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. 2023 Jan 19:1–15. Online ahead of print. doi: 10.1007/s41810-023-00173-w

Insights on Air Pollution During COVID-19: A Review

Sushil Kumar 1,✉
PMCID: PMC9850342

Abstract

Air quality improved due to a sudden reduction in the mass concentration of criteria pollutants (PM2.5, PM10, NOx, CO, SO2) except ozone (O3) over cities of the world during the novel coronavirus diseases (COVID-19) lockdown. Such reduction in pollutants concentration during the lockdown period is an indicator of pollutants contributed from human-induced sources. The elevated ozone level during the lockdown period is explained by shifted NOx-mediated reaction towards volatile organic carbon (VOCs) mediated reaction. The reduction in pollutants concentration and improved air quality is not uniform for outdoor and indoor environments. The indoor air quality is quite poor compared to outdoor throughout the lockdown period. The degradation in indoor air quality is associated with increased human activities and the degree of ventilation inside the home. The number of active COVID-19 cases is associated with air quality over a region. The improved air quality helped in a reduction in COVID-19 virus transmission among the people. Present review articles provide detailed insight into current research progress, the impact of lockdowns on outdoor and indoor air quality in different cities of the world. Further, this review articles provide a detailed overview of an elevated O3 level during the lockdown period and the mechanism of formation.

Keywords: Criteria pollutants, Air quality, Human health, Particulate matter, Lockdown

Introduction

The exponential spread of novel coronavirus (COVID-19) over different countries of the world severely affects human health and socioeconomic status (Muhammad et al. 2020; Sarmadi et al. 2021). After, the continuous increase in the number of cases in different countries, the world health organization (WHO) on 11th March 2020 declared COVID-19 as a global pandemic. To reduce the human to human transmission among the people, every country imposed a nationwide lockdown in a phased manner on account of restrictions on industrial activities, road transport, railway transport, air transport including restrictions on schools, colleges, universities, unnecessary public movement (Sharma et al. 2020; Singh et al. 2020; Zoran et al. 2020). However, during the entire period of lockdown essential services (milk shops, medicine stores, hospitals, grocery stores, and essential items-producing industries) remained operational. The implementation of the lockdown resulted in a sudden reduction in the average mass concentration of pollutants in different cities due to restrictions on anthropogenic activities. The sudden reduction in pollutants concentration established the linkage between pollutants emission and exponential industrial growth. The noticeable reduction is mostly confined to urban areas of the world (Zhao et al. 2020; Zhang et al. 2020; Faridi et al. 2021). On contrary, implemented lockdowns in different cities of the world resulted in huge economic losses.

The reduction in atmospheric pollutants concentration level has been assessed based on the estimation of criteria pollutants such as particulate matter (PM2.5, PM10), nitrogen dioxide (NO2), sulfur dioxide (SO2), ozone (O3), and carbon monoxide (CO) over different monitoring station (Adam et al. 2021; Jain and Sharma 2020; Xu et al. 2020). Several studies have been discussed using comparative analysis before lockdown, during the lockdown, and after a lockdown in different cities of the world (Zheng et al.2020; Cui et al. 2020; Hicks et al.2021; Chatterjee et al.2021; Giardi et al.2022; Clemente et al. 2022). The selection of time duration before the lockdown period or reference period varies with variation in the studies. The variation in air quality and chemical composition was studied using criteria pollutants concentration (PM2.5, PM10, SO2, NO2, O3, and CO) and data obtained from filter-based measurement, chemical analysis, real-time monitoring using personal instruments, satellite data, and installed monitoring stations by government agencies in different cities of the world (Baldasano 2020; Zangari et al. 2020; Cazorla et al. 2021; Nakada and Urban 2020; Kumari et al. 2020; Singh et al. 2020; Jain and Sharma 2020; Ordonez et al. 2020; Otmani et al. 2020; Rathod et al. 2021; Siciliano et al. 2020). The reduced pollutants concentration and improved air quality during the lockdown period are different for outdoor and indoor environments (Ezani et al. 2021; Bhat et al.2022; Li et al.2022). Further, the lockdown significantly reduced pollutant concentration for outdoor air quality (OAQ) while indoor air quality (IAQ) remain poor during the entire period of lockdown due to increased indoor activities. The variation in pollutants reduction during the lockdown period could be due to variations in geographical location, meteorological conditions, and atmospheric chemistry. Recently, it has been observed that ozone concentration level in different cities increases during the lockdown period due to alteration in atmospheric chemistry and emission pattern (Zoran et al. 2020; Zheng et al. 2020; Zhang et al. 2021a). The tropospheric ozone formation is largely governed via NOx-VOCS-HOx cycle in the atmosphere. The phase-wise implementation of lockdowns over different cities of the world directly affects the emission pattern of NOx and VOCs on account of restrictions in vehicular and industrial activity (Wang et al. 2021a). However, VOCs have mixed types of sources (natural and anthropogenic) that largely maintain the atmospheric budget. Therefore, ozone formation during the lockdown period shifted towards VOCs limited mechanism in the atmosphere and amplify the atmospheric O3 concentration level (Rathod et al. 2021; Zheng et al. 2020). The phasewise implemented lockdown resulted in a stay-at-home and people spent maximum time inside their home to avoid the virus transmission among the people over different cities of the world. The indoor air quality is largely affected by indoor activities such as cooking, cleaning, and smoking activities. Further, the degree of ventilation in the room largely affects indoor air quality. In the literature, few studies have been carried out to establish indoor air quality and human health during the lockdown period.

The present review articles have extensively analyzed the literature published during the lockdown period implemented in phase manner in different cities over the world mentioned in Table 1. The selection of research articles is based on various criteria such as reduced air quality in the outdoor environment during the lockdown period. In addition, selected articles also describe changes in emission patterns, indoor air quality, link between air pollution and COVID-19 cases, O3 formation chemistry, and the possible scope of research in the future.

Table 1.

The average reduction (-) or increased ( +) in pollutants concentration during lockdown over different cities of country in the world

Name of Cities Data Source Study Period PM2.5 PM10 NO2 SO2 CO O3 Others References
Patiala, Ghaziabad, India Monitoring station and satellite 24th March 2020 to 31st May 2020  − 57%  − 58%  − (3–79%)  − (2–61%)  + (Increase) AQI (151–200) Kumari et al. (2020)
India (2 cities) Monitoring station and satellite 25th March 2020 to 3rd May 2020  − (40–60%)  − (40–60%)  − (30–70%)  − (20–40%) Singh et al. (2020)
India (4 Megacities) Monitoring station 25th March 2020 to 6th April 2020  − 41%  − 52%  − 51%  − 28% Jain and Sharma (2020)
India (Delhi) Monitoring station 27th March to 10th April 2020  − 50%  − 37% VOCs (− 38%) Rathod et al. (2021)
Anand Vihar, Delhi,India Monitoring station 25th March 2020 to 14th April 2020  − 57.6%  − 59.2%  − 65.6%  − 9.1%  + 81.7% Chaudhary et al. (2021)
Milan, Italy Monitoring station and satellite January to April 2020  − 64.7%  + 2.3 time from base line Zoran et al. (2021)
Mainland, China Monitoring station 23rd to 30th January 2020  − 13.7%  − 21.8%  − 47.3%  − 4.6%  − 12.2% Zhao et al. (2020)
East, China Satellite December 2019 to March 2020  − 12.5%  − 40.5%  + 36.5% Zhang et al. (2021b)
New York, USA Monitoring station January to May 2020  − 36%  − 51% Zangari et al. (2020)
Ecuador Monitoring station March to April 2020  − 5.6 time compared to 2018 and − 4.8 times compared to 2019 Zambrano-Monserrate and Ruano (2020)
Beijing, China Monitoring station and satellite 1st January to 12th February 2020  − 20%  − 20% EC (− 50%); OC (− 10%);NOx (− 50%);VOCs (− 30%) Wang et al. (2020)
Yangtze River Delta (YRD), China Monitoring station and satellite January to 29th February 2020  + up to 12% NOx (− > 50%) Wang et al. (2021a)
Dhaka city, Bangladesh Monitoring station and satellite 8th March to 15th May 2020 − 26%  − 20%  − 17.5%  − 8.8%  − 9.7% Rahman et al. (2021)
European cities (Nice, Rome, Valencia and Turin) Monitoring station 1st January to 31 December 2020  − 8%  − 8%  − 53%  + (2.4–27.0%) NO (− 63%) Sicard et al. (2020)
Sale City (Morocco) Field sample collection and monitoring station 11th March to 2nd April 2020  − 75%  − 96%  − 49% Otmani et al. (2020)
European City Monitoring station 15 March to 30 April 2020  − (5–55%)  + (5–22%) Ordonez et al. (2020)
Sao Paulo state, Brazil Monitoring station and satellite 24th March to 20th April 2020  − 54.3%  − 64.8% NO(− 77.3% Nakada and Urban (2020)
United Kingdom Monitoring station April 2020  − 16.5%  − 38.3%  + 7.6% Jephcote et al. (2021)
50 Capital cities of World Monitoring station Phase-wise lockdown in different countries  − 12% Rodriguez-Urrego and Rodriguez-Urrego (2020)
Milan, Italy Monitoring station 23rd March 2020 to 5th April 2020  − (32.7 − 40.5%)  − 47%  − 57.6% Collivignarelli et al. (2020)
Wuhan, China Monitoring station and satellite Jan. 23 to Apr. 8 2020  − 45%  − 49%  − 56%  − 39%  − 18%  + 43% NOx (− 43%) Yin et al. (2021b)
Mexico City, Mexico Monitoring station and satellite April 2020  − 18.7% NOx (− 34.8%) Peralta et al. (2021)
Turkey Monitoring station April to May 2020  − 21.2%  − 28.6%  − 16.6%  − 18.8% NOx (− 39.9%) Alemdar et al. (2021)
Seoul and Daegu, Korea Monitoring station March 2020  − (31–36%)  − 25.4% Seo et al. (2020)
Xi’an, China Field sample collection and monitoring station 1st Jan 2020 to 7th March 2020  − 17%  − 27%  − 52%  − 16%  − 25%  + 160% WSIs (− 16%) Feng et al. (2021)
Elche, Spain Field sample collection and monitoring station 14th March to 18th May during 2015 to 2019 and 2020  − (decrease)  + (increase) NOx (− 60%);EC (− 65%) in PM1; OC (− 40%) in PM1 Clemente et al. (2022)
Tuscany, Italy Field sample collection and monitoring station 16th March 2020 to 12th April 2020  − 14%  − 39% EC (− 66%);OC (− 24%);S (+ 50%);Ca (− 38%); Cr (− 69%); Fe (− 66%); Cu (− 74%); Zn (− 40%); NH4+ (+ 206); K+ (+ 15%); NO3− (− 26%) Giardi et al. (2022)
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Overview of Studies (Outdoor and Indoor) During COVID-19 Lockdown Period

