COVID-19 Lockdown and the Aerosphere in India: Lessons Learned on How to Reduce Air Pollution

The giant increase in COVID-19 infection across India forced the government to impose strict lockdown in order to curb the pandemic. Although the stringent restrictions crippled India’s economy and poor people’s livelihood, it significantly improved the air quality of most of the polluted cities of India and rejuvenated the atmosphere. Thus, the major objective of this study is to provide a comprehensive overview of lockdown on pollutants prevailing in the atmosphere. A prominent decline in primary pollutants such as Particulate matter (PM), Black carbon (BC), Oxides of nitrogen (NOx), Carbon monoxide (CO) is observed across the country. However, lockdown had a trifling impact on Sulphur dioxide (SO2) concentration over some parts of India due to the constant operation of coal-fired thermal plants as a part of essential service. Furthermore, the sudden decline in NOx concentration disturbed the complex atmospheric chemistry and lead to an enhancement of surface ozone (O3) (secondary pollutant) in many cities of India. Thus, lockdown emerged as a unique opportunity for the atmospheric researchers, policymakers as well as stakeholders to collect baseline data of pollutants and their major sources. This will help to set new targets of air quality standards and to develop various mitigation processes to combat air pollution.


Introduction
Air pollution has created wreaking havoc in many parts of the world.Over the decades, a rapid increase in population, especially in developing countries like India and China, has led to rapid urbanization, industrialization, energy consumption, and exponential growth in vehicles.Consequently, anthropogenic emissions have increased 2-6 folds, thus deteriorating the regional air quality and turning most of the Indian cities into non-attainment cities by the National Green Tribunal of India.Moreover, as per the latest IQAr 2020 report, India is the third most polluted country and is a home for 22 of the world's 30 worst polluted cities in 2020 [1].Besides, they also stated Delhi as the most polluted capital that resides 10 most polluted cities of the world.Apart from this, there have been several reports in recent times on ambient air criteria pollutants, like, particulate matter, O3, CO and NOx that are at the helm of triggering several environmental health threats leading to comorbidity and premature deaths [2,3].According to the 2019 Global Burden disease report, $1.67 million deaths were attributed to air pollution, which ascribed $18% of the country's total death [4].Short-term air pollution exposure is often associated with COPD (Chronic Obstructive Pulmonary Disease), shortness of breath, dizziness, nausea, asthma, cough, and an increase in rates of co-morbid symptoms [5].However, long-term exposure leads to cardiovascular diseases, lung cancer, and respiratory diseases such as emphysema and is often also accountable for impairment of the nervous system, kidney, liver, and other organisms.Exposure of women to severe ambient pollution during pregnancy is related to a high risk of congenital disabilities, miscarriage and adverse long-term postnatal health [6,7].Air pollutants also affect plant growth by inducing changes in soil pH, decrease in photosynthesis rate (trace metals block the stomata opening) and retartded growths [8].
Over the last five decades, an increase in greenhouse gas emissions, especially CO2, has tremendously increased earth temperature globally by preventing solar radiation from getting reflected on the outer surface.Besides, short-lived climate pollutants (SLCPs) such as black carbon (BC), tropospheric/surface ozone (O3), methane (CH4) also significantly contribute to global warming, which in turn lead to prolonged heat waves, temperature variability, forest fires, droughts, flood, melting of ice, icebergs, and glaciers along with depletion of the stratospheric ozone layer [5,9].Also, a rampant rise in pollutants such as oxides of sulfur and nitrogen has led to acid rain (pH<5) which in turn affects surface water, soil, and lowers biodiversity.Acid rain damages limestone and marble buildings.
This havoc scenario has raised concern among researchers and environmentalists to focus on minor constituents of the atmosphere such as particulate matter and trace gases (mainly SO2, NOx, O3).These species vary enormously with space, time, meteorology, and climate.In this context, the Ministry of Environment, Forest and Climate Change initiated a National Air Quality Monitoring Program (NAMP) all over India.The network comprised of 543 operating stations in 240 cities/towns in 26 states and 5 Union Territories where monitoring is carried out collectively by the Central Pollution Control Board (CPCB); State Pollution Control Board (SPCB); Pollution Control Committees (PCC); National Environmental Engineering Research Institute (NEERI), Nagpur.Apart from these, the Space Physics Laboratory (SPL) of VSSC has set up more than 42 monitoring stations all over India under the project ISRO-GBP -ARFINET to study regional aerosol characterization, heterogeneous features, and radiative forcing.Robust monitoring helps assess the quality of air along with significant sources of pollutants and its negative impact on the ecosystem and humankind.It also aids in guarding against extreme events by alerting people through designing and implementing new air quality management programs into urban city planning platforms.
The main sources contributing to air pollution in India are transportation, industrial emission, commercial and residential biomass burning, coal-burning for energy production, agricultural stubble burning, brick kilns, municipal waste burning, etc. [10,11].Besides, the use of solid fuel such as wood, cow dung cakes, agricultural residue, coal, charcoal for cooking purposes in rural India is adding an extra burden of pollutants to the air [12].In this challenging scenario, the Central government has recently launched National Clean Air Programme (NCAP) to develop a city-based clean air plan.In the first phase, 122 non-attainment cities were selected from Indo -Gangetic Plain (IGP) based on ambient air quality monitoring data of 2011-2015 [13].The major objective is to reduce 20-30% of particulate pollution by 2024 compared to 2017.As a primary initiative of NCAP, the government has restricted older vehicles (10-year-old petrol vehicle, 15-yearold diesel vehicle), prioritized public transport, increased infrastructure for CNG and electric mobility, ordered to close older power plants in the vicinity ($300 km) of non-attainment cities and promoted conversion of brickkilns into induced draft zig-zag technology.In addition to the foregoing plan, the Central Government launched Pradhan Mantri Ujjawala Yojana in 2016 to distribute LPG connection to below poverty line women in order to reduce household pollution due to solid fuels.Another significant step taken by the Delhi government was the odd-even scheme for four-wheeler vehicles to reduce the city's vehicular fleet volume.However, a recent study at Delhi and Kolkata revealed that these policies could facilitate a maximum $20% reduction of the primary emission, which in turn may not facilitate in achieving the desired goal of good air quality in these cities [14].In such a situation, the national-level lockdown during 2020 to combat the spread of the novel coronavirus came up as a natural experiment to simulate the sensitivity of air pollutants.Several reports showed a significant plummet in primary emission sources or SLCPs (>30%) leading to the emergence of the blue sky after several decades in India [11,15,16].Furthermore, a recent model-based study by our group predicted the drastic decline in fine particulate pollutant level during the lockdown had potentially avoided 73,853-92,116 mortalities during April-June 2020 that might have surpassed the fatality due to the pandemic [17].Thus, lockdown provides a unique opportunity for environment researchers to collect baseline data of various pollutants, which could help develop new mitigation strategies in the future.In this context, we have provided a comprehensive overview of the impact of lockdown on different air pollutants in India.

