The Impact of COVID-19 Lockdowns on Air Quality—A Global Review

The outbreak of the COVID-19 pandemic has emerged as a serious public health threat and has had a tremendous impact on all spheres of the environment. The air quality across the world improved because of COVID-19 lockdowns. Since the outbreak of COVID-19, large numbers of studies have been carried out on the impact of lockdowns on air quality around the world, but no studies have been carried out on the systematic review on the impact of lockdowns on air quality. This study aims to systematically assess the bibliographic review on the impact of lockdowns on air quality around the globe. A total of 237 studies were identified after rigorous review, and 144 studies met the criteria for the review. The literature was surveyed from Scopus, Google Scholar, PubMed, Web of Science, and the Google search engine. The results reveal that (i) most of the studies were carried out on Asia (about 65%), followed by Europe (18%), North America (6%), South America (5%), and Africa (3%); (ii) in the case of countries, the highest number of studies was performed on India (29%), followed by China (23%), the U.S. (5%), the UK (4%), and Italy; (iii) more than 60% of the studies included NO2 for study, followed by PM2.5 (about 50%), PM10, SO2, and CO; (iv) most of the studies were published by Science of the Total Environment (29%), followed by Aerosol and Air Quality Research (23%), Air Quality, Atmosphere & Health (9%), and Environmental Pollution (5%); (v) the studies reveal that there were significant improvements in air quality during lockdowns in comparison with previous time periods. Thus, this diversified study conducted on the impact of lockdowns on air quality will surely assist in identifying any gaps, as it outlines the insights of the current scientific research.


Introduction
The novel coronavirus (SARS-CoV-2) originated from Wuhan in December 2019, and later spread to many countries across the world [1]. The World Health Organization (WHO) declared this virus a "global pandemic" on 11 March 2020. In many parts of the world, severely affected countries imposed lockdowns to prevent the transmission of COVID-19 by restricting transportation, economic, and industrial activities. Thus, restrictions on human activities and the various productive activities of industries and farms resulted in unforeseen impacts and improved the health of the environment to a great extent. The air quality also significantly improved across the globe because of the restricted emissions from different sources during lockdowns. In many previous research studies, it is welldocumented that many countries of the world are facing serious public health problems due to extreme air pollution [2][3][4][5]. More than 60% of the populations living in urban areas are severely exposed to the serious problem of air pollution [1]. Both high-income (56%) and low-income (98%) countries of the world fail to meet the guidelines proposed by WHO [1]. As per the reports published by WHO [1], more than 4.2 billion people have lost their lives because of health risks related to air pollution [1]. Air pollution has become one of the most significant health risks [6][7][8][9] and results in great loss of life. According to an estimation Arabia (1) 117

Geographical Distribution and COVID-19 Studies
In the present analysis, about 70% of the total papers were published in 2020 (particularly from March to December) and the rest of the papers were published (30%) in 2021 (considered up to the month of April). This study encompasses five continents: Asia, Europe, Africa, North America, and South America. About 65% of the total studies were surveyed from Asia, followed by Europe (18%), Africa (7%), North America (6%), South America (6%), and Australia (1%). In the case of countries, the highest number of studies were surveyed from India (29%), followed by China (23%), the U.S. (6%), the UK (5%), Italy (3.8%), and Bangladesh (2.78%) (Figure 3).

Impact of COVID-19 on Air Quality over Asian Countries
From the overall studies surveyed, it was observed that the greatest number of studies on the impact of the COVID-19 pandemic on air quality was performed on Asian countries (65%). As per our literature survey, it is documented that about 45% of the research studies on the impact of COVID-19 on air quality was identified from India, followed by China (36%), Bangladesh (4%), Pakistan (2.6), Korea (2.6%), and Thailand (1.7%). In India, most of the studies were performed on polluted cities and large megacities, such as Delhi, Mumbai, Kolkata, Chennai, Bengaluru, Varanasi, Patna, Gaya, Hyderabad, and Pune. About 60% of the studies were performed over the megacity, Delhi (the capital city of India). In China, most of the studies were performed on large cities, such as Wuhan, Beijing, the megacity Hangzhou, Anqing, Hefei, and the city of Suzhou (Table 3). The studies focusing on the concentrations of PM 2.5 are shown in Figure 4. Country 2020 There was a remarkable decline in the ambient air quality index (AQI) (17.75% and 20.70%, respectively) during post-lockdown periods as compared to pre-lockdown periods (ii) poor air quality had a positive correlation with COVID-19 mortalities (r = 0.435 for AQI) [31].

