Researchers studying California, USA found varying levels of air pollution from January to June 2020 may have been the result of COVID-19 lockdowns. 
- NO2 levels dropped 38%, CO levels dropped 49%, and PM2.5 levels dropped 31% 
- PM10 levels increased 21% and O3 levels increased 14%
- After lockdowns were lifted, PM2.5 levels increased 17%, PM10 levels increased 114%, and O3 levels increased 4% 
- Monitoring by NASA's Aura satellite showed overall NO2 levels decreased 33% during lockdown, with a lower mean value than in the previous four years 

Kenyon College

Humans contribute to air pollution by releasing harmful substances into the earth's atmosphere. Over many decades, harmful substances have accumulated to produce areas with severely polluted air. Good air: 
- Has low NO2, CO, SO2 and BC (black carbon) levels. BC may also be represented by BCff (fossil fuels) or BCbb (biomass burning) 
- Has low THC (total hydrocarbons) and low NMHC (non-methane hydrocarbons)
- Has no formaldehyde 
- Has low PM10 and PM2.5 levels, which measure fine particulates in the air 
- Has moderate O3 (ozone) levels, which can be harmful at too high or too low of a concentration 
- Has low aerosol optical depth (AOD) levels (<1), which measure electromagnetic radiation in the air as caused by pollutants 
- Has a low AQI value (0-100) which indicates good to moderate air quality 
- Has a low BTEX value (benzene, toluene, ethylbenzene, xylene), as these compounds are hazardous

Air Pollution (Post-COVID-19)

Liu, Q., T. Harris, J., S. Chiu, L., Sun, D., R. Houser, P., Yu, M., Q. Duffy. D., M. Little, M., & Yang, C. (2020). Spatiotemporal Impacts of COVID-19 on Air Pollution in California, USA. Science of the Total Environment, 750. https://doi.org/10.1016/j.scitotenv.2020.141592 

Spatiotemporal Impacts of COVID-19 on Air Pollution in California, USA

Studies have found that in the first half of 2020, i.e during the onset of the coronavirus, global CO2 emissions declined by 8.8% as compared with 2019. There was a total decrease of 1551 million tonnes of CO2. This is due to the shut down of public life and parts of the economy worldwide. This gives us hope and new ideas of how to decrease the CO2 emissions and help slow down global warming. 

Global CO2 Levels 

Air Pollution: California (USA) (Post-COVID-19)

Researchers studying the Dwarka River Basin in India found air quality may have improved due to decreased quarrying as a result of the COVID-19 outbreak. 
- PM10 levels decreased from over 200 μg/m3 in some locations to as low as 49.58 μg/m3 after only 4 days 

Air Pollution: Dwarka River Basin (India) (Post-COVID-19)

Researchers studying Sigrauli, India found air quality varied significantly during lock downs due to COVID-19. This variety may be attributed to the ongoing use of coal-based power plants, even throughout lock down. 
- NO2 levels decreased ~12%
- PM10 levels increased ~58%, PM2.5 levels increased ~15%, , SO2 levels increased ~11%, and O3 levels increased ~35%

Air Pollution: Sigrauli (India) (Post-COVID-19)

Air quality in Wuhan, China may have improved following lock downs during the COVID-19 outbreak. 
- NASA satellite images showed NO2 levels decreased up to 30% in comparison to values in January 2020, February 2020, and from previous years 
- NO2 decreased by 22.8 μg/m3 and PM2.5 decreased by 1.4 μg/m3 
- One research group found AQI values for various pollutants may have varied from January 23, 2020 - April 7, 2020 due to COVID-19. 
- NO2 AQI decreased 58.3% compared to the 2019 average (statistically significant) 
- SO2 AQI decreased 11.2% compared to the 2019 average 
- CO AQI decreased 13.8% compared to the 2019 average (statistically significant)
- O3 AQI increased 54.2% compared to the 2019 average (statistically significant)
- PM2.5 AQI decreased 23.2% compared to the 2019 average (statistically significant) 
- PM10 AQI decreased 30.4% compared to the 2019 average (statistically significant) 

Air Pollution: Wuhan (China) (Post-COVID-19)