Lockdown in India in brief
The Lockdown 1 marked the total suspension of all industrial activities, transportation, agricultural activities, educational institutions, government, and nongovernment offices.In addition to these, spiritual gatherings, parties, functions, and other non-essential gatherings were banned.This lead to a considerable reduction in anthropogenic emissions for a short period.During the 2nd lockdown, some relaxations were made in the movement of essential items like goods, medical equipment, private vehicles in the local area, and agricultural activities.However, public transportation such as bus, train, airways remained suspended along with inter-district and inter-state movement.Restrictions were further relaxed during lockdown-3 to 5 by allowing movement of vehicles (to a limited extent), re-opening of government offices and IT companies, small and medium-based industries, etc.However, educational institutions, cinema halls, public gathering places were closed during the entire lockdown.A more detailed description can be found in our recent publication [18,19].Whenever "lockdown" is mentioned in the running text it refers to all phases i.e. from 24th March to 31st May 2020.However, different researchers have taken different pre-lockdown periods for their reference.So, we have mentioned the pre-lockdown periods in the bracket.

Literature selection
This study includes relevant papers indexed in Google Scholar, a prominent online database for accessing scientific articles.The search string was intentionally designed to provide appropriate coverage of the diverse research.The search strategy used the following keywords: COVID-19 lockdown, the impact of lockdown on air quality, India.An initial search in Google Scholar returned 7500 articles.A brief screening revealed that many articles were not related to air quality but included only the term COVID-19 and its impact on water, health, etc. Therefore those articles were excluded.At the end of this stage, 457 articles remained in the database.

Particulate matter (PM)
PM refers to the mixture of solid particles and liquid droplets suspended in the air.It is made up of a large variety of components like dust, smoke, soot (or BC), acids (such as nitrates and sulfate), metals, organic compounds.Some of the big particles or dark particles like soot/BC can be seen through the naked eye, while others can be detected under the microscope.The particles are introduced in the atmosphere from a wide variety of natural (sea-salt, mineral dust, volcanic eruptions, forest fires, etc) and anthropogenic sources (construction sites, unpaved roads, vehicles, industries, biomass burning, coal and oil combustion, solid waste burning) either directly (known as, primary sources) or through complex chemical reactions with gaseous precursors (known as, secondary sources), or both.Furthermore, based on aerodynamic diameter, PM can be classified as coarse (PM10; diameter ≤ 10 μm), fine (PM2.5;diameter ≤ 2.5 μm), and ultrafine (PM <1 μm).The different modes of PM originate separately, are transferred separately, and are removed by different mechanisms from the atmosphere [20].Furthermore, the particles are known to influence climate and weather patterns by acting as cloud condensation nuclei (CCN), thus altering the microphysical properties of clouds and the hydrological cycle as well.In addition to this, aerosols disturb the earth's radiation budget, biogeochemistry of oceans and lakes, impair visibility, etc.The daily and annual threshold limit recommended by National ambient air quality (NAAQ) for PM10 is 60 and 100 μg.m À3 while for PM2.5 is 40 and 60 μg.m À3 , respectively.In many recent studies, it is also hypothesized that chronic exposure to PM induces oxidative stress in the respiratory tract, triggering several health conditions starting from coughing and wheezing to severe asthma attacks, bronchitis to high blood pressure, heart attack, stroke, and premature deaths.