State 2020
There was a substantial reduction in air pollutants during different phases of lockdowns (ii) PM 2. 5 and PM 10 decreased by about 17.76% and 20.66%, respectively, during consecutive periods of lockdowns [32].
City scale 2021 PM 2.5 was reduced by about 40 to 45% during lockdown periods in comparison to the previous two years [33].
City scale 2020 Particulate matter concentration decreased by about 40% during lockdown in comparison to previous years [34].
City scale 2020 The lockdown measures reflected a significant reduction in air pollutants; the most significant fall was estimated for NO 2 (29.3-74.4%), while the least reduction was noticed for SO 2 [35].
City scale 2020 The average value of AQI at Punjab Bagh was noticed as 212 before the lockdown, which dropped down to 74 during the lockdown, indicating a significant improvement in air quality [23].

City scale 2020
The results indicate the lowering of PM 2.5 , PM 10 , and NO 2 concentrations in the city by 93%, 83%, and 70%, respectively, from 25 February 2020 to 21 April 2020 [17].
City scale 2020 The concentration of NO 2 and PM 2.5 significantly decreased due to lockdowns across cities [36].
City scale 2020 These two cities observed a substantial decrease in nitrogen dioxide (40-50%) compared to the same period last year [37].

City scale 2020
Major negative effects on the social and surrounding environment have been reported due to COVID-19, however positive effects have also been observed with respect to air quality. The results have been taken from the National Aeronautics and Space Administration (NASA), and indicate a significant reduction (50%) in the air quality of the Indian region [23].

City Scale 2020
A considerable reduction (∼30-70%) in NO 2 was found, except for a few sites in the central region. A similar pattern was observed for CO having a ∼20-40% reduction. The reduction observed for PM 2.5 , PM 10 , NO 2 , and the enhancement in O 3 was proportional to the population density [38].

India
City scale 2021 PM 2.5 has declined by 14%, by about 30% for NO 2 in million-plus cities, and a 2.06% CO, SO 2 within the range of 5 to 60%, whereas the concentration of O 3 has increased by 1 to 3% in the majority of cities compared with pre-lockdown. On the other hand, CPCB/SPCB data showed a more than 40% decrease in PM 2.5 and a 47% decrease in PM 10 in north Indian cities, more than a 35% decrease in NO 2 in metropolitan cities, more than an 85% decrease in SO 2 in Chennai and Nagpur, and a more than 17% increase in O 3 in five cities during 43 days of pandemic lockdown [39].

City scale 2020
The lockdown effect due to COVID-19 in the city: the complete closure of industries, transports, markets, shopping malls, recreation units, construction works, etc., which are the main sources of CO 2 emissions [40].

City scale 2021
Highest levels of PM 10 and PM 2.5 were observed near sunrise, with little change in the time of maximum levels between 2019 and 2020 [41].

City scale 2020
A reduction of almost 60% in the particulate matter pollution, and up to 40% in the NO x pollution, were observed, while the ozone levels were reduced by 30-40%, as compared to the same period during the previous two years [42].

City scale 2021
The air quality has improved across the country and the average temperature and maximum temperature were connected to the outbreak of the COVID-19 pandemic [43].
City scale 2021 (b) During entire periods of lockdown, the average concentration of PM 2.5 declined by 50% [46].
City scale 2020 Suspended particulate matter (SPM) was reduced by about 36%. The concentration of NO 2 was also reduced during lockdown periods [46].

City 2020
The concentration of PM 2.5 , PM 10 , and NO 2 declined by about 50%, with a significant increase in O 3 in Delhi (p < 0.05) [35].

Regional 2020
Mumbai recorded the highest decrease of NO 2 (34%) with a seasonal decrease of SO 2 in western and southern India [48].

City 2021
During lockdown periods, the concentration of PM 2.5 and PM 10 declined by about 43% and 59%, respectively, in Delhi, and by 50% and 49%, respectively, in Kolkata [49].

City 2020
During the initial periods of lockdown, the concentration of PM 2.5 declined by about 40 to 70% (from 25 March to 31 March 2020) [50].
City 2020 NO 2 was reduced by about 46% and the air quality index (AQI) improved by about 27% [52].
City 2020 Air pollutants (PM 2.5 , PM 10 , NO 2 , and CO) were reduced by about 50% across the megacities of India [54]. City 2020 PM 10 was reduced by more than 46% across five cities [57].
City scale 2020 The concentrations of PM 2.5 , PM 10 , SO 2 , CO 2 , and NO 2 decreased due to lockdown [17].
Country and City scale 2020 Air quality improved by about 25% during lockdown periods [61].

