Impacts of COVID-19 on Air Quality in India

The COVID-19 pandemic spread all over the world in early 2020. India imposed a nationwide lockdown on March 25, 2020, for more than a month to contain the COVID-19 infection. During the nationwide lockdown, transport, industries, and commercial activities were suspended, except for essential services. We made a detailed analysis of the impacts of COVID-19 on air quality in India by using the data from more than 200 Continuous Ambient Air Quality Monitoring Stations (CAAQMS) and reported a change in the National Air Quality Index (NAQI), spatial distribution and concentration levels of PM10, PM2.5, CO, NO2, SO2, and O3 from January to April 2020 nationwide and in five major cities, namely, Delhi, Mumbai, Kolkata, Chennai, and Hyderabad. We defined the period between February 25 to March 24, 2020, as ‘before lockdown’ and March 25 to April 30, 2020, as ‘during lockdown’. The NAQI and satellite visual maps of AOD, NO2, CO, and SO2 from January to April 2020 showed a significant decrease in air pollution levels in India. The average concentration levels of PM10, PM2.5, CO, NO2, and SO2 have decreased nationwide by 33, 34, 21, 47, and 21%, respectively, during the nationwide lockdown compared to their concentration levels before the lockdown. While comparing their concentration levels of the nationwide lockdown period with those observed in April 2019 at the same CAAQMS, it was found that the nationwide average concentration levels of PM10, PM2.5, CO, NO2, and SO2 were decreased by 53, 45, 27, 54, and 35%, respectively. The trends of decreasing air pollutants during the lockdown in five major cities were almost the same as nationwide. The concentration levels of O3 have shown an increasing trend from January–April 2020 including during the nationwide lockdown. The COVID-19 has provided a rare opportunity for India for the collection of air pollution baseline data which could be useful in the formulation of air pollution reduction policies in the future.


INTRODUCTION
India is facing serious air pollution problems. The air quality in most Indian cities has been deteriorating over the past many years (WHO, 2016;Bernard and Kazmin, 2018;Chowdhury et al, 2019;HEI, 2019;Mishra, 2019). The emissions of air pollutants from anthropogenic sources, such as transport, industry, power generation, construction, residential, and commercial activities have been increasing significantly during the past many years (Guttikunda et al., 2014;Kurokawa and Ohara, 2020). Open burning of municipal waste and agricultural residuals are adding extra emission burden of air pollutants in the region which significantly deteriorates air pollution problems, especially during winter when stagnant meteorology promotes accumulation of air pollutants in the atmosphere (Badarinath et al., 2009;Rastogi et al., 2016;Kumari et al., 2017;Liu et al., 2018;Bray et al., 2019;Sawlani et al., 2019). Increasing air pollution in India is significantly impacting air quality, increasing disease burdens, and incurring economic loss (Bhome, 2012;Rizwan et al., 2013;Bunett et al., 2018;Gordon et al., 2019;Reddy and Roberts, 2019).
The Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) or commonly known as coronavirus disease 2019  pandemic, emerged from the city of Wuhan, China, in December 2019, spread around the world in early 2020. Many countries imposed the lockdown in the cities, as well as, nationwide to minimize transmission of the COVID-19 infection. In the lockdown, commercial activities including transport, industries, and public gathering places were closed. As a result, the emissions of air pollutants were significantly decreased in many parts of the world (He et al., 2020;Patel and Stevens, 2020;Sulaman et al., 2020). For example, the total nitrogen dioxide (NO2) column was reduced by about 40 percent (%) over the Chinese cities and 20-38% over Western Europe (Bauwens et al., 2020). Sicard et al. (2020) reported a reduction in oxides of nitrogen (NOx) and particulate matters (PM) by about 56% and 42%, respectively, in Chinese cities. In the United States and Spain, a decrease in NO2 concentrations by 25.5% and 50-62% were observed, respectively, (Berman and Ebisu, 2020;Baldasano, 2020).
India imposed a nationwide lockdown from March 25, 2020, onward for a few months to minimize the spread of the COVID-19 infection. During the lockdown, all transport services (road, airplane, and railway), schools, colleges, industrial operations, hotels, restaurants, shopping malls, markets were suspended, except for essential services (CPCB, 2020). The real-time National Air Quality Index (NAQI) published by the Central Pollution Control Board (CPCB, https://app.cpcbccr.com/AQI_India/) and CPCB's preliminary report on the NAQI suggested that the air pollution in India has been substantially decreased during the nationwide lockdown. This encouraged us to study in detail the impacts of the COVID-19 on air quality in India.