A remarkable reduction is observed in PM concentration across all cities during the lockdown.Table 1 depicts the mean percentage change in PM loading over different Indian cities.A study over India with data from 200 air monitoring stations revealed PM10 and PM2.5 concentration decreased by 33% and 34% respectively, during lockdown (25th Mar-30th Apr 2020) as compared to the normal days (25th Feb-24th Mar 2020) [11].New Delhi, the capital of India with the most hazardous air pollution level showed a drastic decline in PM10 and PM2.5 concentration (>50%) during the confinement period [21].A recent study over Kolkata, the largest megacity in eastern India revealed that the average concentration of PM10 shows a significant plummet of 51% from pre-lockdown (17th Feb -23rd Mar 2020) to lockdown (24th Mar-20th May 2020) with a decrement of 1 μg.m À3 daily during the lockdown [33].Other studies reported the impact of lockdown on Kolkata taking data from seven air quality monitoring stations [34].
A reduction in 33% and 42% for PM10 and PM2.5 respectively were observed over Kolkata during Jan-May 2020 as compared to the same time frame of 2019.Also, the average PM2.5 concentration was 20 μg.m À3 during lockdown (1st Jan-23rd Mar 2020) while it was 80 μg.m À3 in pre-lockdown days (1st Jan-23rd Mar 2020) representing a reduction of 75%.Similarly, a significant plummet in PM2.5 concentration in the peak hour (07:00-11:00 hour) during lockdown over Kolkata (63.4%),Mumbai (56.4%),Chennai (48.5%),New Delhi (21.3%) and Hyderabad (23.8%) compared to the pre-lockdown period (1st-24th Mar 2020) [22].More studies over metro cities during the pandemic revealed that reduction in PM2.5 concentration shows large variability varying from 3-50% over Hyderabad, Delhi, Chennai, Ahmedabad, Pune compared to the previous year of the same time [23,26,27].As per some studies, PM2.5 decreased from 102.17 μg.m À3 to 51.66 μg.m À3 (>50%) over Lucknow during lockdown compared to the previous year [16].The number of monitoring stations is mentioned within the bracket.[35].A continuous drop in the concentration of PM2.5 and PM10 is also observed in eastern India i.e.Kolkata from 25th Mar -15th May 2020 in comparison to the previous 3 years (2017-2019) [32].PM2.5 and PM10 loading curbed down to 38% and 33% respectively during lockdown (Mar-May 2020) over the former two years of the same time over Bhubaneswar, an urban coastal site in eastern India [19].A significant reduction of PM2.5 in the range of 24-65% is observed at five different monitoring sites in Chennai, a tropical coastal site in South India [29].However, the weekly analysis revealed that the decrement is not constant throughout the lockdown.There are some weeks with a higher value of PM2.5 due to the operation of coal-fired thermal power plants as a part of emergency services [29].Nevertheless, in Chandigarh, the PM2.5 concentration was 20 μg.m À3 just 21 days before lockdown which reduced to 14.3 μg.m À3 ($28.5%) during the first phase of lockdown and then increased to 15.4 μg.m À3 during the second phase because of some relaxations [42].Maharastra, the worst-hit state by COVID-19 showed that during 1st-24th Mar 2020 the PM10 concentration was above the NAAQS threshold limit (> 100 μg.m À3 ) which reduced sharply up to 51% during lockdown while PM2.5 dropped down by 46% over normalcy [31].PM measurements over Gadanki, a rural site in south India revealed that PM2.5 and PM10 dropped down by $47% and 50% respectively from pre-lockdown (15th Feb -21st Mar 2020) to lockdown [36].Similarly, studies over south India reported that PM2.5 reduced by 53%, 15-22%, and 24-47% over Kunnur, Bengaluru and Kerala respectively during lockdown over pre-lockdown (1st Jan-22nd Mar 2020) [30,39,40].Thus, a decrease in traffic density, human mobility, shutting of industries drastically reduced PM pollution over India leading to the emergence of blue sky after a decade [25,43].

