| Khurram Shehzad et al. (2020) | Mumbai, New Delhi | NO2 | ~ 10 μg/m3 (Mumbai)12 – 25 μg/m3 (New Delhi) | Sentinel-SP satellite images ESA and NASA | January 1–March 24 2020 (before lockdown)March 25–April 20 2020 (during lockdown) | Reduction in NO2 due to decrease in electric consumption |
| Yichen Wang et al. (2020) | 366 urban areas in China's mainland | AQI, PM2.5, PM10, NO2, SO2, CO, O3 | Average AQI for all stations reduced from 89.6 to 71.6 | China's National Environmental Monitoring Centre. | January 1–January 23 2020 (before control period)January 24–February 9 2020 (control period) |
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| Pengfei et al. (2020) | 10 major cities in China | PM2.5 and its components SO4, NO3, NH4, and SOA |
| China's National Environmental Monitoring CentreNational Climate Data Centre | January 1–February 12, 2020 (High and low pollution days)Lockdown in Wuhan from January 23 and in Hubei province from January 24, 2020 |
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| H. Zheng et al. (2020) | Wuhan | PM2.5 and other air pollutants, PM2.5 chemical species, Meteorological parameters | PM2.5 reduced by 27.0 μg/m3 (Compared to previous year, same time period) | In situ observations |
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January 23–February 22 2020 (During lockdown and comparison with previous year, same period)
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Residential/commercial with no industrial emissions at surroundings
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| Zhipeng Pei et al. (2020) | Beijing Wuhan Guangzhou | NO2, SO2, O3, HCHO, PM2.5 |
| Lockdown period (same period as previous year) | NO2 and HCHO by remote sensing satellite - Other indices – In-situ measurements | Different responses for different air pollutants in different places. |
| Hao Xu et al. (2020) | 33 locations in China | AQI | -- | January 29–February 15 2020 (Lockdown period) |
| AQI effect on confirmed cases in temperature range 10–20 °C is stronger and spread of COVID enhanced under low relative humidity. |
| Hui Chen et al. (2020) | Shanghai, China | NR-PM2.5, SO2, CO, Sulphate and OOA |
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Nitrate decreased by 60%, SO2 by 15%, CO by 22%.
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Sulphate and OOA barely decreased
| January 8–January 23 (before lockdown) January 24–February 8 (lockdown) |
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A Time of Flight- Aerosol Chemical Speciation Monitor (Tof- ACSM) - NR PM2.5
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PM2.5, SO2, NOx, CO and O3-Air quality station
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| Feng Liu et al. (2021) | Globally, 597 major cities from 76 countries | PM2.5, PM10, NO2, SO2, CO, O3 | NO2 falls more precipitously, followed by PM10, SO2, PM2.5, and CO, but O3 increases relative to pre lockdown period. | January 1–July 5 2020 (lockdown period) | Air quality open data platform | Improvement in air quality due to lockdown |
| Rui Bao et al. (2020) | 44 cities in northern China | AQI, PM2.5, PM10, NO2, SO2, CO | The concentrations of SO2, PM2.5, PM10, NO2 and CO decreased by 6.76%, 5.93%, 13.66%, 24.67% and 4.58% respectively. | January 1–March 21 2020 |
| Travel restriction measures significantly reduced air pollution. |
| Yongjian Zhu et al. (2020) | 120 cities in China | PM2.5, PM10, NO2, SO2, CO, O3, Covid-19 confirmed cases | Increase in air pollutants except SO2 was associated with increase in daily counts of COVID-19 confirmed cases. | January 23–February 29 2020 |
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Local health commissions on official websites
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Air pollution data from online platform
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Meteorological data from National Meteorological Information centre
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Positive association of PM2.5, PM10, CO, NO2 and O3 with COVID-19 confirmed cases
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SO2 was negatively associated with confirmed cases
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| Farrukh Shahzad et al. (2020) | 10 most affected provinces in China | Temperature COVID-19 confirmed cases | Positive relationship with temperature and COVID-19 | January 22–March 31 2020 |
| Positive, negative, and mixed trends |
| Dongyang Nie et al.(2021) | 31 provincial cities in China | PM2.5, PM10, NO2, SO2, CO, O3 | All air pollutants except O3 reduced | January 1–May 2, 2020 | Website of National Environmental Monitoring Centre of China | Increase in O3 is of primary concern |
| Shelby Zangari et al. (2020) | New York City 15 central monitoring stations | PM2.5 and NO2 | 36% and 51% reduction in PM2.5 and NO2 after shutdown | January–May 2015 to 2020 | DEC's air monitoring website (New York state Department of Environmental Conservation, 2020) | Short-term decline in pollution levels in 2020 |
| Manu Sasidharan et al. (2020) | London borough | PM2.5 and NO2 | Correlation of PM2.5 and NO2 with Covid cases | As of March 31 2020 | Public Health England 2020National Health services 2020Air pollution data-King's college London 2020 | Correlation between increment in PM2.5 and NO2 and increase in risk of COVID-19 transmission |
| Jose M. Baldasano (2020) | Cities of Barcelona and Madrid, Spain | NO2 | Reduction compared to previous years | March 2018–2020 | Air Quality Monitoring NetworksClimatological reports from Spanish State Meteorological Agency | Under lockdown, reduction of NO2 in Barcelona and Madrid were 50% and 62%, respectively. |
| Maria Cristina Collivignarelli et al. (2020) | Metropolitan city of Milan, Italy | PM2.5, PM10, NOx, SO2, NO2, CO, O3, BC, benzene |
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PM2.5, PM10, NOx, BC, Benzene and CO reduced and O3 increased.