Outdoor Air Qualities During COVID-19 Lockdown Period

Many studies have been carried out in different cities of the world to understand the impact of lockdown on criteria pollutants (PM2.5, PM10, NO, NO2, NOx (NO + NO2), SO2, O3) concentration levels. The list of studies with the time duration, % reduction of pollutants concentration, location, and method of data collection was summarized in Table 1. The emission pattern over different cities changed during the lockdown period compared to before the lockdown period was summarized in Fig. 1. Among all criteria pollutants, NOX shows the highest reduction (− 3 to − 79%) followed by the PM2.5 and PM10 (− 40 to − 58%), CO (− 2 to − 60%) in different cities of India (Kumari et al. 2020; Singh et al. 2020; Chaudhary et al. 2021; Kumar and Yadav 2021). The average concentration level of PM2.5, PM10, NO, NO2, NOx (NO + NO2), SO2 decreases except for ozone over different cities of India (New Delhi, Kolkata, Chennai, Bangalore, Mumbai) (Mahato et al. 2020; Singh et al. 2020; Kumari et al. 2020; Rathod et al. 2021; Chaudhary et al. 2021; Kumar and Yadav 2021). The average reduction in pollutant concentrations over the monitoring station shows variability and is specific to the location of monitoring in different cities. The diversity in pollutant reduction over the monitoring station could be associated with the location of the monitoring station, nearby sources, and the regional meteorology of the monitoring station. Similarly, a significant reduction in criteria pollutants concentrations level is observed in several cities of China (Beijing, Yangtze River Delta, Wuhan, Eastern part of China, Mainland of China) during the lockdown period (Zhao et al. 2020; Wang et al. 2021a, 2021b; Zhang et al. 2021c).

Fig. 1.

Fig. 1

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The comparative analysis of emission pattern during the lockdown and before the lockdown period

The variation in reduced pollutant concentration is specific to monitoring stations and mainly associated with source-specific, location-specific, and meteorological conditions-specific over regions. Furthermore, pollutants concentration in the atmosphere mainly depends on strength of emission, chemical transformation mechanism, and rate of removal processes. The emission inventory analysis and literature survey suggest that air pollutants sources in the urban area are mainly vehicular emission, coal-based combustion, biomass burning activity, and resuspension of road dust (Fig. 1) (Yadav et al.2016; Yushin et al.2020; Hicks et al.2021; Soba et al.2022). In addition, natural sources such as windblown dust, dust storm, sea salt are also dominant contributors to natural aerosols in the atmosphere (Fig. 1) (Moschos et al. 2022; Liu et al.2022; Song et al.2022). On phase-wise implementation of worldwide lockdown largely affect the emission pattern on account of restriction in vehicular movement, public transport, train movement, airplane, and public movement. Further, emission patterns are not confined only to vehicular movement activity it also depends upon the number of vehicle, type of vehicle, resuspension of road dust activity, and public movement.

Zhao et al (2020) studied the effect of lockdown on criteria pollutants concentration and observed that NO2 shows the highest reduction followed by PM2.5, SO2, and CO except ozone. This sudden reduction in concentration level is largely regulated via a controlled emission pattern with significant meteorological factors. Few studies explain the reduction of pollutants levels using traffic volume and vehicular type (heavy and light vehicles) used in the area. The two Brazil cities (Sao Paulo and Rio de Janeiro) have extensively studied during the lockdown period for understanding the impact of the lockdown on PM10, CO, and NO2 (Siciliano et al. 2020). The highest reduction in CO (up to 100%) during the lockdown period is associated with light vehicle emissions. However, NO2 (9.1 to 41.8%) also shows a significant reduction followed by PM10 reduction only during the first phase of the lockdown. Such variation in reduction pattern is not only due to vehicular composition (type of vehicle) it is also influenced by air masses transported from the industrial area or rural area. The significant percentage reduction in PM10 (− 75%), SO2 (− 49%), and NO2 (− 96%) during the lockdown period (11th March to 2nd April 2020) compare to before the lockdown period (11th to 20th March 2020) was observed over Sale City (Morocco) (Otmani et al. 2020). The sudden reduction in SO2, NO2, and PM10 is attributed due to restrictions on vehicles, and public movement on-road (Ordonez et al. 2020; Otmani et al. 2020). However, in the trend of decreasing pollutants concentration, ozone shows either an increasing trend or remains unchanged in most of the cities during the lockdown period (Yin et al. 2021a; Peralta et al. 2021). The study was carried out over Mexico city during (March, April, and May 2020) and found the precursor of ozone decreased but the concentration remains the same compared to the previous year for a similar time duration. Further, a study suggested that with the implementation of the lockdown the ozone formation shifted from NOX-sensitive to VOCs-sensitive regions due to a sudden drop in NOx level.

Indoor Air Quality (IAQ) During COVID-19 Lockdown Period

On implementation of lockdown, people spend maximum time (up to 90–100%) within the home to break the chain of human-to-human transmission (Hassan et al. 2021; Phongphetkul et al. 2021; Adam et al. 2021). In the initial phase of lockdown due to strict guidelines, people are confined to home expect some essential services. Therefore, human beings within the home largely affect indoor air quality due to routine activities. Indoor activities such as cooking, cleaning, sweeping, and smoking affects the indoor air quality (Domínguez-Amarillo et al. 2020; Megahed and Ghoneim 2021; Elsaid and Ahmed 2021; Fadaei 2022) (Fig. 2).

Fig. 2.

Fig. 2

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The comparative analysis of indoor air quality emission pattern and risk of exposure in natural ventilation and poor ventilation condition

However, indoor air quality is also affected by outdoor activities and the degree of ventilation. Moreover, the presence of dust storm particles, vehicular emissions signature, and plant particle residue in the indoor environment is a clear indicator of indoor air quality affected by outdoor activities (Kuo and Shen 2010; Leung 2015; Peng et al. 2017). Kim et al (2015) have studied the indoor air quality (IAQ) of subway platforms is largely influenced by the poor air quality of the outdoor environment. Further, in extremely poor outdoor air quality conditions particulate matter gets entered the indoor environment through the ventilation system. Syafei et al (2020) have reported increased PM10 and total suspended particles (TSP) while reduced gaseous pollutants in a naturally ventilated building in 37 residential buildings in Indonesia. Further, they suggest exhaust and fan-induced ventilation provide the mixed result of reduced particle concentration and increased gaseous pollutants concentration. Blocken et al (2021) have observed that proper ventilation and cleaning in the Gym room reduced 80–90% of aerosol particles. Further, they suggest on the implementation of such a mechanism in indoor environments could improve indoor air quality. A significant reduction in CO2 concentration compared to formaldehyde was observed in the study of two ventilation approaches (exhaust and routine ventilation) strategies in 10 homes in Gainesville, Florida (Widder and Haselbach 2017). The positive association between di (2-ethylhexyl) phthalate (DEHP) and child respiratory diseases in an indoor environment indicates a high risk for child health (Sun et al. 2022). Studies based on field measurements of 454 residential buildings indicate quite high concentration levels compared to prescribing an indoor standard for CO2, PM2.5, PM10 followed by total volatile organic compounds (tVOCs) and formaldehyde (HCHO) in Shanghai city, China (Sun et al. 2022). The indoor environment of the home depends on the degree of ventilation and outdoor air quality of the atmosphere. However, indoor air is directly linked with the outdoor environment and affected via the chemical composition of outdoor air, and advection transport. The indoor practices (cooking, cleaning, smoking, biomass burning) largely affect the indoor chemical composition. The indoor air is constituted of ultrafine particles (UFP), PM2.5, VOCs, NOX, CO, polyaromatic hydrocarbon (PAHs) and other toxic hydrocarbons, and potentially toxic metals (Pb, As, Cd, Cr, Fe, Mn, Se, Zn, Ni) emitted from domestic activities (Pietrogrande et al. 2021; Mainka and Fantke 2022). The continuous exposure to PM2.5 and several toxic metals through ingestion, inhalation, and dermal pathways within the indoor environment is a major concern for preschool children's health (Mainka and Fantke 2022). Mechanical ventilation within indoor healthcare clinic significantly reduce the PM2.5, PM10, VOCs and CO2 level and shows a positive effect on COVID-19 case reduction (Tzoutzas et al. 2021). In other studies carried out on the air and surface of the intensive care unit general ward of the hospital indicate a significant role in the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) among the patient (Tan et al. 2020). Further, this study suggests that highly touched areas have a quite high number of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) compared to the least touched area. The indoor air quality severely degraded during the lockdown period resulting in negative health effects in South Texas during (May–July 2019) (Roh et al. 2021). Further, this study suggests the office is more safer compared to home in South Texas, USA during the lockdown period on account of lower PM2.5 and total volatile organic compounds (tVOCs). The comparative study to understand the variation in PM2.5, PM10, CO, CO2, temperature, relative humidity in the indoor and outdoor environments in Alexandria, Egypt is largely associated with socioeconomic status (Abdel-Salam 2022). In addition, variation in indoor air quality also functions the ventilation rate of the home, fuel quality used for cooking purposes and frequency of closing and opening the door (Korsavi et al. 2022). IAQ is mainly affected by the room size, meteorological parameters (temperature, relative humidity), level of occupancy, and activities performed inside the home such as cooking, and cleaning. (Lee and Chang 2000; Goyal and Khare 2011; Vicente et al. 2017; Hassan et al. 2021). In addition, cooking fuels (firewood, stove type, fuel moisture content), and ventilation of the room determined the IAQ (Parajuli et al. 2016; Mannan and Al-Ghamdi 2021). Further, it is also affected by outdoor activities such as local emissions, biomass burning, and the resuspension of road dust. The short-term emission (firecrackers burning, dust storms) of pollutants significantly affects the outdoor as well as the indoor environment (Kim et al. 2015; Kumar and Yadav 2016; Kumar et al. 2016).The Asian dust storm originated from an arid region and severely affect indoor and outdoor air quality on account of three-time elevated levels of PM2.5 and PM10 mass concentration (Kuo and Shen 2010).Similarly, dust storms originated from the Thar desert, India due to strong prevailing south-westerly (SW) wind degraded air quality over the downwind region (Delhi, India) (Yadav and Rajamani 2006; Sarkar et al. 2019; Kumar and Yadav 2020). Tippayawong et al (2009) monitored the outdoor and indoor size-resolved particulate matter within the school during November and December for weekends and weekdays in Chiang Mai, Thailand. Further, they suggest the highest contributor of particles in indoor environments due to exchange from the outdoor environment and it depends upon the rate of exchange of air. In addition, environmental and building variables such as building temperature, insulation, building walls, humidity, and seasonal variation affect the concentration of the pollutant in an indoor environment (Mendes et al. 2015; Ma et al. 2021). Matthaios et al (2022) have studied the possible factor affecting exposure to PM2.5, black carbon (BC), and NO2 in school classrooms in the northeast, United State of America (USA). Further, they suggested considerable mechanical ventilation, air exchange rate, and appropriate air exchange device, air conditioners reduced the outdoor contribution.