City scale 2020
The over-standard multiples method and a grey relational analysis to study the individual and overall change trends of pollutants in Wuhan during the same period in the past seven years. The results show that the concentrations of SO 2 and O 3 increased because of the pandemic, but still met the standard [62].

City Scale 2020
Urban aerosols decreased from 27.1% for pre-C19Q aerosols to only 17.5% during C19Q. WRF-Chem reported ã 0.2 • C warming across east-central China that represented a minor, though statistically significant, contribution to C19Q temperature anomalies. The largest area of warming is concentrated south of Chengdu and Wuhan, where temperatures increased between +0.2-0.3 • C [63]. Table 3. Cont.

Country Study Area Publication Year Major Findings
City scale 2021 The increment in secondary organic and inorganic aerosols under stationary weather reached up to 36.4% and 10.2%, respectively, which was further intensified by regional transport. PRD was quite the opposite. The emission reductions benefited PRD air quality, while regional transport corresponded to an increase of 17.3% and 9.3% in secondary organic and inorganic aerosols, respectively. In different regions, the maximum daily 8 h average ozone (O 3 ) soared by 20.6-76.8% in YRD but decreased by 15.5-28.1% in PRD. In YRD, nitrogen oxide (NO x ) reductions enhanced O 3 accumulation and, hence, increased secondary aerosol formation [64].
City scale 2020 It was found that the COVID-19 pandemic caused PM 2.5 and AQI to decrease by about 7 µg/m 3 and 5-points, respectively [65].

City scale 2021
The precipitous decrease of AQI and PCDI in Q1 2020, and the peaks of the AQI during the epidemic period were closely related to people's activities. AQI, PM 2.5 , and NO 2 were significantly positively correlated with PCDI [66].

City scale 2020
The average concentrations of PM 2.5 , PM 10 10 concentrations, they were still more than four times higher than the World Health Organization standards (10µg/m 3 and 20 µg/m 3 , respectively) [68].

City scale 2020
Average concentrations of PM 2.5 and PM 10 across China were 10.5% and 21.4% lower, respectively, during the lockdown period. The largest reductions were in Hubei province, where NO 2 concentrations were 50.5% lower than expected during the lockdown [69].
City scale 2020 The AQIs in these cities were brought down by 6.34 points (PM 2.5 was down by 7.05 µg m −3 ) relative to the previous year. The lockdown effects were greater in colder, richer, and more industrialized cities [61].

City scale 2020
The pandemic promoted a decrease in PM 2.5 , PM 10 , and NO 2 concentrations, but it had just reached the standard or even exceeded the standard [71]. Table 3. Cont.

Country Study Area Publication Year Major Findings
China City scale 2020 The concentrations of SO 2 and O 3 increased but still met the standard. However, the pandemic promoted a decrease in PM 2.5 , PM 10 , and NO 2 concentrations, but it had just reached the standard or even exceeded the standard [62].
Country and City scale 2020 O 3 responses to NO 2 declines can be affected by the primary dependence on its precursors [72].

City scale 2021
The air quality index (AQI) during the lockdown period decreased by 7.4%, and by 23.48%, compared to pre-lockdown levels and the identical lunar period during the past 3 years, respectively, which exhibited optimal air quality due to reduced emissions [73].

City scale 2020
A causal relationship between P and R across 31 provincial capital cities in China was established via matching. A higher P resulted in a higher R in China. A 10 µg/m 3 increase in P produced a 0.9% increase in R (p < 0.05). An interaction analysis between P and absolute humidity (AH) showed a statistically significant positive relationship between P × AH and R (p < 0.01). When AH was ≤8.6 g/m 3 , higher P and AH produced a higher R (p < 0.01) [74].

City scale 2021
The number of days with NO 2 , PM 10 , and PM 2.5 as the primary pollutants decreased by approximately 10, 9, and 15%, respectively. We compared the wind direction, wind speed, temperature, and relative humidity from January-April 2020, 2019, 2018, and 2017, and found no obvious correlation between meteorological factors and improved air quality during the 2020 lockdown [75].
Country 2020 The concentrations of CO and NO 2 were reduced by about 20% and 30%, respectively [76].