Black carbon (BC)
In recent years, with rapid modernization, India has become the second-largest emitter of BC in the world [44].BC or soot (as discussed above) is an important anthropogenic component of atmospheric aerosol with residence time varying from several days to a week in the lower troposphere [45].It is released into the atmosphere due to incomplete combustion of fossil fuel, biomass burning, and biogenic sources by the multiphase reaction [46,47].Furthermore, BC strongly absorbs solar radiation, affecting the Earth's radiation budget [48,49], thermodynamics of the atmosphere [50], lifetime and optical properties of clouds [51], resulting in global warming.Moreover, inhalation of BC particles causes serious respiratory problems in humans [52] and aquatic animals [53] and is also responsible for crop damage.The real-time mass concentration of BC is measured using a seven-channel (370, 470, 520, 590, 660, 880, and 950 nm) instrument called Aethelometer.The instrument is based on spot technology and it measures the attenuation of light at seven different wavelengths passing through a quartz fiber filter tape, on which aerosols get deposited.The total mass concentration of BC is estimated from 880 nm wavelength.Being chemically inert under atmospheric conditions, BC is removed mostly by wet deposition from the atmosphere [52].A recent study on primary BC measurement from ARFINET (Aerosol Radiative Forcing over India Network) reported 10-40% diminution in BC concentration during lockdown (24th Mar-31st May 2020) compared to pre-lockdown (4th -23rd Mar 2020) [54].Table 2 shows the reduction percentage of BC over different cities during confinement periods over normal days.Along with this, a significant reduction in the range of 16-60% over central and peninsular India and Himalayan and sub-Himalayan regions was observed during lockdown-2 compared to 2015-2020 at the same time [54].However, Indo Gangetic Plain (IGP) showed a maximum reduction of >60% followed by north-eastern India (>30%) during lockdown-2 with respect to the preceding 5 years of the same period.Similarly, a study over Bhubaneswar depicted a 33% reduction in BC concentration during March-May 2020 compared to 2017-2018 of the same time frame [19].BC concentration also showed a decline of 34% during lockdown over the same period of 2019 over Gadanki [36].A notable dwindling in BC concentration (> 60%) during lockdown-1 over pre-lockdown (1st Jan-14th Mar 2020) is observed at Bengaluru, a megacity in southern India [55].Nevertheless, a  marginal reduction in BC concentration over Challakare (a remote rural location situated $230 km away from Bengaluru) divulged that lockdown had a minimum impact on aerosol concentration over the rural location.Moreover, they reemphasized that emissions from industries and the transport sector are the major sources of aerosols of urban and semi-urban locations whereas rural sites are mostly dominated by meteorological conditions and synoptic wind [55].The study from ARFINET also disclosed that the most remarkable plummet is observed at the urban locations than remote and rural places [54].Additionally, it was revealed that the sharp decline in BC concentration is due to strict restrictions in vehicular movement and other anthropogenic activities which resulted in a decrement of fossil fuel contribution to the overall BC load [19,55].However, no precipitable change is observed in BC from biomass burning because of domestic cooking activities as well as mass cooking by different Non-Governmental Organisations to feed the needy and homeless people.