Several studies have been published on the impacts of the COVID-19 on air pollution level in India (Jain and Sharma, 2020;Kumar, 2020;Kumari and Toshniwal, 2020;Majumdar, 2020;Mahato et al., 2020;Sharma et al., 2020;Shehzad et al., 2020;Singh and Chauhan, 2020). However, most of the studies have reported measurements of air pollution in metro cities and urban regions. In this article, we reported the nationwide air pollution measurements, using the data from more than 200 Continuous Ambient Air Quality Monitoring Stations (CAAQMS) located all over the country, from January to April 2020, the trends in the change in NAQI in Indian states, the spatial distribution of concentrations of Aerosol Optical Depth (AOD) and Carbon Monoxide (CO), NO2, Sulfur Dioxide (SO2), and Ozone (O3) using the satellite visual maps, and made a comprehensive statistical analysis of the change in the concentrations of PM (both PM10, PM2.5), CO, NO2, SO2, and O3 at nationwide and in major Indian cities, namely, Delhi, Mumbai, Kolkata, Chennai, and Hyderabad. For a better understanding of the impacts of the nationwide lockdown on the air pollution level, we defined, the period from February 25, 2020, to March 24, 2020, as "before lockdown" and from March 25, 2020, to April 30, 2020, as "during lockdown". We compared time-series and average concentrations of key air pollutants observed from January to April 2020 with those observed from January to April 2019.

Air Pollutants
India operates a network of 231 CAAQMS located in various parts of the country. Fig. 1 shows a spatial distribution of the CAAQMS network in India, blue dots are the location of each CAAQMS. These CAAQMS are operated by CPCB, State Pollution Control Boards (SPCBs), Delhi Pollution Control Committee (DPCC), and Indian Meteorological Department (IMD). Air pollution data from these CAAQMS acquired at CPCB. For the present study, we downloaded 24-hr averaged data from the CPCB website (https://app.cpcbccr.com/ccr/#/caaqm-dashboard-all/caaqmlanding/data) for PM10, PM2.5, CO, NO2, SO2, and O3 for January to April 2019 and January to April 2020. The data from January to April 2019 has been used only for comparison of temporal variations in the concentration levels of air pollutants for 2019 and 2020, while data observed in April 2019 included in statistical analysis for comparison of concentration levels of air pollutants that observed during the nationwide lockdown. We would like to clarify that quality control of data was beyond our reach since these CAAQMS are operated by different organizations and there is no information available on data quality control. However, we assumed that data quality control has been performed by the respective CAAQMS operators.

National Air Quality Index
CPCB published a real-time NAQI of 231 CAAQMS (https://app.cpcbccr.com/AQI_India/). NAQI is calculated by a weighted average of 8 criteria pollutants, namely, PM10, PM2.5, SO2, NO2, CO,  O3, NH3, and Lead (Pb) into a single value matrix. The basic scheme used by CPCB for the calculation of NAQI is summarized in Fig. 2. Briefly, NAQI has been calculated in 2 steps. In step 1, the sub-index of each criteria pollutant was calculated, and then in step 2, all sub-indexes were aggregated to a single index. Details of equations used for the calculation of NAQI have been discussed in the Report on National Air Quality Index (CPCB, 2014). For the present study, 24hour averaged NAQI data was downloaded from the CPCB website from January to April 2020 from the CAAQMS, shown in Fig. 1. If the number of CAAQMS in a city is more than one, one 24hour averaged NAQI value from all CAAQMS has been published for that city. Also, not all CAAQMS are operated all the time.

Satellite Visual Maps
Combustion is the main source of NO2, generally emitted from the transport, industrial, commercial, and residential sectors. In Fig. 5(b), the NO2 hotspots were seen in the urban regions of India. The NO2 hotspots were largely seen in north India including Delhi, Haryana, Punjab, Western Uttar Pradesh, and in the eastern part of India including Bihar, Jharkhand, West Bengal, and Odisha. Although the sources of NO2 emissions exist all over India, however, in the states of Delhi, Haryana, Punjab, Uttar Pradesh, Bihar, Jharkhand, West Bengal, and Odisha, they were predominantly more due to the dense population and industries. Also, these states are part of the IGP corridor where frequent stagnant meteorological conditions favor the accumulation of pollutants in the atmosphere (Dekker et al., 2019;Sawlani et al., 2019;Kanawade et al., 2020;Ojha et al., 2020).