City 2021
The PM 2.5 and SO 2 were reduced from 37 to 26 ug/m 3 and from 6 to 4 ug/m 3 , respectively, during restricted lockdown periods [79].
City 2020 The concentration of PM 2.5 was higher during New Year holidays in 2020 (73%) than New Year holidays in 2019 (59%) [80].

Country 2021
Lockdown resulted in about a 50% reduction in NO 2 [82].

Country 2021
The NO 2 was reduced by about 53, 50, and 30% in Wuhan, Hubei province, and China, respectively. The concentration of PM 2.5 declined by about 35, 29, and 19%, respectively, in comparison to last year [83].
Country 2020 NO 2 declined by about 24% during the Chinese New Year (CNY) holiday [84].
Country 2020 The concentration of NO 2 was reduced by about 20 to 50% for cities, 15 to 40% for maritime transport, and 40% for power plants [85].  [86].
City 2020 A substantial reduction of PM 2.5 , PM 10 , CO, and SO 2 were reported during lockdown periods [87].
Country 2020 Air pollution was reduced by up to 90% during city lockdown [88].
City 2020 During lockdown periods, the concentration of PM 2.5 and NO 2 decreased by about 36% and 53%, respectively, and O 3 increased by about 116% [92].
Country 2020 During lockdown periods, the concentration of PM 2.5 decreased by up to 23 ug/m 3 [93].

Thailand
City scale 2020 Air quality improved by about 50% to 70% during lockdown periods due to restricted emissions from transportation [94].
City scale 2020 The environmental benefits documented in major urban agglomerations during the lockdown may extend to medium-sized urban areas as well [95].

City scale 2021
Due to lockdown measures, significant differences between PM 2.5 , SO 2 , NO 2 , CO, and O 3 in 2019 and 2020 were observed in Dhaka city. We used lag-0, lag-7, lag-14, and lag-21 days on daily COVID-19 cases to look at the lag effect of different air pollutants on meteorology [25].

City scale 2021
The concentration of PM 2.5 and PM 10 decreased by 40% and 32% during lockdown periods in comparison to previous dry seasons [96].
Country scale 2020 The concentration of NO 2 and SO 2 decreased by about 40% and 43%, respectively [97].
City scale 2020 Air quality during lockdown was found to be 5.30% lower than 2019 [98].

Malayasia
Country and City scale 2020 PM .5 and PM 10 decreased by about 25% during lockdown [99].
City scale 2020 Differences between PM 10 , PM 2.5 , SO 2 , NO 2 , CO, O 3 , and solar radiation in 2019 and 2020 since the movement control order (MCO) was implemented on 18 March 2020 [100].

Singapore
Country and City scale 2020 The concentrations of the following pollutants PM 10 , PM 2.5, NO 2 , CO, and SO 2 decreased by 23, 29, 54, 6, and 52%, respectively, while that of O 3 increased by 18%. The Pollutant Standards Index decreased by 19% [101]. In its earlier closest period, the pollution from transport, based on nitrogen oxides, had reduced by 40% on average, whereas the pollution from industry, based on Grand-level ozone had increased by 34% on average [105].

Iran
City scale 2020 PM 2.5 increased by 0.5-103, 25, and 2-50%. In terms of the national air quality, SO 2 and NO 2 levels decreased, while AOD 26 increased during the lockdown [106].

Country 2021
There were no significant improvements of air quality in Lahore and Karachi during lockdown periods, as compared to 2019 [107].

City scale 2021
With the reduction in human activity (known to be the biggest source of air pollution) during the COVID-19 pandemic, changes in air pollution values were observed. The year 2020, compared with 2018 and 2019, in order to observe this change and to compare it with other years: 1 January-15 March, considered the pre-pandemic process; 16 March-31 May, considered the pandemic process; 1 June-30 June, considered the normalization process [108].

Teheran
Country 2020 There were increases in PM 2.5 and PM 10 (by 20.5% and 15.7%) during the first month of the COVID-19 outbreak [113].

Impact of COVID-19 on Air Quality over European Countries
After Asia, the second highest number of studies was performed on European countries. As per the literature survey, 33% of the total studies were collected from European countries. In this review, studies were collected from 13 countries in Europe with the highest number of studies on the UK (23%), followed by Italy (20%), Spain (20%), Turkey (8.8%), and France (5.5%). The cities in Europe on which studies were performed were Barcelona, Madrid, Naples, the city of Novi Sad, Munich, Tirana, Southampton, and Milan. As per the literature survey, most of the studies were performed on the concentrations of NO 2 (more than 50% of the study). As per the results of Anderson and Dirks [114] (2020), Lee et al. [115], and Jephcote et al. [116], the concentration of NO 2 decreased by about 92%, 42%, and 38%, respectively, in the UK Similarly, in Italy, the concentration of NO 2 declined by about 49-62%, and SO 2 decreased by about 70% [117] during lockdown periods. Thus, from the previous studies, it is clear that there was a substantial decline in air pollutants, and air quality significantly improved during lockdown periods. The details of the impact of lockdown on air quality are presented in Table 4. Most of the studies focusing on the concentration of PM 10 are presented in Figure 5. Table 4. Literature on the impact of COVID-19 on air quality in European countries.