Aerosol optical depth (AOD)
AOD is a comprehensive variable to measure the aerosol burden (such as dust, sea salt, smoke, urban haze, etc) distributed within the earth's surface to the top of the atmosphere.In other words, AOD gives information about how much direct sunlight is prevented from reaching the ground by aerosols.Furthermore, it gives an estimation of PM2.5 surface concentration [56].Measurements using satelliteborne observation and aerosol reanalysis products revealed that during lockdown (24th Mar -22nd Apr 2020) maximum decrement in AOD value is observed in eastern IGP ($40% of pre-lockdown; 20th Feb-20th Mar 2020) followed by northwest (27%) and south-India (13%) [4].In disparity, an increment of 20% is observed in central India, the most polluted part of India.It might be due to an increase in secondary aerosol formation, aerosol water content, and other meteorological conditions.Likely researchers observed a maximum reduction in AOD in the IGP region followed by western, southern and eastern parts of India [15].Satellite visual maps over IGP showed a reduction in AOD during March 2020 compared to Jan-Feb 2020 and a much more reduction is observed in April 2020 (Figure 1).However, ground-based measurement revealed that the highest reduction in AOD is observed at Lahore (60%) followed by Kanpur (52%), Gandhi college (33%), Bhola near the Bay of Bengal (BoB) (12%).But Karachi, a coastal site near the Arabian sea showed an enhancement of 4% from pre-lockdown to lockdown.In addition, both ground-based and satellite measurements over Gadanki revealed a decrement of $17% of AOD during lockdown over 2019 [36].AOD at 550 nm from January-April 2020 (source: [11]).

Carbon monoxide (CO)
CO is a very stable, odorless, colorless, and poisonous atmospheric pollutant with a lifetime of two to four months in the atmosphere [57].However, CO is not considered as a direct greenhouse gas due to its low absorption in the infrared region but it increases the concentration of greenhouse gases (such as methane and ozone) by reacting with the hydroxyl radical of the atmosphere.The indirect radiative forcing by CO is 0.23 Wm À2 due to the formation of ozone, carbon dioxide, and methane (IPCC,2013).The major emission sources of CO include vehicular exhaust, waste incinerators, wildlife fires, power stations, biomass burning, furnaces, grills, stoves, and incomplete combustion of coal, wood, gasoline, plastics, fuel oils [58,59].The photooxidation of methane and other hydrocarbons also leads to the formation of CO as a by-product [18].However, studies revealed biofuel burning in residential sites to be the major contributor of CO ($86%) followed by the transport section ($13%) in India.According to Occupational Safety and Health Association (OSHA), the personal exposure limit for CO is 50 ppm for 8 hours.However, chronic exposure to CO leads to a neurological disorder, cardiac dysfunction [60] in addition to fatigue, dizziness, food poisoning, stomach pain, etc.
[61] Satellite images showed a significant reduction in surface CO concentration during the lockdown (Figure 2).Nationwide average CO concentration was observed to drop upto$21% during lockdown (25th Mar-30th Apr 2020) over prelockdown (25th Feb-24th Mar 2020) [11].As per studies, CO concentration dropped by 15% during Jan-May 2020 over 2019 at Kolkata [34].The lowest monthly average concentration of CO was reported during April and May 2020 compared to the preceding 3 years of the same month [32].Studies over western India during the lockdown revealed a plummet of $21% and 25% over Pune and Gujrat respectively, during 2020 in comparison to the same period during 2019, while no significant change is observed at Mumbai [38,62].Over Bhubaneswar, the eastern coastal city showed a reduction of $14% in CO concentration during complete lockdown period compared to the earlier lockdown period (1st Mar-23rd Mar 2020) whereas it showed a diminution of 11.1% during March-May 2020 compared to the previous year of the same time [18,37].A lower reduction of CO compared to the early phase of lockdown might be attributed to its long residence time in the atmosphere.Similarly, a lower reduction in CO ($10%) compared to other primary pollutants is observed at Gadanki during lockdown over the previous year [36].However, CO measurement at five different sites of Chennai showed a prominent plummet in the range of 5.60-90.72%during lockdown (Table 3) [29].Similarly, a remarkable plummet of 4-44% is observed over Delhi, Uttar Pradesh and Haryana Change in surface CO concentration during January-April 2020 (source: [11]).
during the lockdown.Chandigarh showed a decrement of 16.3% from 21-days before lockdown to the first phase of lockdown and then showed an enhancement of 5.4% in the second phase of lockdown compared to the first phase.A sharp reduction in the range of 24-67% is also reported over Kerala amid lockdown and meteorological changes [39].