Air quality in Spain may have improved following lock downs during the COVID-19 outbreak. 
- In Spain, notably in Madrid, Barcelona, and Seville, ESA satellite images showed NO2 levels decreased 20-30% in March 2020 compared to values from previous years 

Air Pollution: Spain (Post-COVID-19)

Researchers studying Delhi, India found air quality may have improved during lock downs due to COVID-19. 
- PM10 levels decreased ~55%, PM2.5 levels decreased ~49%, NO2 levels decreased ~60%, and SO2 levels decreased ~19% 
- O3 levels increased ~37% 
- A separate study found AQI values for various pollutants may have varied from March 24, 2020 - May 31, 2020 due to COVID-19. 
- NO2 AQI decreased 60% compared to the 2019 average (statistically significant)
- SO2 AQI decreased 31.7% compared to the 2019 average (statistically significant) 
- CO AQI decreased 34.0% compared to the 2019 average (statistically significant)
- O3 AQI increased 36.3% compared to the 2019 average (statistically significant) 
- PM2.5 AQI decreased 27.6% compared to the 2019 average (statistically significant)
- PM10 AQI decreased 45.9% compared to the 2019 average (statistically significant) 

Air Pollution: Delhi (India) (Post-COVID-19)

Researchers studying Barcelona, Spain found air quality improved during the first two weeks of lock down due to COVID-19. Researchers believe these numbers can be attributed to reduced human activity during the lock down, though they caution other factors may be responsible. 
- NO2 levels decreased ~47% and BC levels decreased ~45%
- PM10 decreased ~27%
- O3 levels increased ~28% 


Air Pollution: Barcelona (Spain) (Post-COVID-19)

Researchers studying Mumbai, India found air quality may have improved during lock downs due to COVID-19.
- PM10 levels decreased ~44%, PM2.5 levels decreased ~37%, NO2 levels decreased ~78%, and SO2 levels decreased ~39%
- O3 levels increased ~20%

Air Pollution: Mumbai (India) (Post-COVID-19)

Researchers studying Malaysia during the 2020 Malaysia Movement Control Order (MCO) found air quality may have improved as a result of their COVID-19 lock down. Though other factors may have contributed, researchers believe the MCO was the main factor in reduced air pollution. 
- PM2.5 levels decreased ~28.3% in "red zone" areas where COVID-19 infections were highest 
- Overall, PM2.5 levels decreased ~58.4% 
- Decreases in PM2.5 levels as compared to levels pre-MCO were seen at 50% of all air quality monitoring stations across the country 

Air Pollution: Malaysia (Post-COVID-19)

Researchers studying South Korea found air quality may have improved from March-April 2020 due to COVID-19 lock downs. 
- Daily average PM2.5 levels were recorded at 20.39 μg/m3 (45.45% decrease)
- Daily average PM10 levels were recorded at 39.16 μg/m3  (35.56% decrease)
- Daily average NO2 levels were recorded at 16.22 ppb (20.41% decrease)
- Daily average CO levels were recorded at 0.387 ppm (17.33% decrease) 

Air Pollution: South Korea (Post-COVID-19)

Researchers studying Eastern China found air quality may have both improved and worsened in certain respects during COVID-19 lock downs. 
- While there was a major decrease in NO2, one study determined O3 and PM2.5 levels increased unexpectedly. Their results showed decreasing NO2 levels were directly responsible for increased O3, which in turn directly contributed to formation of secondary PM pollutants. 

Air Pollution: Eastern China (Post-COVID-19)

Air quality in France may have improved following lock downs during the COVID-19 outbreak.
- Across northern France, ESA satellite images showed NO2 levels decreased 20-30% in March 2020 compared to values from previous years 

Air Pollution: France (Post-COVID-19)

Air quality in Italy may have improved following lock downs during the COVID-19 outbreak. 
- Across northern Italy, ESA satellite images showed NO2 levels decreased 20-30% in March 2020 compared to values from previous years 

Air Pollution: Italy (Post-COVID-19)