In January 2020, there were many NO2 hotspots in the north and eastern part of India, which slightly decreased in February 2020. In March 2020, the NO2 hotspots almost disappeared from north India, while in April 2020 they disappeared from entire India, except few hot spots in the eastern part of India ( Fig. 5(b)). The disappearance of NO2 hotspots in March and April 2020 could be attributed to the decreasing NO2 emissions from transport and industries, as both emission sources were suspended during the nationwide lockdown. The emissions of CO link with the combustion sources, hence the pattern of the CO in the satellite visual maps were the same as those of NO2 (Figs. 5(b) and 5(c)).
The hotspots of SO2 emissions were visible along the IGP corridor (Delhi, Uttar Pradesh, Bihar, Jharkhand) and in the eastern states of Odisha and West Bengal and south-western industrial regions in the states of Maharashtra and Gujrat (Fig. 5(d)). The main source of SO2 is coal-fired power plants and industries that use coal in the boilers. The intensity of SO2 emission hotspots in India was almost the same in January and February 2020. They decreased slightly in March 2020, however, still visible during April 2020 (Fig. 5(d)) despite the nationwide lockdown. As mentioned, power supply was classified as emergency service during the nationwide lockdown, so power generation was continued in the coal-fired power plants. Interestingly, high AOD and emission hotspots of NO2, CO, and SO2 did not disappear in the states Odisha and Jharkhand in April 2020 ( Fig. 5(a)-5(d)). Also, the NAQI was moderate in these states ( Fig. 3(a) and 3(b)). This might be because in the states of Jharkhand and Odisha, large-scale coal mining is done, and these mines were likely operating during the nationwide lockdown to meet the demand for coal for power generation. Recently published regional emission inventory in Asia (REAS, version 3.1) by Kurokawa and Ohara (2020) has shown that the emissions of NO2, CO, and SO2 were large in the eastern part of India where the states of Jharkhand and Odisha are located.
The nationwide lockdown period (25 March-30 April 2020) was also indicated as a blue dashed line in Fig. 6. In Figs. 6(a)-6(f) the nationwide average concentrations of PM10, PM2.5, CO, NO2, SO2, and O3 observed from January to April 2019 at the same CAAQMS were also included for showing a better comparison of the concentration levels of air pollutants, especially during the nationwide lockdown period.
The temporal variations in the concentrations of PM10, PM2.5, CO, and NO2 show a similar trend which shows proximity in the emission sources of these pollutants, such as fossil fuels combustion in transport, industries, residential and commercial activities (Figs. 6(a)-6(d)). The PM10 and PM2.5 concentrations were quite high at a few CAAQMS during January and February 2020 (Figs. 6(a) and 6(b)), however, their concentrations systematically decreased with time. The nationwide average and nationwide median concentrations of PM10 and PM2.5 continuously shown a decreasing trend from January to mid-March 2020. The nationwide average concentrations of PM10 and PM2.5 observed from January to April 2019 have also shown a similar decreasing trend, but their levels were higher in January and February 2019 compared to those in January and February 2020. From February to mid-March, the nationwide average concentration of PM10 and PM2.5 both in 2019 and 2020 were almost the same. Afterward, the nationwide average concentrations of PM10 and PM2.5 of 2019 have remained almost steady, whereas the nationwide average concentrations of PM10 and PM2.5 of 2020 decreased significantly, which could be attributed to the impacts of the nationwide lockdown (Figs. 6(a) and 6(b)).
The nationwide average and nationwide median concentrations of PM10 and PM2.5 in 2020 and the nationwide average concentrations PM10 and PM2.5 in 2019 were higher than the 24-hour NAAQS for PM10 and PM2.5 from January to mid of March. Afterward, from March 21, 2020, both the nationwide average and nationwide median concentrations of PM10 and PM2.5 were lower than the 24-hour NAAQS (Figs. 6(a) and 6(b)). The lower nationwide average and nationwide median concentrations of PM10 and PM2.5 during the nationwide lockdown as compared to their 24-hour NAAQS provided a new nationwide baseline data for the PM10 and PM2.5.