Nitrogen oxides (NOx)
Nitrogen oxides (NOx = NO + NO2) are primary criteria air pollutants that play a vital role in tropospheric chemistry as the precursor of surface ozone and secondary aerosols apart from being responsible for acid rain and reddish-brown haze [64].Besides, NOx affects human health causing several heart and lung diseases as well as unseasonable death worldwide.The NOx measurement over different parts of the country revealed that the major anthropogenic emitters of NOx are from vehicles followed by power plants [65,66].Apart from these, soil, lightning, and wildfire are some of the natural sources of NOx [67].Owing to a very short lifetime ($2 days) NOx showed maximum reduction in concentration across all parts of India as compared to other primary pollutants (Table 4).Satellite data revealed major NO2 hotspots of India are Delhi, Haryana, Punjab, and in the eastern part of Bihar, Jharkhand, West-Bengal, and Odisha which disappeared completely during April 2020 [11] (Figure 3).Similarly, a 60-66% reduction in NO2 concentration was found over Delhi, Ahmedabad, Mumbai, and Pune [27].Analysis of NO2 concentration at 63 locations of Delhi, Uttar Pradesh, Harayana revealed a plummet of 27-58% during lockdown [24].In Lucknow, NO2 concentration reduced from 43.06 μg.m À3 to 14.57 μg.m À3 (66%) during lockdown 2020 compared to 2019.According to a recent study over Kolkata NOx showed a decline of 68% during lockdown (24th Mar-20 May 2020) in comparison to pre-lockdown (17th Feb -23rd Mar 2020) with a daily decline of 0.373 μg.m À3 during the lockdown [33].Moreover, NO2 reduced up to 45% during Jan-May 2020 with respect to Jan-May 2019 over Kolkata [34].On the same note, it was stated a sharp reduction in the average concentration of NO2 from 41.58 μg.m À3 in Mar 2019 to 18.01 μg.m À3 ($57%) in Mar 2020 [32].A significant plummet in average NOx concentration is also observed on the eastern coast of India.Studies over Bhubaneswar revealed that average NOx concentration decreased by $67% during quarantine days (24th Mar-31st May 2020) compared to pre-lockdown (1st -23rd Mar 2020) whereas 12.3% during Mar-May 2020 in comparison to 2019 of the same period [18,37].Similarly, NO and NO2 reduced by $55 and 59% respectively, over Gadanki during lockdown with respect to 2019 [36].Significant reduction in the range of 41-55% in NOx concentration is also observed at Chennai, the southern coastal site of India.Studies over Hyderabad and Kerala specified NO2 concentration dropped down by 33% and 48% respectively during lockdown over pre-lockdown [39,63] .NO2 concentration curbed down by $60% and 30-84% from pre-lockdown to lockdown over Maharashtra and Gujrat respectively [31,38].

