard the world's environment and stabilize the climate. Besides, people in China quickly and positively respond to the Blue Carbon Initiative. The plan's aim is to ensure economic growth on the basis of recover good natural environment. It is a good way to control carbon releasing to the atmosphere as ocean is a big sink to the greenhouse gases, such CO, CO2 and CH4 [52][53][54] . for Shanghai. Its total area is 1413 km 2 and had 818,100 permanent residents [10] with density of about 13,310 person km -2 by the end of 2018. Xuhui is the typical area to study how concentrations of the major air pollutants vary in populated urban environment. There are 176 companies registered in Jinshan industrial zone and most of them are chemical companies. In addition, 59 of those firms exhale pollutants [10] .

Methods
Its total area is 50.94 km 2 Fig. 1 Changes of major pollutants from 1st Jan to 9th Feb with lockdown beginning date of 23 rd Jan at different parts of Shanghai