Researchers studying East Asia found air quality may have improved in February 2020 when compared to February 2019 due to COVID-19 lock downs. 
- Monthly AOD levels decreased 62% in Wuhan and 31% in the Beijing-Tianjin-Hebei area. 
- NO2 levels decreased 54% in Wuhan; 83% in the Beijing-Tianjin-Hebei area; 33% in the Soul Metropolitan Area; and 19% in the Tokyo Metropolitan Area
- Formaldehyde levels decreased 10% in Wuhan; 13% in the Beijing-Tianjin-Hebei area, 22% in the Soul Metropolitan Area; and 8% in the Tokyo Metropolitan Area 
- SO2 levels decreased ~71% in Wuhan. All other areas of study showed increased SO2 levels 
- CO levels decreased 4% in Wuhan; 8% in the Beijing-Tianjin-Hebei area; 6% in the Soul Metropolitan Area; and 1% in the Tokyo Metropolitan Area 
- O3 levels increased ~20% in East China, ~17% in the Beijing-Tianjin-Hebei area, and ~50% in Wuhan 

Air Pollution: East Asia (Post-COVID-19)

Air quality in the United States may have improved following lock downs during the COVID-19 outbreak. 
- Across several northeastern states, NASA satellite images showed NO2 levels decreased as much as 30% in March 2020 compared to values from previous years 

Air Pollution: The United States of America (Post-COVID-19)

Researchers studying Rio de Janeiro, Brazil found air quality may have improved as a result of a six week partial lock down due to COVID-19. 
Despite the results, researchers noted several factors contributed to air pollution, and some values fluctuated greatly. 
- CO levels dropped the most of any pollutant 
- NO2 levels decreased slightly 
- PM10 remained roughly the same 
- O3 levels increased 


Air Pollution: Rio de Janeiro (Brazil) (Post-COVID-19)

Researchers studying China found air quality may have improved following strict lock downs during the COVID-19 outbreak. 
- Satellite imaging showed NO2 levels above China decreased in early February as compared to January, but increased in late February as industrial activity resumed
- Numerous cities saw improvements in general air quality and 120 cities achieved standard air quality levels 
- On average, PM2.5 levels decreased by 14.8%, PM10 levels decreased by 20.5%, NO2 levels decreased by 25% (12.9 μg/m3), SO2 levels decreased by 21.4%, CO levels decreased by 6.2%, and O3 levels remained consistent 
- ESA satellite images showed NO2 levels decreased 20-30% from February 10, 2020 to February 25, 2020 
-  The US NASA Aura and European Copernicus Sentinel-5 Precursor satellites found NO2 levels decreased ~48% from late January to early February 2020, a larger decrease than could be attributed to celebration of Lunar New Year alone 

Air Pollution: China (Post-COVID-19)

Researchers studying Ahmedabad, India found air quality may have improved from March to May 2020 due to COVID-19 lock downs. 
- PM2.5 values decreased to levels below 100 μg/m3 
- PM10 values decreased to levels below 50 μg/m3
- NO2 values decreased to levels below 50 μg/m3, with concentrations the lowest they'd been in the previous three years 

Air Pollution: Ahmedabad (India) (Post-COVID-19)

Researchers studying Baghdad, Iraq found air quality may have varied from March 16 to July 24, 2020 due to COVID-19 lockdowns. 
- Average NO2 levels dropped as low as 73 μg/m3 
- Daily O3 levels rose over 100 μg/m3 in mid-July
- Average PM2.5 levels fluctuated, dropping as low as 37 μg/m3, with daily averages as high as 117 μg/m3 
- Average PM10 levels fluctuated, dropping as low as 101 μg/m3 and as high as 186 μg/m3 
- AQI values were recorded at 105, 151, 161, and 167 as the lockdown progressed 
- S-5P/TROPOMI satellite data found NO2 levels across Iraq decreased ~35-40% 

Air Pollution: Baghdad (Iraq) (Post-COVID-19)

Researchers studying Almaty, Kazakhstan found air quality may have changed due to COVID-19 lockdowns from March 19 - April 14, 2020. 
- Average PM2.5 levels decreased by 18% compared to 2018 and 23% compared to 2019 (March 19 - April 14) 
- PM2.5 levels varied by monitoring station, with reductions ranging from 6-34%
- 18/27 days of lockdown exceeded the WHO daily limit for PM2.5 (a 25% reduction compared to 2018 and 2019) 
- BTEX: Benzene levels were 3x higher, toluene levels were 2x higher, ethylbenzene levels were 4x lower, and o-xylene levels were 2.7x lower (compared to 2015-2019) 
- BTEX levels varied by elevation, with benzene levels increasing as much as 274% and toluene levels increasing as much as 241% depending on location
- NO2 levels decreased 35%, SO2 levels increased 7%, CO levels decreased 49%, O3 levels increased 15% (compared to pre-lockdown period) 