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SO2 dropped only in Milan not in adjacent areas.
| 9 to 22 March 2020 (partial lockdown) 23 March to 5 April 2020 (total lockdown) |
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Air quality data from local environmental protection agency
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Data for estimation of sources of PM2.5, PM10, CO, NO2, SO2, NOx from regional inventory of emissions from Lombardy
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Meteorological data by local environmental protection agency
| Increase in O3 due to presence of benzene even though VOCs emissions from vehicular traffic and industrial combustion decreased. |
| Antonio Frontera et al. (2020) | 21 Italian regions | PM2.5 and NO2 | High correlation between PM2.5 and total number of cases, ICU admissions, deaths and hospitalizations | February 2020 |
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Air pollution data from Air-Matters app
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Patient data- Italian civil protection website
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Population data – Italian Statistical agency
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| Yaron Ogen (2020) | European countries – Italy, Spain, France and Germany (66 administrative regions) | NO2 | Long-term exposure to this pollutant and Covid-19 fatality | January to February 2020 |
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Sentinel -5 precursor space borne satellite
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NCEP/NCAR reanalysis
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Information about fatalities from Ministry of Health (Italy)
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Ministry of Health, Social Services and Equality (Spain)
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The National Agency of Public Health (France)
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Robert-Koch-Institute and the State Health Offices (Germany)
| Chronic exposure to NO2 could be an important contributor to high Covid fatality rate. |
| Matthew D. Adams (2020) | Ontario, Canada 32 stations | PM2.5, NO2, NOX, O3 | NO2 and NOX reduced | January 3–February 6, 2020 (five weeks), compared with previous five years same period. | Ontario Ministry of the Environment, Conservation and Park's air pollution data portal | Ozone reduced and PM2.5 has not varied. |
| Bruno Siciliano et al. (2020) | Rio de Janeiro, Brazil | NO2, NO, O3, NHMC | Reduction in NOx | March 1 to April 16 2020 | Automatic monitoring stations of the Municipal Department of the Environment | Increase in O3 due to increase in NMHC/NOX ratios. |
| Kasturi Devi Kanniah et al. (2020) | Southeast Asian countries and Malaysia | AOD, PM2.5, PM10, SO2, NO2, and CO | Notable reduction in pollutants in urban areas | March 18–30 April for three years 2018, 2019, 2020 |
| A large decrease in NO2 levels occurred in industrial sites and urban |
| Ismail Anil and Omar Alagha (2020) | Eastern Province, Saudi Arabia | PM10, SO2, NO2, CO, O3 | Markable reduction in NO2 by 12–86%. | September 15, 2019–March 22, 2020 (pre-lockdown) March 23–June 20, 2020 (during lockdown) June 21–July 18, 2020 (post lockdown) |
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| Akhtar Shareef and Durdana Rais Hashmi (2020) | Karachi, Pakistan | PM10, SO2, NO2, CO | PM10 reduced to 50% during lockdown SO2, NO2, CO reduced about 60%–70% | Feb25March 23, 2020 (before lockdown)March 24April 20, 2020 (during partial lockdown)February, March, April of five previous years | Analysers | AQI during lockdown is either moderate or good |
| M. Bigdeli et al. (2020) | 31 provinces in Iran | CO, SO2, NO2, O3 | Negative and positive correlations with density of confirmed COVID cases | Feb 19 March 22, 2020 | Sentinel 5P | SO2 was correlated more negatively |
| Aiymgul Kerimray et al. (2020) | Almaty, Kazakhstan | PM2.5, SO2, NO2, CO, O3 Benzene, Toluene, Ethylbenzene-xylene |
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CO and NO2 reduced by 49% and 35% during lockdown.
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O3 increased by 15% and SO2 by 7%.
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PM2.5 reduced by 21% compared to 2018–2019 same period.