UK
England 2021 PM 2.5 was a major contributor to COVID-19 cases in England, as an increase of 1 m 3 in the long-term average of PM 2.5 was associated with a 12% increase in COVID-19 cases [118].
Southampton 2020 NO 2 decreased by about 92% during lockdown, as compared with the previous two years [114].

Country 2021
The concentration of NO, NO 2 , and NO x decreased 32% to 50% at roadsides during lockdown [119].

Country 2021
The concentration of NO 2 decreased by about 50%, and O 3 increased by about 10% [121].

Country 2021
The concentration of Ox emissions declined nationwide by~20% during the lockdown [122].

Spain
City 2020 The 4-week lockdown had a significant impact on reducing the atmospheric levels of NO 2 in all cities, except for the small city of Santander, as well as the levels of CO, SO 2 , and PM 10 in some cities, but resulted in an increase of the O 3 level [123].

Country 2020
Changes in the concentration of the pollutant nitrogen dioxide (NO 2 ) during the lockdown period were examined, as well as how these changes relate to the Spanish population [124].

Country 2021
Decreases in PM 10 levels were greater than in PM 2.5 because of reduced emissions from road dust, vehicle wear, and construction/demolition activities. The averaged O 3 daily maximum of 8-h (8hDM) experienced a generalized decrease in the rural receptor sites in the relaxation (June-July) with −20% reduced mobility [20].

City 2020
The concentration of NO 2 in Barcelona and Madrid decreased by about 50% and 62%, respectively, during lockdown periods [127].

Regional 2020
Potentially, it is the spatially confounding factors related to urbanization that may have influenced the spreading of novel coronavirus. Our epidemiological analysis uses geographical information (e.g., municipalities) and Poisson regression to assess whether both the ambient PM concentration and the excess mortality have a similar spatial distribution [129].

Regional 2020
The estimate of the time series slope, i.e., the expected change in the concentration associated with a time unit increase, decreased from −0.25 to −1.67 after the lockdown [130].

Scale of Study Publication Year Major Findings
Italy Country 2021 The model finds that there is a positive nonlinear relationship between the density of particulate matter in the air and COVID-19 transmission, which is in alignment with similar studies on other respiratory illnesses [131].
City 2020 There were significant reductions in PM 2.5 , PM 10 , CO and NO, respectively [21].

Regional 2021
The concentration of PM 2.5 and NO 2 declined by about 16% and 33%, respectively [132].

Country 2021
To determine the effects of COVID-19 measures on air quality in Turkey, for this investigation, the daily means of PM 10 , PM 2.5 , NO 2 , CO, O 3 , and SO 2 air pollutant data were used [133].
Country 2020 By the end of April, the PM 2.5 index had improved by about 35% during lockdown [134].

City 2021
The NO 2 concentrations were reduced by about 11.8 % in the after-virus period [135].

Russia City 2020
Just under half were from changes in surface transport. At their peak, emissions in individual countries decreased by -26% on average. The impact on 2020 annual emissions depends on the duration of the confinement, with a low estimate of -4% (-2 to -7%) if pre-pandemic conditions return by mid-June, and a high estimate of -7% (-3 to -13%) [137].

Germany City 2021
The concentration of NO 2 reduced by about 15-25% and 34-36% from traffic sites during lockdown periods [138].

Macedonia
Country 2020 PM 2.5 in Kumanovo and carbon monoxide in Skopje (7% and 3% higher concentrations, respectively). The most notable decrement was for NO 2 , with a concentration 5-31% lower during the COVID-19 period [139].
Poland Country 2021 Aerosols concentrations were reduced by about 23% and 18% in April and May, respectively [76].