Air Pollution and COVID-19 Cases

A significant correlation between air pollution with confirmed COVID-19 cases was observed and further suggested that polluted air act as a medium for virus transmission among peoples (Fattorini and Regoli 2020; Velasquez and Lara 2020; Fareed et al. 2020; Ram et al. 2021; Marques and Domingo 2022). Polluted air is a complex mixture of toxic gases and particulate matter with a wide range of sizes. However, aerosols are suspensions of solid or liquid in the atmosphere. The severely infected person has flu-like symptom and their sneezing activity release a huge amount of water droplets in an atmosphere loaded with the virus. Further, released liquid droplets in the atmosphere on interaction with suspended particulate matter in the atmosphere at breathing level height act as a carrier for virus transmission to another human through inhalation processes. Zhu et al (2020) have observed a statistically significant relationship between air pollution and the number of COVID-19 confirmed cases from 23rd January 2020 to 29th February 2020 over 120 cities, China. Further, an additive model-based study confirmed that 10 µg m−3 increased mass concentration imposed diverse effects on a number of increased confirmed COVID-19 cases. Further, another study carried out over 235 Chinese cities using generalized additive models (GAMs) suggests a reduction in pollutants concentration reduced virus transmission (Zhang et al. 2021c). However, on adjustment of PM2.5 mass concentration with population age, sex, and a number of the food market, shows clear conclusive evidence for a positive association of air pollution with COVID-19 confirmed cases. Another study suggests the elevated PM2.5 and NO2 level in the atmosphere amplify the rate of COVID-19 infection and associated comorbidity (Velasquez and Lara 2020; Sharma and Balyan 2020). In addition, a study carried out over 29 provinces, in mainland China between 21st January 2020 and 3rd April 2020 indicates that increased CO level amplifies the transmissibility of the COVID-19 virus among human beings (Lin et al. 2020). The increasing risk of infection with the COVID-19 virus is directly associated with a high mass concentration of PM2.5 followed by PM10 over nine Asian cities (Gupta et al. 2021). Frontera et al (2020) proposed the double-hit hypothesis with elevated PM2.5 mass concentration and their impact on overexpression of angiotensin-converting enzyme-2 (ACE-2) which is an important receptor for severe acute respiratory syndrome coronavirus-2(SARS-CoV-2) infection. Domingo et al (2020) reviewed the research articles on the airborne transmission of SARS-CoV-2 during COVID-19 and observed the positive association of the severity of COVID-19 infection with certain air pollutants. The case fatality rate (CFR) in 49 Chinese cities due to COVID-19 is directly linked to elevated PM2.5 and PM10 mass concentration (10 µg m−3) (Yao et al. 2020). The reduced air pollution and social distancing behavior could control virus transmission among human beings during the COVID-19 pandemic (Vasquez-Apestegui et al. 2021).

Ozone Level COVID-19 Lockdown Period

As mentioned in Table 1 the mass concentration of ozone increases during the lockdown period in different cities of the world. However, the ozone concentration during the lockdown period is largely associated with the location, source, and meteorological condition of a region. The rate of emissions was significantly reduced during the lockdown period in most regions of the world (Sicard 2021). Therefore, the concentration level of ozone remains unchanged or increases in the atmosphere in most of the cities. The increasing pattern of ozone is not uniform over all the cities. The formation of ozone in the troposphere is largely regulated via the NO/NO2, VOCs, and HOx cycles (Fig. 3).

Fig.3.

Fig.3

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The general overview of O3 formation in the atmosphere. The reaction (R1, R2, R3) shows the formation and loss of O3 while the reaction (R4, R5, R6) shows the role of NO2 in O3 formation. The reaction (R7, R8) shows the VOCs and OH radicals in O3 formation. (Figure are modified from Source: Kroll et al. 2020)

Therefore, the photochemistry of O3 in the troposphere is most likely similar to low-temperature combustion induced via VOCs and NOx released from multiple sources. The formation of ozone is favored by NO2 while NO participates in the destruction of ozone. The high NO2/NO ratio during the lockdown period favored ozone formation on account of reduced high-temperature burning activities. Further, VOCs fueled the formation of NO2 and amplify the ozone level in the troposphere. The whole photochemistry in the troposphere is either NOx-limited or VOCs limited and associated with their emission pattern (Zheng et al. 2020; Liu et al. 2021; Fu et al. 2021). The atmospheric oxidation capacity (AOC) is largely affected by the change in emission pattern during the COVID-19 lockdown period. The sudden restriction in human activities, industrial emissions, and vehicular emissions reduce the NOx emission resulting change in AOC. The atmospheric oxidation governs via VOCs, NOx (NO + NO2), and HOx (OH + HO2) concentration levels over a region (Lelieveld et al. 2008; Moiseenko et al. 2021; Wang et al. 2021a). During the lockdown period, NOX is reduced manifold (> 45%) of time over different cities of the world (Singh et al. 2020; Shehzad et al. 2020). Wang et al. (2021a) have studied the AOC using Community Multi-scale Air Quality (CMAQ) model over the Yangtze River Delta (YRD) and observed more than (> 50%) reduction in NOx emission while O3 increased (+ 12%) during the lockdown period over different locations. Further, they have suggested the increased O3 level could be due to an increased level of atmospheric oxidants (up to 25%) in the ambient atmosphere. The NOx (NO + NO2) is a key component of atmospheric chemistry and acts as a precursor for the formation of ground-level O3 (Mousavinezhad et al. 2021). It is mainly emitted from vehicular emissions, high-temperature burning, and fossil fuel burning. The NO is emitted into the atmosphere either from natural or anthropogenic sources. Further, vehicular emissions, thermal power plants, and high-temperature burning activities are one of the dominant sources of NO in the atmosphere. The emitted NO on oxidation produce NO2 which is harmful to human health and the ecosystem. In addition, biologically emitted N2O from the soil, swamp and anaerobic condition in reaction with reactive oxygen produce the NO and further on oxidation convert it to NO2. In the presence of light, NO2 undergoes dissociation to produce reactive oxygen. The produced reactive oxygen atom in reaction 1 (R1) on interaction with oxygen molecule form ozone in the atmosphere (Liu and Shi 2021).

NO2+hυ→NO+O R1
O2+O→O3 R2
NO+O3→NO2 R3

The interaction of VOCs with HOx (OH + HO2) in the presence of NO2 significantly increased the rate of O3 formation. The VOCs include C5 hydrocarbon (isoprene), C10 (monoterpenes), C15 (sesquiterpenes) mainly emitted from natural sources (Lelieveld et al. 2008). The emitted VOCs undergo a series of chemical reactions and amplify the tropospheric O3 level in the atmosphere (Lelieveld et al. 2008; Zheng et al. 2020).

RH+OH→RO2+H2O R4
RO2+NO→RCHO+HO2+NO2 R5

The reaction (R1, R2, R3, R4, R5) clearly shows the O3 formation is either NOx driven or VOCs driven in the troposphere. The O3 formation is explained on the basis of the formation of oxygen free radical on photo-dissociation as well as the formation of NO2. The oxygen-free radical further reacts with the O2 molecule to form O3. On the other hand, hydroxyl radicals produced in the atmosphere react with VOCs and form peroxy radicals. The generated peroxy radicals undergo a reaction with nitrogen oxide and produce nitrogen dioxide. The VOCs-induced NO2 undergoes further photochemical dissociation and produce O3 in the troposphere. Overall, on the linear increase in O3 level with an increase in NOx concentration in the atmosphere is considered as NOx limited O3 formation while on a linear increase O3 concentration with VOCs is considered as VOCs limited O3 formation.