Air Pollution: Almaty (Kazakhstan) (Post-COVID-19)

Researchers studying Dhaka City, Bangladesh found air pollution may have improved following COVID-19 lockdowns. 
- A full lockdown occurred between 26 March - 10 April, 2020. PM2.5 levels decreased 26.0%, 03 levels decreased 9.7%, CO levels decreased 8.8%, SO2 levels decreased 17.5%, and NO2 levels decreased 20.4%. The average AQI values was ~150. 
- A partial lockdown occurred between 11 April - 15 May, 2020. Compared to pre-lockdown values: PM2.5 levels decreased 38.2%, O3 levels decreased 23.6%, CO levels decreased 16.2%, SO2 levels decreased 37.1%, and NO2 levels decreased 44.2%. The average AQI value was ~120. 

Air Pollution: Dhaka City (Bangladesh) (Post-COVID-19) 

Researchers studying Milan, Italy, found varying levels of air pollution throughout areas of differing population density, likely due to COVID-19 lockdowns. Partial lockdown (PL) began on March 8, 2020 and total lockdown (TL) began on March 23, 2020. 
- In several municipalities, including Milan, PM10 levels decreased ~32-40% during PL and decreased a further ~13-19% during TL 
- In Milan and another municipality, PM2.5 levels decreased ~37% - 44% during PL and a further 47.1–47.4% during TL 
- BC levels decreased 57.5% in PL and 71% in TL 
- In Milan, benzene levels decreased 49.6% 
- CO levels varied by municipality; in Milan, CO levels decreased 57.6%
- In Milan, SO2 levels decreased 19.2% during PL and a further 6.8% during TL 
- Average NO2 levels were 31.9 μg/m3 in PL and 22.1 μg/m3 in TL 
- Average O3 levels were 43.7 μg/m3 in PL and 59.0 μg/m3 in TL 

Air Pollution: Milan (Italy) (Post-COVID-19) 

Researchers studying Salé City, Morocco found air pollution may have decreased as a result of COVID-19 lockdowns. Lockdown occurred from March 21 - April 2, 2020. 
- Average PM10 levels were 28.3 μg/m3 (75% decrease)
- Average NO2 levels were 0.2 μg/m3 (96% decrease) 
- Average SO2 levels were 6.6 μg/m3 (49% decrease) 

Air Pollution: Salé City (Morocco) (Post-COVID-19)

Researchers studying Ontario, Canada found air pollution may have varied during a COVID-19 state of emergency. The state of emergency was declared on March 17, 2020. Data was compared to air quality statistics from the previous five years. 
- Average PM2.5 levels were 5.6 μg/m3 (no change) 
- Average NO2 levels were 5 ppb (decrease) 
- Average nitrogen oxide levels were 7ppb (decrease)
- Average O3 levels were 33 ppb (decrease)

Air Pollution: Ontario (Canada) (Post-COVID-19) 

Researchers studying New York City, USA found air quality was likely not affected by COVID-19. 
- In the first 17 weeks of 2020, PM2.5 levels dropped 36%. Statistical analysis found this decrease to be insignificant 
- In the first 17 weeks of 2020, NO2 levels dropped 51%. Statistical analysis found this decrease to be insignificant 
- Researchers concluded these decreases are in accordance with general air quality trends and can not be attributed to COVID-19
- However, a separate group of researchers found AQI values for various pollutants in Manhattan may have varied from March 22, 2020 - June 7, 2020 due to COVID-19. 
- NO2 AQI decreased 27.5% compared to the 2019 average (statistically significant) 
- CO AQI decreased 20.7% compared to the 2019 average (statistically significant) 
- O3 AQI decreased 6.3% compared to the 2019 average 
- PM2.5 AQI decreased 27.6% compared to the 2019 average (statistically significant)

Air Pollution: New York City (USA) (Post-COVID-19) 