Serbia City 2021
The average daily concentrations of PM 2.5 , NO 2 , PM 10 , and SO 2 were reduced by 35%, 34%, 23%, and 18%, respectively. In contrast, the average daily concentration of O 3 increased by 8%, even if the primary precursors were reducing, thus representing a challenge for air quality management [142]. Viruses may persist in the air through complex interactions with particles and gases depending on: (1) chemical composition; (2) the electric charges of the particles; and (3) meteorological conditions, such as relative humidity, ultraviolet (UV) radiation, and temperature. In addition, by reducing UV radiation, air pollutants may promote viral persistence in the air and reduce vitamin D synthesis [143].
Europe 2020 The lockdown effect on atmospheric composition, in particular through massive traffic reductions, has been important for several short-lived atmospheric trace species, with a large reduction in NO 2 concentrations, a lower reduction in particulate matter (PM) concentrations, and a mitigated effect on ozone concentrations due to nonlinear chemical effects [144].
Europe 2020 The concentration of NO 2 was reduced by about 25% during lockdown periods, when compared to the same periods of previous years [145].

Impact of COVID-19 on Air Quality over North American Countries
In North America, the greatest number of studies on air quality were performed on the United States and Canada. In this literature review, about 90% of the total studies were surveyed from the U.S., followed by Canada (10%). Most of the studies in the U.S. were performed at the national level and the city scale (such as California and New Jersey). In the U.S., there were substantial reductions in air pollutants due to lockdown. For example, according to Goldberg et al. [146], NO 2 declined by about 9% to 42%, with the highest decline in San Jose and Los Angeles, and the lowest decrease (<12%) in Miami, Minneapolis, and Dallas. As per the study of Jiang et al. (2020), PM 2.5 concentration was reduced by more than 68% after lockdown. Moreover, other studies in the U.S. also revealed that there was a significant decrease in the air pollutant concentrations during lockdown periods that resulted in a substantial improvement in air quality ( Table 5). The studies focusing on the assessment of CO are shown in Figure 6.   If the air quality improvements persist over a year, it could potentially lead to 3970-8900 preventable premature deaths annually (note: the estimates of preventable premature deaths have large uncertainties). Public transit demand showed dramatic declines (~80%) [147].
City 2020 COVID-19 prevalence and fatality (plotted as logarithm-transformed prevalence/fatality on the y-axis) as a function of mean ozone/PM 2.5 AQI (plotted on the x-axis). Coefficients were not statistically significant for ozone (p = 0.212/0.814 for prevalence/fatality) and PM 2.5 (p = 0.986/0.499) [148]. Country 2020 The concentration of NO 2 was reduced by about 25% in comparison to past years [149]. Country 2020 The NO 2 concentration was reduced by about 5 to 49%, with a mixed impact on O 3 (±20%) [24].

City 2021
As per ground-based observation, it was reported that the concentration of NO 2 , CO, and PM 2.5 dropped by about 38%, 49%, and 31%, respectively, during lockdown periods (19 March to 7 May 2020) [152]. Canada City 2020 The concentration of nitrogen dioxide and nitrogen oxides reduced across Ontario [153].

Impact of COVID-19 on Air Quality over South American Countries
In South America, the greatest number of studies were performed on Brazil, Ecuador, and Mexico. In this review, about 40% of the total studies were surveyed from Brazil and Ecuador, followed by Mexico (20%). As per the results of the study, it was documented that there was a substantial improvement in air quality during lockdown periods. According to Hernández-Paniagua et al. [154], the concentration of NO 2 was reduced by between 10% and 35% in Mexico during lockdown periods. Zalakeviciute et al. [155] performed a study in Quito (Ecuador), and the findings of the study show that air quality improved by about 26% to 68%. According to Nakada and Urban [156], NO, NO 2 , and CO decreased by about 70%, 50%, and 60%, respectively, in Sau Paulo (Brazil). The details of the findings on the impact of lockdown on air quality are presented in Table 6. The studies focusing on the assessment of NO 2 are shown in Figure 7.  Table 6. Literature on the impact of COVID-19 on air quality in South American countries.

Brazil
City 2020 There was a substantial decrease of NO (more than 70%), CO (more than 60%), and NO 2 (more than 50%). Ozone concentration increased by about 30% during partial lockdown periods, as compared to previous years [11].
City Scale 2020 During lockdown, CO reported the highest decline of up to 100%. NO 2 decreased by about 9 to 41% [158].

Ecuador
City 2020 The concentration of NO 2 and PM 2.5 significantly decreased due to the implementation of lockdown. The concentration of PM 2.5 was lower in 2020, as compared to 2018 and 2019 during the same lockdown periods i.e., March [159].