Factors Affecting Air Pollution (Indoor and Outdoor)

Source and Meteorology

The ambient air quality is largely regulated via the source, meteorology, and seasonal variation of the region and atmospheric chemistry (Guttikunda and Gurjar 2012; Hou et al. 2019; Kroll et al. 2020). On phase-wise implementation of lockdowns over different countries mainly affects the emission pattern resulting in variations in local and regional air quality (Tian et al. 2020; Gu et al. 2022). The study carried out on source apportionment, emission inventory analysis suggests the main sources in an urban environment include industries, vehicular emission, coal-based burning industries, biomass burning, and vehicular-induced resuspension of road dust (Chen et al. 2020; Li et al. 2021; Hu et al. 2021; Tohidi et al. 2022; Xiang et al. 2022). In addition, windblown dust, dust storm, volcanic eruption, forest fire are the ultimate source of natural emissions of atmospheric pollutants (Yadav and Rajamani 2006; Lee et al. 2021). In addition, the ambient concentration and chemistry are largely regulated via meteorological factors (wind speed, wind direction, relative humidity, planetary boundary layer or mixing height) of the regions (Kumar and Yadav 2016; Karle et al. 2021). The study conducted over the Yangtze River Delta (YRD) city cluster of China to observe the effect of source and meteorology on ozone formation reported 5.1 ppbv contribution for ozone formation regulated via precursor mediated while 0.5 ppbv contribution via meteorological factor (Zhang et al. 2021a). The other study carried out in Handan and the industrial city in China suggests that ozone formation in the ambient atmosphere is a combined effect of reduction in source and meteorology (Yao et al. 2021). Korhale et al. (2021) have studied the ozone formation over Mumbai and Pune is mainly regulated via VOCs/NOx ratio and during the entire lockdown period, ozone formation governs by VOCs limited. The ozone formation during normal and episodic periods over Pearl River Delta (PRD) region, China is largely regulated via NOx/VOCs ratio. Further, considering the meteorological approach with NOx/VOCs ratio maximizes the ozone control strategies (Yang et al. 2019). Source apportionment study carried out during the lockdown period (24th January 2020 to 10th February 2020) over coastal cities of southeast China conclude that a nonlinear response between the source of emission and chemical formation mechanism under reduced anthropogenic emission (Hong et al. 2021). Querol et al (2021) have observed the impact of lockdown on air quality over 11 metropolises cities in Spain and reported nonlinear reduction on account of reduced traffic-related emissions, biomass burning, and agricultural activity. The hourly mass concentration of PM2.5 was collected and analyzed from 12th January 2020 to 2nd April 2020 over Xianghe a rural sampling site to understand the major sources of atmospheric particles (Cui et al. 2020). Further, air mass cluster analysis and positive matrix factorization suggest firework burning, coal burning, and vehicular emission are the main sources.

COVID-19 Lockdown as the Scope of Future Research

The nationwide lockdown implemented due to a steep surge of COVID-19 cases by the governments of different countries to avoid the human to human transmission provides a unique natural condition to understand air pollution. The sudden lockdown provides a reduced concentration level of NOx, PM2.5, PM10, and other pollutants in the ambient atmosphere. Major cities of the world face severe air pollution problems and annually spend huge amounts of money to achieve the set target. The imposed lockdown provides the natural laboratory to understand the changed atmospheric chemistry in the scientific community. The model-based, monitoring station, and field-based measurement studies try to understand the effect of reduced pollutant concentration on atmospheric chemistry. The reduced criteria pollutants except for ozone in the troposphere are still a mystery for the scientific community to understand the main cause of the elevated condition. However, the indoor air quality during the lockdown period degraded due to increased stay time inside the home and indoor activities. Therefore, suddenly increased pollutant concentration and poor indoor air quality severely affect human health. Such natural condition and their link with the outdoor environment are still limited and need to be explored in the future.

Conclusion

The outdoor and indoor air quality is mainly affected by the phase-wise lockdown in different cities of the world. The outdoor air quality showed a significant reduction and mainly depended upon the location of the installed monitoring station, field-based sample collection point, and selected study area. Further, the air quality during the selected time duration also depends upon the nearby source of the monitoring station as well as local meteorological conditions. During the entire period of lockdown, the average mass concentration of criteria pollutants decreases except ozone. The sudden decreases in pollutants level after the implementation of the lockdown suggest that human-induced emissions are a major contributor. However, the elevated ozone level in the atmosphere is the result of changed atmospheric chemistry. In contrast, indoor air quality showed an entirely different pattern, and most of the studies showed poor indoor air quality during the lockdown period. The indoor air quality is mostly regulated by indoor activities such as cooking, cleaning, and sweeping processes. Further, the air quality is associated with active COVID-19 cases in different cities. The suspended water droplets released from sneezing activity on interaction with atmospheric particles participate in the virus transmission. The change in atmospheric pollutant concentration during the lockdown period is the scope of future research to understand atmospheric chemistry.

Acknowledgements

The author is thankful to anonymous reviewers and the associate editor for their critical comments which have helped to improve the MS. The author is also thankful to the Department of Environmental Studies, Siksha Bhavana, Visva-Bharati, Santiniketan, West Bengal, India. The present work did not use any financial support.

Data availability

The relevant information used in present review article are collected from different published research work. Hence no any primary data are use.