Sulphur dioxide (SO2)
SO2 is one of the potent anthropogenic sulphur-containing air pollutants responsible for smog and acid rain [20].According to a 2019 report by Greenpeace, India has become the largest emitter of SO2 in the world with more than 15% of hotspots as detected by NASA OMI (Ozone Monitoring Instrument) satellite [68].It is also revealed that coal-fired thermal power plants and industries are the major emitters of SO2 in India.In addition, emissions from vehicles, ships, locomotives, ore smelting are also contributing substantially to atmospheric SO2 concentration.lockdown (24th Mar-20 May 2020) with an average reduction of 40%.However, the daily decrement of SO2 was found to be 0.153 μg.m À3 [33].A reduction of 3% in SO2 Jan-May 2020 compared to the previous year of the same time [34].Similarly, the monthly average concentration of SO2 during April 2017, 2018, and 2019 was found to be 7.59, 9.47 and 8.27 μg.m À3 respectively which dwindled up to 5.36 μg.m À3 during complete lockdown (i.e.Apr 2020), depicting a plummet of $37% compared to the past three years [32].A sharp decline in SO2 $63% and $16% is also observed over Gadanki and Lucknow respectively during lockdown than in 2019 [16,36].Kannur (the hotspot of corona) in Kerala showed a decrement of $62% in SO2 concentration during the quarantine days (25th Mar-9th May 2020) over normal days (1st -24th Mar 2020 & 10th -17th May 2020) [40].A reduction of $40% is also observed over the industrial state of western India during Jan-Apr  2020 over the previous year [38].Likely, SO2 concentration reduced from 28.99 to 9.43 μg.m À3 ($67%) and 19.14 to 9.43 μg.m À3 ($51%) over Ankleshwar and Vapi respectively located in Gujrat during the first phase of lockdown than the previous year [35].However, amid lockdown, an increase in SO2 concentration is observed at the commercial cum residential sites of Chennai namely Teynampet (40%) and Velacherry (70%) due to the use of fossil fuels (coal, wood) for cooking purposes and constant operation of thermal power plants [29].Chandigarh also showed an increment in SO2 concentration from 9.9 μg.m À3 (before lockdown) to 10.0 ($1%) and 11.4 μg.m À3 (15%) during the first and second phases of lockdown respectively.Similarly, no significant change in SO2 intensity is observed from satellite data over eastern Odisha, West-Bengal, and South-west industrial regions of Maharashtra and Gujrat [11].This is due to the presence of excessive coal mining and the constant operation of power plants as part of emergency service.Likewise, SO2 measurement over Singrauli, which is home to several coal-fired thermal power plants showed an increment of 12% during lockdown over normalcy (1st-24th Mar 2020).Table 5 describes the mean percentage change in SO2 concentration over India during lockdown.

Ozone (O3)
Surface O3 is a secondary pollutant formed majorly by photooxidation of CO, methane (CH4), or volatile organic carbons (VOCs) in presence of a sufficient amount of NOx.The major formation pathways are shown below During this process, NOx acts as a catalyst until it is removed by surface deposition or converted to other NOy compounds.Also, we can say NO2 helps in O3 formation while NO destroys O3.Being a powerful oxidizing agent, O3 controls the lifetime of many primary pollutants in the atmosphere.Nonetheless, O3 is a potent greenhouse gas that substantially contributes to global warming and affects human health, crop yield, natural ecosystem, and climate.
Table 6 summarises the mean percentage change in O3 concentration over different parts of India.During lockdown (24th Mar-20th May 2020), O3 concentration shows a sizeable daily reduction of 0.571 μg.m À3 with an average of 42% compared to pre-lockdown (17th Feb-23rd Mar 2020) over Kolkata [33].A reduction of $7% is also observed during lockdown over 2019 at Gadanki [36].Similarly, an 18% decrement in O3 concentration is observed over Maharashtra during lockdown with respect to pre-lockdown (1st -24th Mar 2020) [31].In contrast, an enhancement of 4-6% of O3 concentration is observed at 63 different stations of Delhi, Uttar Pradesh, and Haryana during lockdown [24].Some studies revealed concentration of O3 increased by 32% and 6% over Kolkata and Gujarat during the lockdown year as compared to the previous year [34,38].Likely, an increment of 3% and 2% was observed over Mumbai and Pune respectively during lockdown over the previous year [62].Similarly, O3 concentration gets amplified by 5.59%, 9.73%, 3.56% during March, April, and May 2020 compared to the same month of 2019.Two times enhancement in O3 concentration is also observed at Hyderabad during COVID-19 lockdown over normal days [63].An increment of 3% in O3 concentration was also found during the 22nd March-14th April 2020 over the past weeks (1st -21st Mar 2020) at Bhubaneswar [18].Similarly, an 12% increment in O3 concentration was reported during Mar-May 2020 over 2019 [37].O3 concentration enhancement is also observed at the commercial (Teynampat) and residential (Velachery) sites of Chennai by 48 and 5% respectively [29]    Change in Air quality index from January-April 2020 (source: [11]). 18 Biotechnology to Combat COVID-19