Like PM10 and PM2.5, the nationwide average and nationwide median concentrations of CO and NO2 were higher during January 2020 which subsequently decreased with time. The nationwide average of CO and NO2 in January 2019 was higher as compared to those observed in January 2020. However, during February and March in both 2019 and 2020, the nationwide average and nationwide median concentrations of CO and NO2 were almost similar (Figs. 6(c) and 6(d)), which implied that the magnitude of CO and NO2 emissions were not changed significantly during both years. After mid-March, the nationwide average concentrations of CO and NO2 has remained steady in 2019, while the nationwide average and nationwide median concentrations of CO and NO2 in 2020 has been decreased significantly during the nationwide lockdown (Figs. 6(c) and 6(d)). The 24-hour NAAQS of NO2 was always higher than the nationwide average and nationwide median concentrations ( Fig. 6(d)).
The temporal variations in the concentrations of SO2 were quite different than those of PM10, PM2.5, CO, and NO2 (Figs. 6(a)-6(e)The nationwide average and nationwide median concentrations of SO2 observed from January to March 2020 had remained steady. During the nationwide lockdown, a slight decrease in the concentrations of nationwide average and nationwide median of SO2 was observed compared to those observed in April 2019. The 24-hour NAAQS of SO2 was always higher than the nationwide average and nationwide median of SO2 ( Fig. 6(d)).
O3 is a secondary pollutant. Ground-level O3 is formed during photochemical chain reactions between oxides of nitrogen (NOx), volatile organic compounds (VOCs), and CO. The temporal variations in the nationwide average and the nationwide median concentrations of O3 showed a consistently increasing trend from January to April 2020 including during the nationwide lockdown ( Fig. 6(f)). Almost the same trend has been shown by the nationwide average concentrations of O3 observed from January to April 2019. However, the nationwide average and nationwide median concentrations of O3 observed during the nationwide lockdown were slightly lower than those in April 2019. The increasing trend in the temporal variations in the O3 concentrations was quite opposite to that shown by primary key pollutants including during the nationwide lockdown (Figs. 6(a)-6(d)). It was expected that during the nationwide lockdown, since the precursors of O3, (e.g., NO2 and CO) have shown a decreasing trend (Figs. 6(c) and 6(d)), the O3 concentrations were also expected to show a decreasing trend.
To investigate the increasing trend in the concentration of O3, time-averaged satellite visual ORIGINAL RESEARCH Special Issue on COVID-19 Aerosol Drivers, Impacts and Mitigation (XII) https://doi.org/10. 4209/aaqr.200482 maps of O3 and Incoming Shortwave Flux were plotted from January-April 2020 for each month, shown in Figs. 7(a) and 7(b), respectively. As can be seen in Figs. 7(a) and 7(b), the spatial distribution of concentrations of O3 were kept increasing from January-April 2020, in particular over north India, along with incoming shortwave flux. This implied that despite the decreasing trend in the concentrations of O3 precursors (e.g., NO2 and CO) at ground level observations (Figs. 6(c) and 4(d)), there were enough emissions of NO2, CO, and probably VOCs which causes the formation of O3 in northern India. The emissions of NO2, CO, and VOCs might be from the burning of biofuels in domestic cooking in remote or rural regions of India since the burning of biofuels are a big source of O3 precursors in north India (Pallavi et al., 2019;Jat et al., 2020). It may be noted that the observation sites, shown in Fig. 1, are mostly located in urban regions of India which failed to capture the emissions of O3 precursors from rural or remote areas. The 24hour NAAQS of O3 was always higher than the nationwide average concentration of O3 during both 2019 and 2020 ( Fig. 6(f)).