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Benzene and toluene were 3 and 2 times higher and ethylbenzene and 0-xylene were 4 and 2.7 times compared to 20152019 same period
| March 19–April 14, 2020 LockdownCompared with 2015–2019 same period |
| Trafic-free conditions did not improve air quality |
| Li Li et al. (2020) | Yangtze River Delta Region Shanghai, Hangzhou, Nanjing, Hefei | PM2.5, SO2, NOX, VOCs | SO2, NOX, PM2.5 and VOC's reduced by 26%,47%,46% and 57% during the most stringent level response period | January–March 2017–2020 |
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Pollutants: Air Monitoring Data Centre of Ministry of Ecology and Environment of the People's Republic of China
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Meteorological data: NOAA, National climate data centre archive and National Data centre of Chinese Meteorology
| Daily PM2.5 during lockdown still ranged between 15 and 79 μg/m3, and O3 rebounded by 20.5% |
| Pierre Sicard et al. (2020) | Europe (Nice, France, Rome and Turin, Italy, Valencia, Spain) Wuhan, China | NOX, PM, O3 | Reduction in NOX in all cities ~56% PM in Wuhan ~42% and in Europe ~8% Increase in O3 by 17% in Europe and 36% in Wuhan | Before (1 January 2020 until start date of lockdown) and after (from start date to 8 April in Wuhan and until 18 April in European cities) lockdown | Local and regional agencies in charge of air monitoring stations | Reduction in NOX and PM and increase in O3 |
| Jesse D. Berman, Keita Ebisu (2020) | Continental United States (122 counties) | PM2.5 and NO2 | NO2 declined 25.5% and PM2.5 declined | January 8–Mach 12 (pre-lockdown)March 13April 21, 2020 (lockdown) and 2017–2019 same period |
| PM2.5 declined in urban counties |
| Anas Otmani et al. (2020) | Sale city (Morocco) | PM10, SO2, NO2 | Decreased by 75%, 49%, 96% within a few days of lockdown | March 11–April 2, 2020 After and during lockdown | In-situ | Most significant variation in NO2 |
| Akshansha Chauhan, Ramesh P. Singh (2020) | Rome, Shanghai, Mumbai, Dubai, Delhi, Beijing, Los Angeles, New York, Zaragoza | PM2.5 | 32% reduction in New York, 4% in Los Angeles, 58% in Zaragoza, 24% in Rome, 11% in Dubai, 35% and 14% in Delhi and Mumbai, 50% in both Beijing and Shanghai. | Dec 2019–Mar 2020 compared with years 2017–2019 |
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Purple Air
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Air Now
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US EPA
| In the major cities around the world suffering severely with Covid19, a decline in PM2.5 is observed. |
| S. Selvam et al. (2020) | Zone 1: Surat, Ankleshwar, Vadodara Zone 2: Ahmedabad, Gandhi Nagar, Zone 3: Jamnagar, Rajkot Zone 4: Bhuj and Palanpur. | PM2.5, PM10, SO2, NO2, CO, O3, AQI | Major improvements in Zones 2, 3; moderate improvements in Zones 1, 4.Zone 1: Dominant transportation and fabric sectorsZone 2: Pharmaceutical, beverage, textile automobile, steel recycling, auto parts and petroleum/petrochemicalsZone 3: Large cargo ships, ferries and cruisesZone 4: Lowest air pollution and less populated cities. | CPCB | 1 Jan 2020–20 Apr 2020Pre-lockdown: 01 Jan 2020–23 Mar 2020 and lockdown period: 24 Mar 2020–20 Apr 2020 | Overall improvement in AQI of 58% compared to previous year and increase in O3 by 16% to 58% due to less NO emissions. |
| Indrajit Chowdhuri et al. (2020) | Kolkata | PM10, NO2, SO2, O3 | Average of PM10, NO2, SO2, O3 reduced by 40% to 68%. | 24 February 2020 to 23 May 2020 (before lockdown)24 March 2020 to 20 May 2020 (during lockdown) |
| Overall reduction of surface pollution in thunderstorm environment. |
| Susanta Mahato et al. (2020) | Delhi | PM2.5, PM10, SO2, NO2, CO, O3, AQI | PM10 and PM2.5 have witnessed maximum reduction >50%. | 24 Mar 2020 to 14 Apr 2020 (Same period 2019)03 March 2020 to 21 March 2020 |
| About 54%, 49%, 43% 37% & 31% reduction in NAQI observed in Central, eastern, southern, western and northern parts of megacity Delhi. |
| Sneha Lokhandwala and Pratibha Gautam (2020) | Ghaziabad | PM2.5, PM10, NO2, SO2 | Major reduction in PM2.5 – 85.1% | 14 April 2020 Compared with 14 January 2020 | CPCB | Quality of air has started to improve. |
| Abhishek Saxena and Shani Raj (2021) | Agra, Noida, Gurugram, Delhi | PM2.5, PM10, NO2, CO, O3 | PM2.5, PM10, CO reduced and O3 increased. | Before lockdown (02 March 2020–21 March 2020)During lockdown (24 March 2020to 14 April 2020) |
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