Air Quality Index (AQI)
AQI gives information about daily air quality and related public health risks.In general, there are six AQI categories as shown in Table 7.The Indian national predicted the daily air quality data taking 24 hour average of eight criteria pollutants such as (PM10, PM2.5, NO2, SO2, CO, O3, NH3 and Pb), however, for CO and O3 8-hour average.Exposure to an enhanced AQI leads to acute and chronic health issues, especially for older age people and children.

Summary and recommendation
This review clearly illustrates that the strict imposition of lockdown by the Indian Government not only helped to curb the pandemic spread but also significantly improved the air quality during quarantine days across numerous cities nested within different states of India.Thus, this unique scenario emerged as a natural experiment to get the baseline data of pollutants which will be useful for policymakers and stakeholders to develop new targets to tweak air quality standards and various mitigation processes to combat air pollution.However, it is important to keep in mind the atmospheric chemistry while developing mitigation strategies for the abatement of primary pollutants.As it is observed, a sudden reduction in NOx concentration during lockdown leads to an enhancement of O3 concentration over many parts of India.So, time demands a better understanding of pollutant sources and the development of extenuation policies.
We are suggesting a few science-driven measures which can lead to the co-existence of a sustainable economy and environment hand to hand a. Decentralization of metro cities and some urban sites.g.Development of indigenous processes to arrest the PM polluted generated through biomass and stable burning at various locations in the country.
h. Always encouraging and choosing sustainable ways of existence.

Table 4 .
Percentage change in NOx over India during lockdown.
b. Fossil fuel consumption should be replaced with solar/wind/hydro/CNG/ battery/nuclear energy.c.A focal shift from fossil fuel-based economy to bio-energy-based economy.d.Development of clean coal technology to generate electricity.e. Reducing vehicle density by encouraging the use of public transport, carpooling, and bicycle f.Development in geoengineering technique to improve road conditions and substitute metallic brakes with ceramic.

Table 1 .
[41]entage change in particulate matter concentration during the lockdown.Furthermore, a prominent diminution in PM10 concentration is observed just after 4 days of nationwide lockdown at Dwarka river basin harboring 239 stone mining and 982 stone crushing areas[41].PM10 loading was 100 μg.m À3 on 12th Mar 2020 (before lockdown) which dropped to 60 μg.m À3 ($40%) on 28th Mar 2020 (lockdown) with the temporary halt of industries.Similarly, studies over the industrial cities viz.Ankleshwar and Vapi of western India reported a drop in PM10 concentration by 19% and 51% respectively during the first phase of lockdown in comparison to the same period of 2019.Whereas, PM2.5 concentration shows a sharp decline from 72.23 to 43.60 μg.m À3 at Vapi during the first phase of lockdown over the same time of the previous year 19iotechnology to Combat COVID-19

Table 2 .
Declining percentage of total BC during lockdown.8 Biotechnology to Combat COVID-19

Table 5 .
Mean percentage change in the mean percentage change in SO2 concentration over India during lockdown.

Table 6 .
Percentage change in O3 concentration over India amid lockdown.lockdownrespectively.The enhancement in O3 is due to insufficient NO concentration as discussed in the previous section which resulted in the accumulation of O3 in the troposphere.

Table 7 .
Air quality index and possible health impact.