Statistical Analysis
Tables 1(a)-1(f) shows statistical analysis of change in the average concentrations (±1 SD) of PM10, PM2.5, CO, NO2, SO2, and O3 nationwide and in Delhi, Mumbai, Kolkata, Chennai, and Hyderabad during January and April 2020 including before the lockdown and during the lockdown and those observed in April 2019. Figs. 8(a1)-8(f1) and 8(a2)-8(f2) graphically represents the change in the average (±1 SD) concentrations of PM10, PM2.5, CO, NO2, SO2, and O3 during the lockdown with the period before the lockdown (left panels: a1-f1) and the change in average (±1 SD) concentrations of PM10, PM2.5, CO, NO2, SO2, and O3 with average concentrations of these parameters observed in April 2019 (right panels: a2-f2) at nationwide and in Delhi, Mumbai, Kolkata, Chennai, and Hyderabad. Among these cities, Mumbai, Kolkata, and Chennai are the Data from no. of CAAQMS   Table   1.  coastal cities while Delhi and Hyderabad have located hinterland quite far from the coastal environment. These cities are located in different parts of the country. For example, Delhi is situated in the north, Mumbai in the west, Kolkata in the east, Chennai in the south, and Hyderabad in the south but far from the coastal environment. These five cities represented the major urban conglomerates of India. The nationwide average concentrations of PM10 were 160 and 155 µg m -3 in January and February of 2020, respectively. The nationwide PM10 average concentrations before the lockdown and during the lockdown were 117 and 78 µg m -3 , respectively. On comparing, the nationwide PM10 average concentration observed during the lockdown with that before the lockdown, it was found that PM10 nationwide average concentration decreased by 33% (Fig. 8(a1) and Table 1(a)). The nationwide PM10 average concentration observed in April 2019 at the same CAAQMS was 167 µg m -3 . The nationwide PM10 average concentration observed during the lockdown decreased by 53% when compared with PM10 nationwide average concentrations of April 2019 (Fig. 8(a2) and Table 1(a)). In Delhi, Mumbai, Kolkata, and Hyderabad, the average PM10 concentrations observed in January 2020 were 240, 144, 201, and 100 µg m -3 , respectively. The PM10 average concentrations in these four cities in February 2020 were 222, 161, 181, and 96 µg m -3 , respectively. Before the lockdown, the average PM10 concentrations in Delhi, Mumbai, Kolkata, and Hyderabad were 153, 146, 110, and 86 µg m -3 , respectively, while during the lockdown, PM10 concentrations have decreased to 98, 65, 54, and 60 µg m -3 in Delhi, Mumbai, Kolkata, and Hyderabad, respectively. It was found that average PM10 concentrations decreased by 36, 55, 51, and 30% in Delhi, Mumbai, Kolkata, and Hyderabad, respectively, during the lockdown as compared to those before the lockdown in these cities (Fig. 8(a1) and Table 1(a)). In Mumbai and Kolkata, the percentage decrease in PM10 average concentrations during the lockdown was nearly 50%, higher than Delhi and Hyderabad. This could be an additional impact of the coastal environment where the land-sea breeze circulation brought clean air from the ocean to the land which helps in diluting the polluted air. During April 2019, the average PM10 concentrations in Delhi, Mumbai, Kolkata, and Hyderabad were 235, 101, 80, and 92 µg m -3 , when compared these average PM10 concentrations with those observed during the lockdown in these cities, it was found that PM10 concentrations decreased by 58,36,33,and 35% in Delhi,Mumbai,Kolkata,and Hyderabad,respectively (Fig. 8(a2) and Table 1(a)).
The nationwide CO average concentrations during January and February 2020 were 1.18 and 1.06 mg m -3 , respectively. Whereas, the nationwide CO average concentration before the Fig. 8. Change in average concentration (±1 SD) levels of PM10, PM2.5, CO, NO2, SO2, and O3 during the lockdown (DLD) with respect to before the lockdown (BLD) (left panels: a1-f1) and with respect to average concentration (±1 SD) levels of those observed in April 2019 (right panels: a2-f2) nationwide and in Delhi, Mumbai, Kolkata, Chennai, and Hyderabad. lockdown and during the lockdown were 0.91 and 0.72 mg m -3 , respectively. The nationwide average CO concentration decreased by 21% during the lockdown compared to that before the lockdown (Fig. 8(c1) and Table 1(c)). The nationwide average CO concentration during April 2019 was 0.98 in mg m -3 . The nationwide average CO concentration decreased by 27% during the lockdown with that observed in April 2019 (Fig. 8(c2) and Table 1(c)). In Delhi, Mumbai, Kolkata, Chennai, and Hyderabad, the average CO concentrations were 1.57, 1.13, 1.19, 0.84, and 0.65 mg m -3 , respectively, in January 2020 and 1.30, 1.20, 0.89, 0.82, and 0.61 mg m -3 , respectively, in February 2020. Before the lockdown, the average CO concentrations were 1.01, 0.92, 0.70, 0.73, and 0.57 mg m -3 in Delhi, Mumbai, Kolkata, Chennai, and Hyderabad, respectively. While during the lockdown, the average CO concentrations were 0. 77, 0.65, 0.40, 0.60, and 0.46 mg m -3 in Delhi, Mumbai, Kolkata, Chennai, and Hyderabad, respectively. It was found that during the lockdown, the average CO concentrations decreased by 24,29,43,18,and 19% in Delhi,Mumbai,Kolkata,Chennai,and Hyderabad,respectively, with those observed before the lockdown (Fig. 8(c1) and Table 1(c)). The average CO concentrations observed in April 2019 in Delhi, Mumbai, Kolkata, Chennai, and Hyderabad were 1.36, 1.68, 0.53, 0.91, and 0.61 mg m -3 , respectively. When compared the average CO concentrations of these cities during the lockdown period with those during April 2019, the average CO concentrations have decreased by 43,61,25,34,and 25% during the lockdown in Delhi,Mumbai,Kolkata,Chennai,and Hyderabad,respectively (Fig. 8(c2) and Table 1(c)).