Declarations

Conflict of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

  1. Abdel-Salam MM. Relationship between residential indoor air quality and socioeconomic factors in two urban areas in Alexandria. Egypt. Build Environ. 2022;207:108425. doi: 10.1016/j.buildenv.2021.108425. [DOI] [Google Scholar]
  2. Adam MG, Tran PT, Balasubramanian R. Air quality changes in cities during the COVID-19 lockdown: a critical review. Atmos Res. 2021;264:105823. doi: 10.1016/j.atmosres.2021.105823. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Alemdar KD, Kaya O, Canale A, Çodur MY, Campisi T. Evaluation of air quality index by spatial analysis depending on vehicle traffic during the COVID-19 outbreak in Turkey. Energies. 2021;14:5729. doi: 10.3390/en14185729. [DOI] [Google Scholar]
  4. Baldasano JM. COVID-19 lockdown effects on air quality by NO2 in the cities of Barcelona and Madrid (Spain) Sci Total Environ. 2020;741:140353. doi: 10.1016/j.scitotenv.2020.140353. [DOI] [PubMed] [Google Scholar]
  5. Bhat MA, Eraslan FN, Awad A, Malkoc S, Uzmez OO, Dogeroglu T, Gaga EO. Investigation of indoor and outdoor air quality in a university campus during COVID-19 lock down period. Build Environ. 2022 doi: 10.1016/j.buildenv.2022.109176. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Blocken B, van Druenen T, Ricci A, Kang L, van Hooff T, Qin P, Xia L, Ruiz CA, Arts JH, Diepens JFL, Maas GA. Ventilation and air cleaning to limit aerosol particle concentrations in a gym during the COVID-19 pandemic. Build Environ. 2021;193:107659. doi: 10.1016/j.buildenv.2021.107659. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Cazorla M, Herrera E, Palomeque E, Saud N. What the COVID-19 lockdown revealed about photochemistry and ozone production in Quito, Ecuador. Atmos Pollut Res. 2021;12:124–133. doi: 10.1016/j.apr.2020.08.028. [DOI] [PMC free article] [PubMed] [Google Scholar]
  8. Chatterjee A, Mukherjee S, Dutta M, Ghosh A, Ghosh SK, Roy A. High rise in carbonaceous aerosols under very low anthropogenic emissions over eastern Himalaya, India: impact of lockdown for COVID-19 outbreak. Atmos Environ. 2021;244:117947. doi: 10.1016/j.atmosenv.2020.117947. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Chaudhary S, Kumar S, Antil R, Yadav S. Air quality before and after COVID-19 lockdown phases around New Delhi. India. J Health Pollut. 2021;11:210602. doi: 10.5696/2156-9614-11.30.210602. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. ChenY WO, Ryan E, Sahu SK, Lowe D, Archer-Nicholls S, Wang Y, McFiggans G, Ansari T, Singh V, Sokhi RS. Mitigation of PM2.5 and ozone pollution in Delhi: a sensitivity study during the pre-monsoon period. Atmos Chem Phys. 2020;20:499–514. doi: 10.5194/acp-20-499-2020. [DOI] [Google Scholar]
  11. Clemente A, Yubero E, Nicolas JF, Caballero S, Crespo J, Galindo N. Changes in the concentration and composition of urban aerosols during the COVID-19 lockdown. Environ Res. 2022;203:111788. doi: 10.1016/j.envres.2021.111788. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Collivignarelli MC, Abba A, Bertanza G, Pedrazzani R, Ricciardi P, Miino MC. Lockdown for CoViD-2019 in Milan: what are the effects on air quality? Sci Total Environ. 2020;732:139280. doi: 10.1016/j.scitotenv.2020.139280. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Cui Y, Ji D, Maenhaut W, Gao W, Zhang R, Wang Y. Levels and sources of hourly PM2.5 -related elements during the control period of the COVID-19 pandemic at a rural site between Beijing and Tianjin. Sci Total Environ. 2020;744:140840. doi: 10.1016/j.scitotenv.2020.140840. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Domingo JL, Marques M, Rovira J. Influence of airborne transmission of SARS-CoV-2 on COVID-19 pandemic. A review. Environ Res. 2020;188:109861. doi: 10.1016/j.envres.2020.109861. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. Dominguez-Amarillo S, Fernandez-Aguera J, Cesteros-Garcia S, Gonzalez-Lezcano RA. Bad air can also kill: residential indoor air quality and pollutant exposure risk during the COVID-19 crisis. Int J Environ Res Public Health. 2020;17:7183. doi: 10.3390/ijerph17197183. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Elsaid AM, Ahmed MS. Indoor air quality strategies for air-conditioning and ventilation systems with the spread of the global coronavirus (COVID-19) epidemic: improvements and recommendations. Environ Res. 2021;199:111314. doi: 10.1016/j.envres.2021.111314. [DOI] [PMC free article] [PubMed] [Google Scholar]
  17. Ezani E, Brimblecombe P, Ashaari ZH, Fazil AA, Ismail SNS, Ramly ZTA, Khan MF. Indoor and outdoor exposure to PM2.5 during COVID-19 lockdown in suburban Malaysia. Aerosol Air Qual Res. 2021;21:200476. doi: 10.4209/aaqr.2020.07.0476. [DOI] [Google Scholar]
  18. Fadaei A. Investigation of outdoor/indoor air quality during the outbreak of COVID-19: a review study. Eur J Sustain Dev Res. 2022;6:0180. doi: 10.21601/ejosdr/11726. [DOI] [Google Scholar]
  19. Fareed Z, Iqbal N, Shahzad F, Shah SGM, Zulfiqar B, Shahzad K, Hashmi SH, Shahzad U. Co-variance nexus between COVID-19 mortality, humidity, and air quality index in Wuhan, China: new insights from partial and multiple wavelet coherence. Air Qual Atmos Health. 2020;13:673–682. doi: 10.1007/s11869-020-00847-1. [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Faridi S, Yousefian F, Janjani H, Niazi S, Azimi F, Naddafi K, Hassanvand MS. The effect of COVID-19 pandemic on human mobility and ambient air quality around the world: a systematic review. Urban Clim. 2021;38:100888. doi: 10.1016/j.uclim.2021.100888. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Fattorini D, Regoli F. Role of the chronic air pollution levels in the Covid-19 outbreak risk in Italy. Environ Pollut. 2020;264:114732. doi: 10.1016/j.envpol.2020.114732. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Feng R, Xu H, Wang Z, Gu Y, Liu Z, Zhang H, Zhang T, Wang Q, Zhang Q, Liu S, Shen Z. Quantifying air pollutant variations during COVID-19 lockdown in a capital city in Northwest China. Atmosphere. 2021;12:788. doi: 10.3390/atmos12060788. [DOI] [Google Scholar]
  23. Frontera A, Cianfanelli L, Vlachos K, Landoni G, Cremona G. Severe air pollution links to higher mortality in COVID-19 patients: The “double-hit” hypothesis. J Infect. 2020;81:255–259. doi: 10.1016/j.jinf.2020.05.031. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Fu S, Guo M, Fan L, Deng Q, Han D, Wei Y, Luo J, Qin G, Cheng J. Ozone pollution mitigation in guangxi (south China) driven by meteorology and anthropogenic emissions during the COVID-19 lockdown. Environ Pollut. 2021;272:115927. doi: 10.1016/j.envpol.2020.115927. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Giardi F, Nava S, Calzolai G, Pazzi G, Chiari M, Faggi A, Andreini BP, Collaveri C, Franchi E, Nincheri G, Amore A. PM10 variation, composition, and source analysis in Tuscany (Italy) following the COVID-19 lockdown restrictions. Atmos Chem Phys. 2022;22:9987–10005. doi: 10.5194/acp-22-9987-2022. [DOI] [Google Scholar]
  26. Goyal R, Khare M. Indoor air quality modeling for PM10, PM2.5, and PM1.0 in naturally ventilated classrooms of an urban Indian school building. Environ Monit Assess. 2011;176:501–516. doi: 10.1007/s10661-010-1600-7. [DOI] [PubMed] [Google Scholar]
  27. Gu Y, Liu B, Dai Q, Zhang Y, Zhou M, Feng Y, Hopke PK. Multiply improved positive matrix factorization for source apportionment of volatile organic compounds during the COVID-19 shutdown in Tianjin China. Environ Int. 2022;158:106979. doi: 10.1016/j.envint.2021.106979. [DOI] [PubMed] [Google Scholar]
  28. Gupta A, Bherwani H, Gautam S, Anjum S, Musugu K, Kumar N, Anshul A, Kumar R. Air pollution aggravating COVID-19 lethality? Exploration in Asian cities using statistical models. Environ Dev Sustain. 2021;23:6408–6417. doi: 10.1007/s10668-020-00878-9. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Guttikunda SK, Gurjar BR. Role of meteorology in seasonality of air pollution in megacity Delhi, India. Environ Monit Assess. 2012;184:3199–3211. doi: 10.1007/s10661-011-2182-8. [DOI] [PubMed] [Google Scholar]
  30. Hassan A, Zeeshan M, Bhatti MF. Indoor and outdoor microbiological air quality in naturally and mechanically ventilated university libraries. Atmos Pollut Res. 2021;12:101136. doi: 10.1016/j.apr.2021.101136. [DOI] [Google Scholar]
  31. Hicks W, Beevers S, Tremper AH, Stewart G, Priestman M, Kelly FJ, Lanoiselle M, Lowry D, Green DC. Quantification of non-exhaust particulate matter traffic emissions and the impact of COVID-19 lockdown at London Marylebone road. Atmosphere. 2021;12:190. doi: 10.3390/atmos12020190. [DOI] [Google Scholar]
  32. Hong Y, Xu X, Liao D, Zheng R, Ji X, Chen Y, Xu L, Li M, Wang H, Xiao H, Choi SD. Source apportionment of PM2.5 and sulfate formation during the COVID-19 lockdown in a coastal city of southeast China. Environ Pollut. 2021;286:117577. doi: 10.1016/j.envpol.2021.117577. [DOI] [PubMed] [Google Scholar]