City 2021
There was a substantial reduction in NO during lockdown periods [160].

Scale of the Study Publication Year Major Findings
Ecuador Country 2020 Air quality improved by 29-68% due to lockdown [161].

Impact of COVID-19 on Air Quality over African Countries
In this review, four countries were surveyed from Africa, with the highest percentage of studies on Morocco (42%), followed by Egypt (28%), Uganda (14.3%), and Nigeria (14.3%) ( Table 7). In African countries, there were also significant improvements in air quality during lockdown periods. For example, as per the study by Otmani et al. [162], the concentration of PM 2.5 and PM 10 decreased by about 75% and 96%, respectively, during lockdown periods. Similarly, CO and NO 2 decreased by about 46% and 45%, respectively. The studies focusing on the assessment of SO 2 are shown in Figure 8. Table 7. Literature on the impact of COVID-19 on air quality in African countries.

Scale of the Study Publication Year Major Findings
Egypt Country 2020 The whole country is improved as a result of reduced pollutant emissions, with NO 2 reduced by 45.5%, CO emissions reduced by 46.23%, ozone concentration decreased by about 61.1%, and AOD reduced by 68.5%, compared to the previous two years [163].

Morocco
City Scale 2020 PM 10 and NO 2 decreased by about 75% and 96%, respectively [164]. The lockdown resulted in a decrease of SO 2 and NO 2 across the cities. For example, 1.1% and 215.5% of NO 2 and SO 2 , respectively, from the city Port Harcourt [167].

Number of Publications and Journal Distributions
As per literature screening from different sources, it was observed that there were more than 300 studies performed on the impact of lockdown on air quality across the world (as per our observations from April 2020 to March 2021). Among all the countries, the greatest number of studies were performed on Asian countries, followed by European countries. From the literature screening, it was well-recognized that the greatest number of research studies were published by Science of The Total Environment (about 29%), followed by Aerosol and Air Quality Research (23%), Air Quality, Atmosphere & Health (9%), Environmental Pollution (6%), and Environmental Research (4%). From the top ten journals, about 80% of the total studies were included (Figures 9 and 10).

Discussion
The present study mainly focuses on a review on the impact of COVID-19 lockdowns on air quality around the globe. From the results, it was found that most of the studies on the impact of lockdowns on air quality were performed in Asian countries (about 65%), followed by European countries (18%), and North and South American countries (10%). As per the literature screening, it was reported that the greatest number of studies on the impact of lockdown on air quality were reported from India (29%), followed by China (23%), the UK (4.44%), and Italy (3.89%). From the previous research studies, it was recognized that there was a strong association between air pollutants and respiratory disease [168][169][170]. Thus, the areas with high exposure to air pollution are vulnerable to mortalities due to respiratory diseases. With COVID-19 being a respiratory disease, it is obvious that COVID-19 deaths are strongly influenced by air pollutants. Recent studies also show that the concentrations of air pollutants are significant risk factors in COVID-19 deaths [171][172][173][174]. In many studies across the cities of the world, it is reported that concentrations of air pollutants are strongly associated with COVID-19 cases, as well as deaths. The severely affected countries of the world imposed several measures to fight COVID-19 and reduce the transmission of the virus around the world [47]. China was the first country to implement a complete shutdown in commercial fields, restrictions on domestic and international travel, and strict COVID-19 protocols were imposed across affected cities [175]. After that, similar restrictions were implemented by several other countries, such as India, Italy, and France. These restrictions were placed on public transportation, social gatherings, schools and colleges, and emissions from industries as well (Das et al., 2020). Economic activities are the prime factors that contribute to environmental pollution because of the combustion of fuel and the release of air pollutants into the atmosphere [175]. In most of the densely populated cities, particularly the ones in developing countries, air pollution levels are higher than the tolerance limits, and that results in risks to human health. In urban areas, the concentration of primary air pollutants, such as CO, NO 2 , SO 2 , O 3 are significantly higher. The concentrations of particulate matters (PM 2.5 and PM 10 ) were significantly decreased during lockdown periods due to the restriction on emissions from various sources [18]. A substantial improvement in air quality was reported for most polluted cities around the world, such as Delhi (the capital city of India, India), Dhaka (the capital city of Bangladesh), and Beijing (China). Thus, COVID-19 lockdowns have had positive impacts on the environment. Therefore, from the overall results, it can be stated that lockdown, particularly short-term lockdown, can, to some extent, be considered an alternative measure to reduce air pollution level. The outbreak of COVID-19 compelled the affected countries to impose lockdowns to curb COVID-19 transmission, and many countries have remained under partial lockdown since last year. Thus, the environment has had a long time to restore its capacity because of the cessation of many economic activities. However, in reality, it is not possible to completely cease economic activity because it is a matter of human livelihoods. So, planners and policymakers must implement and follow sustainable strategies to reduce air pollution levels. The lockdowns during COVID-19 not only induced the improvement of air quality, but it has also had other positive effects on the environment. Measures regarding social distancing kept people away from resorts and sea beaches, and effluent discharge into the water stopped because of industrial shutdowns. Water quality also improved in many countries, such as in Spain, Equador, and Mexico. For instance, as per the findings of Paital et al. (2020), and Saadat et al. (2020), the water quality of Venice's canals, and the Yamuna River in Delhi, improved significantly in comparison with past years. Apart from environmental perfectives, COVID-19 lockdowns also had a substantial impact on the global economy. As per a World Trade Organization (WTO, 2020) report, economic activity decreased in world trade between 13% and 32%. Thus, it has been well-documented in previous literature that COVID-19 lockdowns result in substantial positive, as well as negative, impacts on the environment.
The present review study on the impact of COVID-19 lockdowns on air quality across the globe can provide a unique work for understanding, as well as implementing, effective strategies at the city, regional, and country scales. For example, among all the regions, Asian countries, such as India, China, and Bangladesh, are more severely affected by extreme air pollution levels. At the country scale, most of the studies were performed in India, followed by China, and the UK Thus, at the global scale, effective strategies can be implemented for the Asian region to fight air pollution levels, and at the country scale (such as with India), cities can be identified through this literature and strategies for them can thus be implemented. Therefore, this study may be very helpful to planners and policymakers for understanding the global scenario and the improvement of air quality due to COVID-19 lockdowns. In this study, few limitations can be identified. Firstly, in this study, the literature was reviewed from March 2020 to April 2021. No studies were taken into consideration that were published after April 2021 in this review assessment. Thus, further research in the future can be performed on literature published after April 2021 for a better understanding of the impact of lockdown on air quality. Secondly, no models or laws were applied in this study for literature screening. Thus, for future researchers, we suggest implementing the Bradford law for literature screening. In spite of these limitations of the study, this is a unique piece of research into the global scenario with respect to the impact of COVID-19 lockdowns on air quality.