Researchers studying Shenyang, China found air pollution may have varied as a result of COVID-19 lockdowns. Lockdown occurred January 26 - March 23, 2020. The post-lockdown period occurred March 24 - June 30, 2020. 
- Researchers concluded lockdown had no affect on O3 levels, which continued to worsen through 2020. O3 levels were 1.4% higher than 2019 and 1.2% lower than 2018
- During lockdown, average AQI values decreased 37.1%, NO2 levels decreased 34.4%, PM2.5 levels decreased 46.4%, PM10 levels decreased 39.5%, SO2 levels decreased 45.6%, and CO levels decreased 43.3% 
- In post-lockdown, O3 levels increased 172.2% compared to pre-lockdown 
- Ozone adopted a positive correlation with other pollutants during lockdown 

Air Pollution: Shenyang (China) (Post-COVID-19)

Researchers studying Sydney, Australia found AQI values for various pollutants may have varied from March 23, 2020 - April 27, 2020 due to COVID-19. 
- NO2 AQI decreased 11.1% compared to the 2019 average 
- CO AQI decreased 24% compared to the 2019 average (statistically significant) 
- O3 AQI increased 5.6% compared to the 2019 average 
- PM2.5 AQI decreased 29.2% compared to the 2019 average (statistically significant) 
- PM10 AQI decreased 17% compared to the 2019 average (statistically significant) 

Air Pollution: Sydney (Australia) (Post-COVID-19)

Researchers studying Johannesburg, South Africa found AQI values for various pollutants may have varied from March 26, 2020 - April 30, 2020 due to COVID-19. 
- NO2 AQI decreased 23.0% compared to the 2019 average (statistically significant)
- SO2 AQI decreased 13.9% compared to the 2019 average 
- CO AQI increased 5.0% compared to the 2019 average 
- O3 AQI increased 9.0% compared to the 2019 average 
- PM2.5 AQI decreased 31.3% compared to the 2019 average (statistically significant)
- PM10 AQI decreased 33.1% compared to the 2019 average (statistically significant)

Air Pollution: Johannesburg (South Africa) (Post-COVID-19) 

Researchers studying Lima, Peru found AQI values for various pollutants may have varied from March 29, 2020 - June 30, 2020 due to COVID-19. 
- NO2 AQI decreased 50.5% compared to the 2019 average (statistically significant)
- SO2 AQI decreased 35.2% compared to the 2019 average (statistically significant)
- CO AQI decreased 61.8% compared to the 2019 average (statistically significant)
- O3 AQI decreased 42.9% compared to the 2019 average (statistically significant)
- PM2.5 AQI decreased 19.4% compared to the 2019 average (statistically significant)
- PM10 AQI decreased 31.7% compared to the 2019 average (statistically significant)

Air Pollution: Lima (Peru) (Post-COVID-19)

Researchers studying Sao Paulo, Brazil found AQI values for various pollutants may have varied from March 24, 2020 - May 10, 2020 due to COVID-19. 
- NO2 AQI decreased 37.8% compared to the 2019 average (statistically significant)
- SO2 AQI decreased 23.2% compared to the 2019 average (statistically significant)
- CO AQI decreased 32.7% compared to the 2019 average (statistically significant) 
- O3 AQI increased 24.9% compared to the 2019 average (statistically significant) 
- PM2.5 AQI decreased 18.3% compared to the 2019 average (statistically significant)
- PM10 decreased 12.9% compared to the 2019 average 

Air Pollution: Sao Paulo (Brazil) (Post-COVID-19) 

Researchers studying Mexico City, Mexico found AQI values for various pollutants may have varied from March 23, 2020 - May 30, 2020 due to COVID-19. 
- NO2 AQI decreased 24.9% compared to the 2019 average (statistically significant) 
- SO2 AQI increased 2.2% compared to the 2019 average 
- CO AQI increased 4.0% compared to the 2019 average 
- O3 AQI decreased 0.1% compared to the 2019 average 
- PM2.5 AQI decreased 8.3% compared to the 2019 average (statistically significant) 
- PM10 AQI decreased 15.5% compared to the 2019 average (statistically significant) 

Air Pollution: Mexico City (Mexico) (Post-COVID-19)