  33. Hou X, Zhu B, Kumar KR, Lu W. Inter-annual variability in fine particulate matter pollution over China during 2013–2018: role of meteorology. Atmos Environ. 2019;214:116842. doi: 10.1016/j.atmosenv.2019.116842. [DOI] [Google Scholar]
  34. Hu C, Kang P, Jaffe DA, Li C, Zhang X, Wu K, Zhou M. Understanding the impact of meteorology on ozone in 334 cities of China. Atmos Environ. 2021;248:118221. doi: 10.1016/j.atmosenv.2021.118221. [DOI] [Google Scholar]
  35. Jain S, Sharma T. Social and travel lockdown impact considering coronavirus disease (COVID-19) on air quality in megacities of India: present benefits, future challenges and way forward. Aerosol Air Qual Res. 2020;20:1222–1236. doi: 10.4209/aaqr.2020.04.0171. [DOI] [Google Scholar]
  36. Jephcote C, Hansell AL, Adams K, Gulliver J. Changes in air quality during COVID-19 ‘lockdown’ in the United Kingdom. Environ Pollut. 2021;272:116011. doi: 10.1016/j.envpol.2020.116011. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Karle NN, Fitzgerald RM, Sakai RK, Sullivan DW, Stockwell WR. Multi-scale atmospheric emissions, circulation and meteorological drivers of ozone episodes in El Paso-Juarez Airshed. Atmosphere. 2021;12:1575. doi: 10.3390/atmos12121575. [DOI] [Google Scholar]
  38. Kim M, Braatz RD, Kim JT, Yoo C. Indoor air quality control for improving passenger health in subway platforms using an outdoor air quality dependent ventilation system. Build Environ. 2015;92:407–417. doi: 10.1016/j.buildenv.2015.05.010. [DOI] [Google Scholar]
  39. Korhale N, Anand V, Beig G. Disparity in ozone trends under COVID-19 lockdown in a closely located coastal and hillocky metropolis of India. Air Qual Atmos Health. 2021;14:533–542. doi: 10.1007/s11869-020-00958-9. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Korsavi SS, Jones RV, Fuertes A. Operations on windows and external doors in UK primary schools and their effects on indoor environmental quality. Build Environ. 2022;207:108416. doi: 10.1016/j.buildenv.2021.108416. [DOI] [Google Scholar]
  41. Kroll JH, Heald CL, Cappa CD, Farmer DK, Fry JL, Murphy JG, Steiner AL. The complex chemical effects of COVID-19 shutdowns on air quality. Nat Chem. 2020;12:777–779. doi: 10.1038/s41557-020-0535-z. [DOI] [PubMed] [Google Scholar]
  42. Kumar P, Yadav S. Seasonal variations in water soluble inorganic ions, OC and EC in PM10 and PM>10 aerosols over Delhi: Influence of sources and meteorological factors. Aerosol Air Qual Res. 2016;16:1165–1178. doi: 10.4209/aaqr.2015.07.0472. [DOI] [Google Scholar]
  43. Kumar S, Yadav S. Chemistry of size-segregated particles: study of sources and processes in N-NW India. Atmos Pollut Res. 2020;11:370–382. doi: 10.1016/j.apr.2019.11.009. [DOI] [Google Scholar]
  44. Kumar S, Yadav S. Environmental resilience and transformation in times of COVID-19. New York: Elsevier; 2021. Air quality index and criteria pollutants in ambient atmosphere over selected sites: impact and lessons to learn from COVID-19; pp. 153–162. [Google Scholar]
  45. Kumar P, Kumar R, Yadav S. Water-soluble ions and carbon content of size-segregated aerosols in New Delhi, India: direct and indirect influences of firework displays. Environ Sci Pollut Res. 2016;23:20749–20760. doi: 10.1007/s11356-016-7313-x. [DOI] [PubMed] [Google Scholar]
  46. Kumari S, Lakhani A, Kumari KM. COVID-19 and air pollution in Indian cities: World’s most polluted cities. Aerosol Air Qual Res. 2020;20:2592–2603. doi: 10.4209/aaqr.2020.05.0262. [DOI] [Google Scholar]
  47. Kuo HW, Shen HY. Indoor and outdoor PM2.5 and PM10 concentrations in the air during a dust storm. Build Environ. 2010;45:610–614. doi: 10.1016/j.buildenv.2009.07.017. [DOI] [Google Scholar]
  48. Lee SC, Chang M. Indoor and outdoor air quality investigation at schools in Hong Kong. Chemosphere. 2000;41:109–113. doi: 10.1016/S0045-6535(99)00396-3. [DOI] [PubMed] [Google Scholar]
  49. Lee JD, Squires FA, Sherwen T, Wilde SE, Cliff SJ, Carpenter LJ, Hopkins JR, Bauguitte SJ, Reed C, Barker P, Allen G. Ozone production and precursor emission from wildfires in Africa. Environ Sci Atmos. 2021;1:524–542. doi: 10.1039/D1EA00041A. [DOI] [Google Scholar]
  50. Lelieveld J, Butler TM, Crowley JN, Dillon TJ, Fischer H, Ganzeveld L, Harder H, Lawrence MG, Martinez M, Taraborrelli D, Williams J. Atmospheric oxidation capacity sustained by a tropical forest. Nature. 2008;452:737–740. doi: 10.1038/nature06870. [DOI] [PubMed] [Google Scholar]
  51. Leung DY. Outdoor-indoor air pollution in urban environment: challenges and opportunity. Front Environ Sci. 2015;2:69. doi: 10.3389/fenvs.2014.00069. [DOI] [Google Scholar]
  52. Li M, Wang T, Shu L, Qu Y, Xie M, Liu J, Wu H, Kalsoom U. Rising surface ozone in China from 2013 to 2017: a response to the recent atmospheric warming or pollutant controls? Atmos Environ. 2021;246:118130. doi: 10.1016/j.atmosenv.2020.118130. [DOI] [Google Scholar]
  53. Li X, Baumgartner J, Harper S, Zhang X, Sternbach T, Barrington-Leigh C, Brehmer C, Robinson B, Shen G, Zhang Y, Tao S. Field measurements of indoor and community air quality in rural Beijing before, during, and after the COVID-19 lockdown. Indoor Air. 2022;32:13095. doi: 10.1111/ina.13095. [DOI] [PMC free article] [PubMed] [Google Scholar]
  54. Lin S, Wei D, Sun Y, Chen K, Yang L, Liu B, Huang Q, Paoliello MMB, Li H, Wu S. Region-specific air pollutants and meteorological parameters influence COVID-19: a study from mainland China. Ecotoxicol Environ Saf. 2020;204:111035. doi: 10.1016/j.ecoenv.2020.111035. [DOI] [PMC free article] [PubMed] [Google Scholar]
  55. Liu C, Shi K. A review on methodology in O3-NOx-VOC sensitivity study. Environ Pollut. 2021;291:118249. doi: 10.1016/j.envpol.2021.118249. [DOI] [PubMed] [Google Scholar]
  56. Liu Y, Wang T, Stavrakou T, Elguindi N, Doumbia T, Granier C, Bouarar I, Gaubert B, Brasseur GP. Diverse response of surface ozone to COVID-19 lockdown in China. Sci Total Environ. 2021;789:147739. doi: 10.1016/j.scitotenv.2021.147739. [DOI] [PMC free article] [PubMed] [Google Scholar]
  57. Liu X, Hadiatullah H, Schnelle-Kreis J, Xu Y, Yue M, Zhang X, Querol X, Cao X, Bendl J, Cyrys J, Jakobi G. Levels and drivers of urban black carbon and health risk assessment during pre-and COVID-19 lockdown in Augsburg, Germany. Environ Pollut. 2022 doi: 10.1016/j.envpol.2022.120529. [DOI] [PubMed] [Google Scholar]
  58. Ma N, Aviv D, Guo H, Braham WW. Measuring the right factors: a review of variables and models for thermal comfort and indoor air quality. Renew Sustain Energy Rev. 2021;135:110436. doi: 10.1016/j.rser.2020.110436. [DOI] [Google Scholar]
  59. Mahato S, Pal S, Ghosh KG. Effect of lockdown amid COVID-19 pandemic on air quality of the megacity Delhi. India. Sci Total Environ. 2020;730:139086. doi: 10.1016/j.scitotenv.2020.139086. [DOI] [PMC free article] [PubMed] [Google Scholar]
  60. Mainka A, Fantke P. Preschool children health impacts from indoor exposure to PM2.5 and metals. Environ Int. 2022;160:107062. doi: 10.1016/j.envint.2021.107062. [DOI] [PubMed] [Google Scholar]
  61. Mannan M, Al-Ghamdi SG. Indoor air quality in buildings: a comprehensive review on the factors influencing air pollution in residential and commercial structure. Int J Environ Res Public Health. 2021;18:3276. doi: 10.3390/ijerph18063276. [DOI] [PMC free article] [PubMed] [Google Scholar]
  62. Marques M, Domingo JL. Positive association between outdoor air pollution and the incidence and severity of COVID-19. A review of the recent scientific evidences. Environ Res. 2022;203:111930. doi: 10.1016/j.envres.2021.111930. [DOI] [PMC free article] [PubMed] [Google Scholar]
  63. Matthaios VN, Kang CM, Wolfson JM, Greco KF, Gaffin JM, Hauptman M, Cunningham A, Petty CR, Lawrence J, Phipatanakul W, Gold DR. Factors influencing classroom exposures to fine particles, black carbon, and nitrogen dioxide in inner-city schools and their implications for indoor air quality. Environ Health Perspect. 2022;130:047005. doi: 10.1289/EHP10007. [DOI] [PMC free article] [PubMed] [Google Scholar]
  64. Megahed NA, Ghoneim EM. Indoor air quality: rethinking rules of building design strategies in post-pandemic architecture. Environ Res. 2021;193:110471. doi: 10.1016/j.envres.2020.110471. [DOI] [PMC free article] [PubMed] [Google Scholar]
  65. Mendes A, Bonassi S, Aguiar L, Pereira C, Neves P, Silva S, Mendes D, Guimarães L, Moroni R, Teixeira JP. Indoor air quality and thermal comfort in elderly care centers. Urban Clim. 2015;14:486–501. doi: 10.1016/j.uclim.2014.07.005. [DOI] [Google Scholar]
  66. Moiseenko KB, Vasileva AV, Skorokhod AI, Belikov IB, Repin AY, Shtabkin YA. Regional impact of ozone precursor emissions on NOX and O3 levels at ZOTTO tall tower in Central Siberia. Earth Space Sci. 2021 doi: 10.1029/2021EA001762. [DOI] [Google Scholar]
  67. Moschos V, Dzepina K, Bhattu D, Lamkaddam H, Casotto R, Daellenbach KR, Canonaco F, Rai P, Aas W, Becagli S, Calzolai G. Equal abundance of summertime natural and wintertime anthropogenic Arctic organic aerosols. Nat Geosci. 2022;15:196–202. doi: 10.1038/s41561-021-00891-1. [DOI] [PMC free article] [PubMed] [Google Scholar]
