Table 1.

Recent studies on the impact of air quality, especially considering particulate matter, during the COVID-19 pandemic.

Study area (city, country)	Key findings	Author (year)
Region of India (134 monitoring stations)	•They estimated changes in air pollution, including particulate matter (PM2.5 and PM10) during the strict lockdown phases (March 25–May 3, 2020) through an analysis of daily variation and diurnal variation. •The results show average reductions of PM2.5 (northwest region and Indo-Gangetic plain up to 50%, southern region up to ∼40% and central region up to ∼25%) and PM 10 (northwest region and Indo-Gangetic plain up to 60%, southern and central region up to ∼40%).	(Singh et al., 2020)
United States (California, Florida, Louisiana, North Carolina, and Alabama) and China (Hubei, Beijing, Shanghai, Guangdong, and Zhejiang).	•Through data obtained between January 1 and April 30, 2019 and 2020 from the EPA online platform (https://www.epa.gov/) and the MEE (http://datacenter.mee.gov.cn/) the concentrations of five contaminants were analyzed. •The results in general show that in the United States, PM2.5 levels decreased by 1.10%, while PM10 levels increased by 3.81%. Meanwhile, in China the concentrations of PM2.5 and PM10 were reduced by 17.78% and 37.85%, respectively.	(Shakoor et al., 2020)
Spain (Barcelona, Bilbao, Lleida, Madrid, Pamplona, Santander, Santiago de Compostela, Seville, Valencia, Vigo, Zaragoza)	•The study showed that short-term lockdowns (March 15 to April 12, 2020) are not sufficient to improve air quality. Although a significant decrease in the environmental levels of PM10 is observed during the quarantine period, the influence of meteorological factors influences surface concentration values.	(Briz-Redón et al., 2020)
India (Chandigarh)	•They investigated the impact of the emergency closure from March 25, 2020 to May 17, 2020 by examining the levels of 14 contaminants through the analysis of principal components and the combined trajectory analysis. •The results show decreases of up to 28.8% and 36.8 for PM2.5 and PM10, respectively. Identifying the greatest variations in environmental levels due to vehicular emissions.	(Mor et al., 2020)
Saudi Arabia (Jubail, Qatif, Dammam and Al Ahsa)	•Using descriptive statistical analysis, they analyzed PM10 and meteorological behavior during September 15, 2019 to July 18, 2020 for 7 surface stations. •The average concentrations of PM10 contaminants in several monitoring stations were reduced to lower levels than the WHO reference values due to the imposed lockdown. Reductions reached within the range between 21 and 70% in most monitoring sites.	(Anil and Alagha, 2020)
India	•They quantified the changes in the hourly pollution levels, as well as the weather during the 6-week lockdown of COVID-19 in 17 cities in India. •The greatest decrease is observed in Ahmedabad (68%) and Delhi (71%) for PM2.5 and PM10, respectively. The greatest decline observed •It was during the hours of the day from 7 to 10 am and from 7 to 10 pm with a reduction of more than 40%.	(Navinya et al., 2020)
China (Beijing-Tianjin-Hebei region)	•The authors explored the variability of large-scale air pollutants from January 23 to April 8, 2020 using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT), potential source contribution function (PSCF) and concentration weighted trajectory models (CWT). •The contamination by PM2.5 was mainly affected by local emission sources that contributed from 51.6% to 60.6% of the total trajectories in the region with a concentration of PM2.5 that oscillates between 146.2 μg m−3 and 196.7 μg m−3 during the first episode presented during this period.	(Zhao et al., 2020)
Iraq (Baghdad)	•They evaluated the concentrations of PM2.5 and PM10 before the closure of January 16 to February 29, 2020, and during four partial and total closure periods from March 1 to July 24, 2020, in Baghdad. •In comparison with the period prior to the closure, the following results are presented: In the first closure, the concentrations decreased by 8 and 15% of PM2.5 and PM10, respectively. •In the second closure, PM2.5 concentrations decreased by 2.5, while there was an increase of 56% for PM10.	(Hashim et al., 2020)
New Zealand (Auckland)	•Its analysis period is focused on the time of the emergency closure that includes from March 27 to April 17, 2020. •The reductions observed in PM2.5 and PM10 were significantly lower (8–17% for PM2.5 and 7–20% for PM10). Sea salt, traffic emissions and seasonality are significant factors in the behavior of these pollutants.	(Patel et al., 2020)
India (Rajasthan)	•Using trend analysis for the 24-h daily average data for five pollutants, they found maximum decreases of PM10 and PM2.5 in the industrial center of Rajasthan of ~58.2% (178.54 to 98.74 μg m−3) and ~44.69% (95.98 to 40.09 μg m−3), respectively.	(Sharma et al., 2020)
China (Wuhan)	•They evaluated the daily data of the air quality index (AQI) from January 1, 2016 to February 31, 2020. These values were compared with values from 2015 to 2020. •The AQI of Wuhan City decreased significantly, the higher the population density, the more significant the decrease.	(Lian et al., 2020)
South Korea (Seoul)	•They evaluated during the two 30-day periods before and after the onset of social distancing due to the pandemic. The mean concentration of PM2.5 at 30 days decreased by 10.4% in 2020. The decrease in PM2.5 concentration was more prominent during the day than at night.	(Han et al., 2020)
Western Europe	•Using the WRF-CHIMERE modeling system, they determined the impact of the measures to contain the Covid-19 pandemic on air quality during the month of March 2020. Time data for 799 stations for PM10 and 399 stations for PM2.5. •Reductions in PM2.5 concentrations are weak, between −5 and −10%. The sources of primary particle emissions from residential heating and emissions from the agricultural sector contribute to the formation of secondary fine particles in the atmosphere.	(Menut et al., 2020)
India	•They used data from the AOD product of the Terra-Agua/MODIS satellite sensors, Level 2 with spatial resolution of 1 km, to investigate the changes in the level of AOD during the lockdown phases throughout the Indian territory compared to the level of AOD medium to long-term (2000–2019) of the same periods. •Almost 45% of the level of AOD fell on the Indian territory during the periods of lockdown. The metropolitan cities showed a negative average AOD anomaly throughout the lockdown period.	(Ranjan et al., 2020)
Northern South America	•For the analysis of particulate matter, 4 stations located in the cities of Medellín and Bogotá in Colombia were used. The authors show that the environmental levels of PM2.5 and PM10 are reduced by 40% and 44%, respectively, during quarantine compared to the days previous to closure.	(Mendez-Espinosa et al., 2020)
Eastern China	•Employed MODIS-derived AOD at 550 nm obtained 154 by merging Dark Target and Deep Blue retrievals. •During the period from January 23 to April 8, 2020, it was 30% lower than the average of 2011–2019.	(Field et al., 2020)