The nationwide average SO2 concentrations during January and February 2020 were 14 and 15 µg m -3 , respectively, and the nationwide average SO2 concentrations before the lockdown and during the lockdown were 14 and 11 µg m -3 , respectively. The nationwide average SO2 concentration was decreased by 21% with that observed before the lockdown (Fig. 8(e1) and Table 1(e)). The nationwide average SO2 concentration during April 2019 was 17 µg m -3 . The nationwide SO2 average concentration was decreased by 35% during the lockdown period with that observed in April 2019 (Fig. 8(e1) and Table 1(e)). In Delhi, Mumbai, Kolkata, Chennai, and Hyderabad, the average SO2 concentrations were 11, 14, 17, 15, and8 µg m -3 , respectively, in January 2020. While in February 2020, average SO2 concentrations in Delhi, Mumbai, Kolkata, Chennai, andHyderabad were 15, 17, 17, 13, and 7 µg m -3 , respectively. Before the lockdown, average SO2 concentrations in Delhi, Mumbai, Kolkata, Chennai, and Hyderabad were 16, 18, 15, 9, and 6 µg m -3 , respectively. During the lockdown, average SO2 concentrations in Delhi, Mumbai, Kolkata, Chennai, and Hyderabad have reduced to 14,11,9,6, and 6 µg m -3 , respectively. The average SO2 concentrations were decreased by 12, 39, 40, 33%, and nil during the lockdown, in Delhi, Mumbai, Kolkata, Chennai, and Hyderabad, respectively, with those before the lockdown The nationwide average O3 concentrations during January and February 2020 were 27 and 35 µg m -3 , respectively. Whereas, the nationwide average O3 concentrations before the lockdown and during the lockdown were 36 and 39 µg m -3 , respectively. The nationwide average O3 concentration was increased by 8% when compared with that before the lockdown period ( Fig. 8(f1) and Table 1(f)). The nationwide average O3 concentration in April 2019 was 45 µg m -3 . By comparing, the nationwide average O3 concentration of the lockdown period with that observed in April 2019, it has found decreased by 13% ( (Fig. 8(f2) and Table 1(f)). In Delhi, Mumbai, Kolkata, Chennai, and Hyderabad, the average O3 concentrations were 19, 37, 33, 31, and 29 µg m -3 , respectively, in January 2020, and 31, 45, 39, 25, and 28 µg m -3 , respectively, in February 2020. Before the lockdown, the average O3 concentrations in Delhi, Mumbai, Kolkata, Chennai, and Hyderabad were 35, 34, 42, 39, and 27 µg m -3 , respectively. While during the lockdown, the average O3 concentrations in Delhi, Mumbai, Kolkata, Chennai, and Hyderabad were 43, 29, 47, 47, and 29 µg m -3 , respectively. During the lockdown, the average O3 concentrations have increased by 23, 12, 21, and 7% in Delhi, Kolkata, Chennai, and Hyderabad, respectively, with those before the lockdown (Fig. 8(f1) and Table 1(f)). Only in Mumbai, the average O3 concentration has decreased by 15% during the lockdown with that before the lockdown (Fig. 8(f1) and Table 1(f)). This may be due to a large decrease in NO2 concentration (74%) in Mumbai during the lockdown (Fig. 8(d1) and Table 1(d)). The average O3 concentrations observed in April 2019 in Delhi, Mumbai, Kolkata, Chennai, and Hyderabad were 36, 19, 30, 26, and 42 µg m -3 , respectively. When compared with the average O3 concentrations of the lockdown with those observed in April 2019 in Delhi, Mumbai, Kolkata, and Chennai, it was found that the average O3 concentrations of the lockdown were increased by 19, 53, 57, and 81%, respectively ( Fig. 8(f2) and Table 1(f)). It was observed that the formation of O3 in Mumbai, Kolkata, and Chennai was higher as compared to Delhi. This may be due to that Mumbai, Kolkata, and Chennai are coastal cities with a relatively clean environment and receives enough solar radiation, and probably have enough O3 precursors. However, a deeper investigation is needed to understand the reason for O3 formation in Mumbai, Kolkata, and Chennai and is beyond the scope of this study.