Researchers studying Los Angeles, USA found AQI values for various pollutants may have varied from March 19, 2020 - May 7, 2020 due to COVID-19. 
- NO2 AQI decreased 24.2% compared to the 2019 average (statistically significant) 
- CO AQI increased 7.2% compared to the 2019 average 
- O3 AQI decreased 3.5% compared to the 2019 average 
- PM2.5 AQI decreased 13.7% compared to the 2019 average 
- PM10 AQI increased 2.0% compared to the 2019 average 

Air Pollution: Los Angeles (USA) (Post-COVID-19) 

Researchers studying London, UK found AQI values for various pollutants may have varied from March 23, 2020 - May 10, 2020 due to COVID-19. 
- NO2 AQI decreased 37.8% compared to the 2019 average (statistically significant) 
- SO2 AQI increased 0.6% compared to the 2019 average 
- CO AQI decreased 53.5% compared to the 2019 average (statistically significant) 
- O3 AQI increased 48.0% compared to the 2019 average (statistically significant) 
- PM2.5 AQI decreased 14% compared to the 2019 average 
- PM10 AQI decreased 10% compared to the 2019 average 

Air Pollution: London (UK) (Post-COVID-19) 

Researchers studying Madrid, Spain found AQI values for various pollutants may have varied from March 14, 2020 - May 4, 2020 due to COVID-19. 
- NO2 AQI decreased 51.6% compared to the 2019 average (statistically significant)
- SO2 AQI decreased 54.8% compared to the 2019 average (statistically significant) 
- O3 AQI decreased 9.9% compared to the 2019 average 
- PM2.5 AQI decreased 1.6% compared to the 2019 average 
- PM10 AQI decreased 19.8% compared to the 2019 average 

Air Pollution: Madrid (Spain) (Post-COVID-19)

Researchers studying Paris, France found AQI values for various pollutants may have varied from March 17, 2020 - May 10, 2020 due to COVID-19. 
- NO2 AQI decreased 46.4% compared to the 2019 average (statistically significant) 
- SO2 AQI increased 13.5% compared to the 2019 average 
- O3 AQI increased 26.8% compared to the 2019 average (statistically significant) 
- PM2.5 AQI decreased 13% compared to the 2019 average 
- PM10 AQI decreased 22.3% compared to the 2019 average 
- A separate study found that NO2 levels decreased 24% and PM2.5 levels increased 16% from March 17, 2020 - June 1, 2020 as compared to January 1, 2020 - March 17, 2020. Average NO2 and PM2.5 levels during these five months were 12 µg/m3 and 2 µg/m3 respectively 

Air Pollution: Paris (France) (Post-COVID-19) 

Researchers studying Rome, Italy found AQI values for various pollutants may have varied from March 9, 2020 - May 3, 2020 due to COVID-19. 
- NO2 AQI decreased 36.4% compared to the 2019 average (statistically significant) 
- SO2 AQI decreased 3.8% compared to the 2019 average 
- O3 AQI increased 2.9% compared to the 2019 average 
- PM2.5 AQI increased 8.3% compared to the 2019 average 
- PM10 AQI decreased 1.8% compared to the 2019 average 

Air Pollution: Rome (Italy) (Post-COVID-19)

Researchers studying Berlin, Germany found AQI values for various pollutants may have varied from March 17, 2020 - April 19, 2020 due to COVID-19. 
- NO2 AQI decreased 17.6% compared to the 2019 average 
- SO2 AQI increased 29.5% compared to the 2019 average (statistically significant) 
- O3 AQI increased 3.9% compared to the 2019 average 
- PM2.5 AQI decreased 22.5% compared to the 2019 average (statistically significant) 
- PM10 AQI decreased 10.3% compared to the 2019 average 

Air Pollution: Berlin (Germany) (Post-COVID-19)

Researchers studying Moscow, Russia found AQI values for various pollutants may have varied from April 3, 2020 - June 8, 2020 due to COVID-19. 
- NO2 AQI decreased 39.8% compared to the 2019 average (statistically significant) 
- SO2 AQI decreased 25.8% compared to the 2019 average (statistically significant) 
- CO AQI decreased 18.7% compared to the 2019 average (statistically significant) 
- O3 AQI increased 6.4% compared to the 2019 average 
- PM2.5 AQI decreased 25.9% compared to the 2019 average (statistically significant) 
- PM10 AQI decreased 42.5% compared to the 2019 average (statistically significant) 