Conclusions
In this study, an attempt has been made to examine and review the impact of COVID-19 lockdowns on air quality on a global scale. Initially, 237 studies related to the impact of COVID-19 lockdowns on air quality were screened, and 144 studies were finally taken into account for this literature review. The literature was extracted from Scopus, Google Scholar, PubMed, Web of Science, and the Google search engine. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) was also used for the literature screening. As per the results of this study, it is well-documented that most of the studies were performed over the Asian region (65%), followed by Europe (18%), North America (6%), South America (5%), and Africa (3%). At the country scale, the greatest number of studies were conducted in India (29%), followed by China (23%), the U.S. (5%), the UK (4%), and Italy. As per our investigation, it was observed that the greatest number of studies on the impact of the COVID-19 pandemic on air quality was published in Science of the Total Environment (29%), followed by Aerosol and Air Quality Research (23%), Air Quality, Atmosphere & Health (9%), and Environmental Pollution (5%). From the core findings of the literature, it is apparent that there was substantial improvement in air quality due to lockdowns across the world. For example, Naqvi et al. [19] performed a study on India and reported that the air quality index was reduced by about 40% during one month of lockdown. According to Filonchyk et al. [76], and Diamond and Wood [82], the concentrations of CO and NO 2 were reduced by about 20% and 30%, respectively, and by 50% in China. In the UK, NO 2 was reduced by about 42% during lockdown periods [107]. Thus, in all of the countries, it has been well-recognized that there was a substantial improvement in air quality during the lockdowns. The implementation of lockdowns around the world resulted in the improvement of the air quality and provided us with an opportunity to realize the impact of the anthropogenic pressures on the environment. Thus, the findings of the review will surely assist planners and policymakers to understand that the implementation of lockdowns may be an effective measure to restore the environment, and to build quality ecosystems in urban environments. All the affected countries of the world imposed effective measures to try to slow down the transmission of COVID-19. These measures included the closures of industrial activities, strict restrictions on transportation, and the cessation of other productive activities that resulted in the improvement of air quality.  Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy reasons.

Conflicts of Interest:
The authors declare no conflict of interest.