Policy Relevance
The COVID-19 has provided a rare opportunity to countries for the collection of air pollution baseline data since during the nationwide lockdown the emissions of air pollutants from transport, industries, and commercial activities were reduced significantly and have relevance to air pollution reduction policies. India imposed a nationwide lockdown from March 25, 2020, onward for a few months. As a result, the concentrations of air pollutants decreased significantly (Figs. 6 and 8). The observed nationwide average concentrations of PM10 and PM2.5 were well below the NAAQS of PM10 and PM2.5 during the nationwide lockdown (Figs. 6(a) and 6(b)). Also, the observed nationwide average concentrations of NO2, SO2, and O3 were always below the respective NAAQS of NO2, SO2, and O3 (Figs. 6(d), 6(e), and 6(f)). As per the WHO (2005) guidelines, the ambient air quality standards for PM10 and PM2.5 must be close to 50 µg m -3 and 25 µg m -3 , respectively, considering their impacts on human health. The NAAQS of India for PM10 and PM2.5 is 100 µg m -3 and 60 µg m -3 , respectively, which is about 2 times higher than the WHO (2005) prescribed guidelines. Considering the high pollution levels in India (WHO, 2016;Bernard and Kazmin, 2018;Chowdhury et al., 2019;HEI, 2019;Mishra, 2019), there is sufficient ground to review the current NAAQS of air pollutants for mitigation of air pollution. The air pollution measurement data summarized in this study showed that during the nationwide lockdown, emissions of air pollutants from transport, industries, and commercial activities were reduced significantly. As a result, the concentrations of PM10 and PM2.5 attained the NAAQS or even observed lower than the NAAQS. This suggests that if the emissions of air pollutants, particularly PM10 and PM2.5, reduced from transport, industries, commercial activities, including from biomass burning in domestic cooking as well open biomass burning, the current NAAQS, as well as the lower level of PM10 and PM2.5, can be achieved. This can be done by providing clean fuels and implementing 25 mitigations measures proposed by the Science-Based Solutions Report (UNEP, 2019).

CONCLUSION
This study made a detailed analysis of the impacts of the COVID-19 on air quality in India by using the observation data from more than 200 CAAQMS. India imposed a nationwide lockdown on March 25, 2020, for a few months and suspended transport, industries, and commercial activities, except for essential services. We defined the period from February 25 to March 24, 2020, as 'before lockdown' and March 25 to April 30, 2020, as 'during lockdown'. The study found that from January to April 2020, which included the nationwide lockdown period, the monthly NAQI was improved from poor to satisfactory or good; AOD and emission hotspots of CO, NO2, and SO2 were gradually disappeared; and temporal variations in the concentrations PM10, PM2.5, CO, NO2, and SO2 showed a gradually decreasing trend. The nationwide average concentrations of PM10, PM2.5, CO, NO2, and SO2 decreased during the lockdown by 33, 34, 21, 47, and 21%, respectively, compared to their levels before the lockdown. The average concentration levels of PM10, PM2.5, CO, NO2, and SO2 of the lockdown period when compared with those observed in April 2019, it was found that their average concentration levels were decreased by 53, 45, 27, 54, and 35%. The O3 level was slightly increased during the lockdown. The COVID-19 has provided a rare opportunity for India for the collection of air pollution baseline data which could be useful in the formulation of pollution reduction policies.