Air Pollution: Moscow (Russia) (Post-COVID-19) 

Researchers studying Istanbul, Turkey found AQI values from various pollutants may have varied from March 21, 2020 - May 10, 2020 due to COVID-19. 
- NO2 AQI decreased 19.9% compared to the 2019 average 
- SO2 AQI increased 29.3% compared to the 2019 average (statistically significant) 
- CO AQI increased 2.9% compared to the 2019 average 
- O3 AQI decreased 43.6% compared to the 2019 average (statistically significant) 
- PM2.5 AQI decreased 3.4% compared to the 2019 average 
- PM10 AQI decreased 19.0% compared to the 2019 average (statistically significant)  

Air Pollution: Istanbul (Turkey) (Post-COVID-19)

Researchers studying Tehran, Iran found AQI values for various pollutants may have varied from March 13, 2020 - April 17, 2020 due to COVID-19. 
- NO2 AQI decreased 35.2% compared to the 2018 average (statistically significant)
- PM2.5 AQI decreased 21.9% compared to the 2018 average (statistically significant) 
- PM10 AQI decreased 37.9% compared to the 2018 average (statistically significant) 

Air Pollution: Tehran (Iran) (Post-COVID-19)

Researchers studying Tokyo, Japan found AQI values for various pollutants may have varied from April 7, 2020 - May 24, 2020 due to COVID-19. 
- NO2 AQI decreased 19.5% compared to the 2019 average 
- SO2 AQI decreased 28.1% compared to the 2019 average (statistically significant) 
- CO AQI increased 7.3% compared to the 2019 average 
- O3 AQI decreased 6.7% compared to the 2019 average 
- PM2.5 AQI decreased 11.4% compared to the 2019 average 
- PM10 AQI decreased 11.0% compared to the 2019 average 

Air Pollution: Tokyo (Japan) (Post-COVID-19)

Researchers studying Seoul, South Korea found AQI values for various pollutants may have varied from February 24, 2020 - May 6, 2020 due to COVID-19. 
- NO2 AQI decreased 25.8% compared to the 2019 average (statistically significant) 
- SO2 AQI decreased 20.8% compared to the 2019 average (statistically significant)
- CO AQI decreased 14.6% compared to the 2019 average (statistically significant) 
- O3 AQI increased 10.2% compared to the 2019 average 
- PM2.5 AQI decreased 19.1% compared to the 2019 average (statistically significant) 
- PM10 AQI increased 19.5% compared to the 2019 average 

Air Pollution: Seoul (South Korea) (Post-COVID-19)

Researchers studying Beijing, China found AQI values for various pollutants may have varied from February 10, 2020 - March 27, 2020 due to COVID-19. 
- NO2 AQI decreased 33.7% compared to the 2019 average (statistically significant) 
- SO2 AQI decreased 33.5% compared to the 2019 average 
- CO AQI increased 92.4% compared to the 2019 average (statistically significant) 
- O3 AQI increased 5.1% compared to the 2019 average 
- PM2.5 AQI decreased 7.2% compared to the 2019 average 
- PM10 AQI decreased 14.8% compared to the 2019 average 

Air Pollution: Beijing (China) (Post-COVID-19) 

Researchers studying Athens, Greece found air pollution may have varied from March 23, 2020 - May 10, 2020 due to COVID-19 lockdowns and again May 11, 2020 - May 31, 2020 during the post-lockdown period. 
- Average CO2 levels decreased 2.1% during lockdown (statistically significant) and increased 0.6% during post-lockdown 
- Average CO levels decreased 58.0% during lockdown and increased 77.8% during post-lockdown (both statistically significant) 
- Average NO2 levels decreased 31.5% during lockdown and increased 72.1% during post-lockdown (both statistically significant) 
- Average PM2.5 levels decreased 18.0% during lockdown and increased 31.1% during post-lockdown (both statistically significant) 
- Average BC levels decreased 40.1% during lockdown and increased 31.1% during post-lockdown (both statistically significant) 
- Average BCff levels decreased 33.4% during lockdown and increased 52.8% during post-lockdown (both statistically significant) 

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