  68. Mousavinezhad S, Choi Y, Pouyaei A, Ghahremanloo M, Nelson DL. A comprehensive investigation of surface ozone pollution in China, 2015–2019: separating the contributions from meteorology and precursor emissions. Atmos Res. 2021;257:105599. doi: 10.1016/j.atmosres.2021.105599. [DOI] [Google Scholar]
  69. Muhammad S, Long X, Salman M. COVID-19 pandemic and environmental pollution: a blessing in disguise? Sci Total Environ. 2020;728:138820. doi: 10.1016/j.scitotenv.2020.138820. [DOI] [PMC free article] [PubMed] [Google Scholar]
  70. Nakada LYK, Urban RC. COVID-19 pandemic: Impacts on the air quality during the partial lockdown in Sao Paulo state. Brazil. Sci Total Environ. 2020;730:139087. doi: 10.1016/j.scitotenv.2020.139087. [DOI] [PMC free article] [PubMed] [Google Scholar]
  71. Ordonez C, Garrido-Perez JM, Garcia-Herrera R. Early spring near-surface ozone in Europe during the COVID-19 shutdown: Meteorological effects outweigh emission changes. Sci Total Environ. 2020;747:141322. doi: 10.1016/j.scitotenv.2020.141322. [DOI] [PMC free article] [PubMed] [Google Scholar]
  72. Otmani A, Benchrif A, Tahri M, Bounakhla M, El Bouch M, Krombi MH. Impact of Covid-19 lockdown on PM10, SO2 and NO2 concentrations in Sale City (Morocco) Sci Total Environ. 2020;735:139541. doi: 10.1016/j.scitotenv.2020.139541. [DOI] [PMC free article] [PubMed] [Google Scholar]
  73. Parajuli I, Lee H, Shrestha KR. Indoor air quality and ventilation assessment of rural mountainous households of Nepal. Int J Sustain Built Environ. 2016;5:301–311. doi: 10.1016/j.ijsbe.2016.08.003. [DOI] [Google Scholar]
  74. Peng Z, Deng W, Tenorio R. Investigation of indoor air quality and the identification of influential factors at primary schools in the North of China. Sustainability. 2017;9:1180. doi: 10.3390/su9071180. [DOI] [Google Scholar]
  75. Peralta O, Ortinez-Alvarez A, Torres-Jardon R, Suarez-Lastra M, Castro T, Ruiz-Suarez LG. Ozone over Mexico City during the COVID-19 pandemic. Sci Total Environ. 2021;761:143183. doi: 10.1016/j.scitotenv.2020.143183. [DOI] [PMC free article] [PubMed] [Google Scholar]
  76. Phongphetkul P, Mangkang S, Praditsmanont A, Intrachooto S, Choruengwiwat J, Treesubsuntorn C, Thiravetyan P. Evaluation of indoor air quality in high-rise residential buildings in Bangkok and factor analysis. Environ Monit Assess. 2021;193:1–11. doi: 10.1007/s10661-020-08792-3. [DOI] [PubMed] [Google Scholar]
  77. Pietrogrande MC, Casari L, Demaria G, Russo M. Indoor air quality in domestic environments during periods close to Italian COVID-19 lockdown. Int J Environ Res Public Health. 2021;18:4060. doi: 10.3390/ijerph18084060. [DOI] [PMC free article] [PubMed] [Google Scholar]
  78. Querol X, Massague J, Alastuey A, Moreno T, Gangoiti G, Mantilla E, Dueguez JJ, Escudero M, Monfort E, Garcia-Pando CP, Petetin H. Lessons from the COVID-19 air pollution decrease in Spain: now what? Sci Total Environ. 2021;779:146380. doi: 10.1016/j.scitotenv.2021.146380. [DOI] [PMC free article] [PubMed] [Google Scholar]
  79. Rahman MS, Azad MAK, Hasanuzzaman M, Salam R, Islam ARMT, Rahman MM, Hoque MMM. How air quality and COVID-19 transmission change under different lockdown scenarios? A case from Dhaka city. Bangladesh. Sci Total Environ. 2021;762:143161. doi: 10.1016/j.scitotenv.2020.143161. [DOI] [PMC free article] [PubMed] [Google Scholar]
  80. Ram K, Thakur RC, Singh DK, Kawamura K, Shimouchi A, Sekine Y, Nishimura H, Singh SK, Pavuluri CM, Singh RS, Tripathi SN. Why airborne transmission hasn’t been conclusive in case of COVID-19? An atmospheric science perspective. Sci Total Environ. 2021;773:145525. doi: 10.1016/j.scitotenv.2021.145525. [DOI] [PMC free article] [PubMed] [Google Scholar]
  81. Rathod A, Sahu SK, Singh S, Beig G. Anomalous behaviour of ozone under COVID-19 and explicit diagnosis of O3-NOx-VOCs mechanism. Heliyon. 2021;7:06142. doi: 10.1016/j.heliyon.2021.e06142. [DOI] [PMC free article] [PubMed] [Google Scholar]
  82. Rodriguez-Urrego D, Rodriguez-Urrego L. Air quality during the COVID-19: PM2.5 analysis in the 50 most polluted capital cities in the world. Environ Pollut. 2020 doi: 10.1016/j.envpol.2020.115042. [DOI] [PMC free article] [PubMed] [Google Scholar]
  83. Roh T, Moreno-Rangel A, Baek J, Obeng A, Hasan NT, Carrillo G. Indoor air quality and health outcomes in employees working from home during the COVID-19 pandemic: a pilot study. Atmosphere. 2021;12:1665. doi: 10.3390/atmos12121665. [DOI] [Google Scholar]
  84. Sarkar S, Chauhan A, Kumar R, Singh RP. Impact of deadly dust storms (May 2018) on air quality, meteorological, and atmospheric parameters over the northern parts of India. GeoHealth. 2019;3:67–80. doi: 10.1029/2018GH000170. [DOI] [PMC free article] [PubMed] [Google Scholar]
  85. Sarmadi M, Moghanddam VK, Dickerson AS, Martelletti L. Association of COVID-19 distribution with air quality, sociodemographic factors, and comorbidities: an ecological study of US states. Air Qual Atmos Health. 2021;14:455–465. doi: 10.1007/s11869-020-00949-w. [DOI] [PMC free article] [PubMed] [Google Scholar]
  86. Seo JH, Jeon HW, Sung UJ, Sohn JR. Impact of the COVID-19 outbreak on air quality in Korea. Atmosphere. 2020;11:1137. doi: 10.3390/atmos11101137. [DOI] [Google Scholar]
  87. Sharma S, Zhang M, Gao J, Zhang H, Kota SH. Effect of restricted emissions during COVID-19 on air quality in India. Sci Total Environ. 2020;728:138878. doi: 10.1016/j.scitotenv.2020.138878. [DOI] [PMC free article] [PubMed] [Google Scholar]
  88. Sharma AK, Balyan P (2020) Air pollution and COVID-19: Is the connect worth its weight? Indian J Public Health. 64:132. https://www.ijph.in/text.asp?2020/64/6/132/285597 [DOI] [PubMed]
  89. Shehzad K, Sarfraz M, Shah SGM. The impact of COVID-19 as a necessary evil on air pollution in India during the lockdown. Environ Pollut. 2020;266:115080. doi: 10.1016/j.envpol.2020.115080. [DOI] [PMC free article] [PubMed] [Google Scholar]
  90. Sicard P. Ground-level ozone over time: an observation-based global overview. Curr Opin Environ Sci Health. 2021;19:100226. doi: 10.1016/j.coesh.2020.100226. [DOI] [Google Scholar]
  91. Sicard P, De Marco A, Agathokleous E, Feng Z, Xu X, Paoletti E, Rodriguez JJD, Calatayud V. Amplified ozone pollution in cities during the COVID-19 lockdown. Sci Total Environ. 2020;735:139542. doi: 10.1016/j.scitotenv.2020.139542. [DOI] [PMC free article] [PubMed] [Google Scholar]
  92. Siciliano B, Carvalho G, da Silva CM, Arbilla G. The impact of COVID-19 partial lockdown on primary pollutant concentrations in the atmosphere of Rio de Janeiro and São Paulo Megacities (Brazil) Bull Environ Contam Toxicol. 2020;105:2–8. doi: 10.1007/s00128-020-02907-9. [DOI] [PMC free article] [PubMed] [Google Scholar]
  93. Singh V, Singh S, Biswal A, Kesarkar AP, Mor S, Ravindra K. Diurnal and temporal changes in air pollution during COVID-19 strict lockdown over different regions of India. Environ Pollut. 2020;266:115368. doi: 10.1016/j.envpol.2020.115368. [DOI] [PMC free article] [PubMed] [Google Scholar]
  94. Soba D, Gamez AL, Becerril JM, Esteban R, Aranjuelo I. Traffic restrictions during COVID-19 lockdown improve air quality and reduce metal biodeposition in tree leaves. Urban for Urban Gree. 2022;70:127542. doi: 10.1016/j.ufug.2022.127542. [DOI] [PMC free article] [PubMed] [Google Scholar]
  95. Song SK, Shon ZH, Bae MS, Cho SB, Moon SH, Kim HS, Son YB, Lee CR. Effects of natural and anthropogenic emissions on the composition and toxicity of aerosols in the marine atmosphere. Sci Total Environ. 2022;806:150928. doi: 10.1016/j.scitotenv.2021.150928. [DOI] [PubMed] [Google Scholar]
  96. Sun C, Huang X, Zhang J, Lu R, Su C, Huang C. The new model for evaluating indoor air quality based on childhood allergic and respiratory diseases in Shanghai. Build Environ. 2022;207:108410. doi: 10.1016/j.buildenv.2021.108410. [DOI] [Google Scholar]
  97. Syafei AD, Surahman U, Sembiring AC, Pradana AW, Ciptaningayu TN, Ahmad IS, Assomadi AF, Boedisantoso R, Slamet A, Hermana J (2020) Factors affecting the indoor air quality of middle-class apartments in major cities in Indonesia: a case study in Surabaya city. AIP Conf Proc 2296:020008. AIP Publishing LLC. 10.1063/5.0030403
  98. Tan L, Ma B, Lai X, Han L, Cao P, Zhang J, Fu J, Zhou Q, Wei S, Wang Z, Peng W. Air and surface contamination by SARS-CoV-2 virus in a tertiary hospital in Wuhan, China. Int J Infect Dis. 2020;99:3–7. doi: 10.1016/j.ijid.2020.07.027. [DOI] [PMC free article] [PubMed] [Google Scholar]
  99. Tian H, Liu Y, Li Y, Wu CH, Chen B, Kraemer MU, Li B, Cai J, Xu B, Yang Q, Wang B. An investigation of transmission control measures during the first 50 days of the COVID-19 epidemic in China. Science. 2020;368:638–642. doi: 10.1126/science.abb6105. [DOI] [PMC free article] [PubMed] [Google Scholar]
  100. Tippayawong N, Khuntong P, Nitatwichit C, Khunatorn Y, Tantakitti C. Indoor/outdoor relationships of size-resolved particle concentrations in naturally ventilated school environments. Build Environ. 2009;44:188–197. doi: 10.1016/j.buildenv.2008.02.007. [DOI] [Google Scholar]
  101. Tohidi R, Altuwayjiri A, Pirhadi M, Sioutas C. Quantifying ambient concentrations of primary and secondary organic aerosol in central Los Angeles using an integrated approach coupling source apportionment with regression analysis. Atmos Environ. 2022;268:118807. doi: 10.1016/j.atmosenv.2021.118807. [DOI] [Google Scholar]
  102. Tzoutzas I, Maltezou HC, Barmparesos N, Tasios P, Efthymiou C, Assimakopoulos MN, Tseroni M, Vorou R, Tzermpos F, Antoniadou M, Panis V. Indoor air quality evaluation using mechanical ventilation and portable air purifiers in an academic dentistry clinic during the COVID-19 pandemic in Greece. Int J Environ Res Public Health. 2021;18:8886. doi: 10.3390/ijerph18168886. [DOI] [PMC free article] [PubMed] [Google Scholar]
  103. Vasquez-Apestegui BV, Parras-Garrido E, Tapia V, Paz-Aparicio VM, Rojas JP, Sanchez-Ccoyllo OR, Gonzales GF. Association between air pollution in Lima and the high incidence of COVID-19: findings from a post hoc analysis. BMC Public Health. 2021;21:1–13. doi: 10.1186/s12889-021-11232-7. [DOI] [PMC free article] [PubMed] [Google Scholar]
  104. Velasquez RMA, Lara JVM. Gaussian approach for probability and correlation between the number of COVID-19 cases and the air pollution in Lima. Urban Clim. 2020;33:100664. doi: 10.1016/j.uclim.2020.100664. [DOI] [PMC free article] [PubMed] [Google Scholar]
  105. Vicente ED, Ribeiro JP, Custódio D, Alves CA. Assessment of the indoor air quality in copy centres at Aveiro, Portugal. Air Qual Atmos Health. 2017;10:117–127. doi: 10.1007/s11869-016-0401-8. [DOI] [Google Scholar]
  106. Wang P, Chen K, Zhu S, Wang P, Zhang H. Severe air pollution events not avoided by reduced anthropogenic activities during COVID-19 outbreak. Resour Conserv Recycl. 2020;158:104814. doi: 10.1016/j.resconrec.2020.104814. [DOI] [PMC free article] [PubMed] [Google Scholar]
  107. Wang P, Shen J, Xia M, Sun S, Zhang Y, Zhang H, Wang X. Unexpected enhancement of ozone exposure and health risks during National Day in China. Atmos Chem Phys. 2021;21:10347–10356. doi: 10.5194/acp-21-10347-2021. [DOI] [Google Scholar]
  108. Wang Y, Zhu S, Ma J, Shen J, Wang P, Wang P, Zhang H. Enhanced atmospheric oxidation capacity and associated ozone increases during COVID-19 lockdown in the Yangtze River Delta. Sci Total Environ. 2021;768:144796. doi: 10.1016/j.scitotenv.2020.144796. [DOI] [PMC free article] [PubMed] [Google Scholar]
  109. Widder SH, Haselbach L. Relationship among concentrations of indoor air contaminants, their sources, and different mitigation strategies on indoor air quality. Sustainability. 2017;9:1149. doi: 10.3390/su9071149. [DOI] [Google Scholar]
  110. Xiang YK, Dao X, Gao M, Lin YC, Cao F, Yang XY, Zhang YL. Nitrogen isotope characteristics and source apportionment of atmospheric ammonium in urban cities during a haze event in Northern China Plain. Atmos Environ. 2022;269:118800. doi: 10.1016/j.atmosenv.2021.118800. [DOI] [Google Scholar]
  111. Xu K, Cui K, Young LH, Wang YF, Hsieh YK, Wan S, Zhang J. Air quality index, indicatory air pollutants and impact of COVID-19 event on the air quality near central China. Aerosol Air Qual Res. 2020;20:1204–1221. doi: 10.4209/aaqr.2020.04.0139. [DOI] [Google Scholar]
  112. Yadav S, Rajamani V. Air quality and trace metal chemistry of different size fractions of aerosols in N-NW India—implications for source diversity. Atmos Environ. 2006;40:698–712. doi: 10.1016/j.atmosenv.2005.10.005. [DOI] [Google Scholar]
  113. Yadav S, Tandon A, Tripathi JK, Yadav S, Attri AK. Statistical assessment of respirable and coarser size ambient aerosol sources and their timeline trend profile determination: a four year study from Delhi. Atmos Pollut Res. 2016;7:190–200. doi: 10.1016/j.apr.2015.08.010. [DOI] [Google Scholar]
  114. Yang Y, Liu X, Zheng J, Tan Q, Feng M, Qu Y, An J, Cheng N. Characteristics of one-year observation of VOCs, NOx, and O3 at an urban site in Wuhan, China. J Environ Sci. 2019;79:297–310. doi: 10.1016/j.jes.2018.12.002. [DOI] [PubMed] [Google Scholar]
  115. Yao Y, Pan J, Wang W, Liu Z, Kan H, Qiu Y, Meng X, Wang W. Association of particulate matter pollution and case fatality rate of COVID-19 in 49 Chinese cities. Sci Total Environ. 2020;741:140396. doi: 10.1016/j.scitotenv.2020.140396. [DOI] [PMC free article] [PubMed] [Google Scholar]
  116. Yao S, Wei W, Cheng S, Niu Y, Guan P. Impacts of Meteorology and emissions on O3 pollution during 2013–2018 and corresponding control strategy for a typical industrial city of China. Atmosphere. 2021;12:619. doi: 10.3390/atmos12050619. [DOI] [Google Scholar]
  117. Yin H, Liu C, Hu Q, Liu T, Wang S, Gao M, Xu S, Zhang C, Su W. Opposite impact of emission reduction during the COVID-19 lockdown period on the surface concentrations of PM2.5 and O3 in Wuhan. China. Environ. Pollut. 2021;289:117899. doi: 10.1016/j.envpol.2021a.117899. [DOI] [PMC free article] [PubMed] [Google Scholar]
  118. Yin H, Lu X, Sun Y, Li K, Gao M, Zheng B, Liu C. Unprecedented decline in summertime surface ozone over eastern China in 2020 comparably attributable to anthropogenic emission reductions and meteorology. Environ Res Lett. 2021;16:124069. doi: 10.1088/1748-9326/ac3e22. [DOI] [Google Scholar]
  119. Yushin N, Chaligava O, Zinicovscaia I, Vergel K, Grozdov D. Mosses as bioindicators of heavy metal air pollution in the lockdown period adopted to cope with the COVID-19 pandemic. Atmosphere. 2020;11:1194. doi: 10.3390/atmos11111194. [DOI] [Google Scholar]
  120. Zambrano-Monserrate MA, Ruano MA. Has air quality improved in Ecuador during the COVID-19 pandemic? A parametric analysis. Air Qual Atmos Health. 2020;13:929–938. doi: 10.1007/s11869-020-00866-y. [DOI] [PMC free article] [PubMed] [Google Scholar]
  121. Zangari S, Hill DT, Charette AT, Mirowsky JE. Air quality changes in New York City during the COVID-19 pandemic. Sci Total Environ. 2020;742:140496. doi: 10.1016/j.scitotenv.2020.140496. [DOI] [PMC free article] [PubMed] [Google Scholar]
  122. Zhang J, Cui K, Wang YF, Wu JL, Huang WS, Wan S, Xu K. Temporal variations in the air quality index and the impact of the COVID-19 event on air quality in Western China. Aerosol Air Qual Res. 2020;20:1552–1568. doi: 10.4209/aaqr.2020.06.0297. [DOI] [Google Scholar]
  123. Zhang K, Liu Z, Zhang X, Li Q, Jensen A, Tan W, Huang L, Wang Y, de Gouw J, Li L. 2021. Insights into the abnormal increase of ozone during COVID-19 in a typical urban city of China. Atmos Chem Phys Discuss. [DOI]
  124. Zhang Q, Pan Y, He Y, Walters WW, Ni Q, Liu X, Xu G, Shao J, Jiang C. Substantial nitrogen oxides emission reduction from China due to COVID-19 and its impact on surface ozone and aerosol pollution. Sci Total Environ. 2021;753:142238. doi: 10.1016/j.scitotenv.2020.142238. [DOI] [PMC free article] [PubMed] [Google Scholar]
  125. Zhang X, Tang M, Guo F, Wei F, Yu Z, Gao K, Jin M, Wang J, Chen K. Associations between air pollution and COVID-19 epidemic during quarantine period in China. Environ Pollut. 2021;268:115897. doi: 10.1016/j.envpol.2020.115897. [DOI] [PMC free article] [PubMed] [Google Scholar]
  126. Zhao Y, Zhang K, Xu X, Shen H, Zhu X, Zhang Y, Hu Y, Shen G. Substantial changes in nitrogen dioxide and ozone after excluding meteorological impacts during the COVID-19 outbreak in mainland China. Environ Sci Technol Lett. 2020;7:402–408. doi: 10.1021/acs.estlett.0c00304. [DOI] [PubMed] [Google Scholar]
  127. Zheng J, Shi X, Ma Y, Ren X, Jabbour H, Diao Y, Wang W, Ge Y, Zhang Y, Zhu W. Contribution of nitrous acid to the atmospheric oxidation capacity in an industrial zone in the Yangtze River Delta region of China. Atmos Chem Phys. 2020;20:5457–5475. doi: 10.5194/acp-20-5457-2020. [DOI] [Google Scholar]
  128. Zhu Y, Xie J, Huang F, Cao L. Association between short-term exposure to air pollution and COVID-19 infection: Evidence from China. Sci Total Environ. 2020;727:138704. doi: 10.1016/j.scitotenv.2020.138704. [DOI] [PMC free article] [PubMed] [Google Scholar]
  129. Zoran MA, Savastru RS, Savastru DM, Tautan MN. Assessing the relationship between ground levels of ozone (O3) and nitrogen dioxide (NO2) with coronavirus (COVID-19) in Milan Italy. Sci Total Environ. 2020;740:140005. doi: 10.1016/j.scitotenv.2020.140005. [DOI] [PMC free article] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Citations

  1. Zhang K, Liu Z, Zhang X, Li Q, Jensen A, Tan W, Huang L, Wang Y, de Gouw J, Li L. 2021. Insights into the abnormal increase of ozone during COVID-19 in a typical urban city of China. Atmos Chem Phys Discuss. [DOI]

Data Availability Statement

The relevant information used in present review article are collected from different published research work. Hence no any primary data are use.

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