key: cord-0987877-psw1w67y authors: Kumari, Pratima; Toshniwal, Durga title: Impact of lockdown on air quality over major cities across the globe during COVID-19 pandemic date: 2020-10-16 journal: Urban Clim DOI: 10.1016/j.uclim.2020.100719 sha: ed3eefba29455f7434ab6aa685e0032e6801e7d7 doc_id: 987877 cord_uid: psw1w67y In present study, the variation in concentration of key air pollutants such as PM(2.5), PM(10), NO(2), SO(2) and O(3) during the pre-lockdown and post-lockdown phase has been investigated. In addition, the monthly concentration of air pollutants in March, April and May of 2020 is also compared with that of 2019 to unfold the effect of restricted emissions under similar meteorological conditions. To evaluate the global impact of COVID-19 on the air quality, ground-based data from 162 monitoring stations from 12 cities across the globe are analysed for the first time. The concentration of PM(2.5), PM(10) and NO(2) were reduced by 20–34%, 24–47% and 32–64%, respectively, due to restriction on anthropogenic emission sources during lockdown. However, a lower reduction in SO(2) was observed due to functional power plants. O(3) concentration was found to be increased due to the declined emission of NO. Nevertheless, the achieved improvements were temporary as the pollution level has gone up again in cities where lockdown was lifted. The study might assist the environmentalist, government and policymakers to curb down the air pollution in future by implementing the strategic lockdowns at the pollution hotspots with minimal economic loss. The above-discussed literature suggests that the restriction measures implemented during pandemic have positively impacted the air quality in different countries (as shown in Fig. 1 ). It can be observed that pandemic has demonstrated a positive impact on air quality of the world's great economic countries, including European countries (such as Spain, Italy and UK), which remains a tourist's attraction throughout the year, countries in the Americas (US, Brazil and Peru), where large production and consumption projects induce high pollution, and Asian countries, including China and India, which are the most populated and highly polluted countries. These are the most affected regions globally, which collectively account for more than 60% COVID-19 cases and deaths. Since the aforementioned countries are adversely affected by the ongoing pandemic, their governments pro-actively implemented effective restriction policies and lockdown measures to contain the spread of the disease. The air quality in these countries is supposed to be highly influenced by the imposed lockdown. Therefore, the primary objective of the present study is to investigate and quantify the global impact of restriction measures on air quality of several countries that are highly affected by COVID-19 pandemic. The present work has great significance in this ongoing pandemic that how much concentration of major air pollutants reduced during the lockdown. In this study, a comparative analysis of the variation in the air quality of 12 cities from highly COVID-19 affected countries across the globe is presented for the first time. For which, a pre-and post-lockdown comparison in daily concentrations of five key air pollutants, including PM 2.5 , PM 10 , NO 2 , SO 2 and O 3 , is conducted to investigate the impact of social pause caused by COVID-19 on air quality using ground-based data from 162 monitoring stations. In addition, monthly concentrations of air pollutants during lockdown period of 2020 are compared with the previous year (2019) data to unfold the effect of restricted emissions under similar meteorological conditions. Soon after the lockdown was implemented, many researchers reported a significant decrease in atmospheric pollution. However, no study has been reported yet, that has analysed the air quality after restriction measures were removed. The present study also focuses on the air quality in cities where the lockdown has been lifted. The uniqueness of the present study is that it reflects the ground truth regarding air quality (PM 2.5 , PM 10 , NO 2 , SO 2 and O 3 ) not only during the lockdown but also after the lockdown has been lifted. In addition, it compares the air quality during the lockdown period of the year 2020 to that of the year 2019 to investigate the impact of restriction measures under similar climatic conditions. Moreover, the strength of present work has been amplified because it puts forward a global scenario of air quality by selecting the most affected countries by the COVID-19 pandemic. To the best of our knowledge, no study has been performed to examine the impact of COVID-19 on the environmental pollution on a global scale, so far. The results of this study will help in identifying the specific emission sources of major air pollutants. In addition, it will help the policymakers and scientific research community in devising a sustainable environmental management plan to curb down the high level of air pollution in polluted regions. The whole world is dealing with the deadly novel coronavirus disease, which has adversely affected the well being, lives, market and economy of the world. However, the air quality is supposed to be improved, as governments have implemented lockdowns to The AQI of Wuhan City decreased significantly, the higher the population density the more significant the decline. Baldasano (2020) Barcelona and Madrid (Spain) NO 2 The NO 2 concentrations in Barcelona and Madrid (Spain) were reduced by % and 62%, respectively. Gautam (2020b) India, Italy, Spain, France NO 2 NO 2 concentration reduced during lockdown period. Adams (2020) Ontario P. Kumari and D. Toshniwal Urban Climate 34 (2020) 100719 contain the spread of the virus. Therefore to investigate the global impact of lockdown on air quality during the COVID-19 pandemic, cities from different continents have been selected. In this study, 12 major cities around the world, namely Beijing, Bengaluru, Delhi, Las Vegas, Lima, London, Madrid, Moscow, Mumbai, Rome, Sao Paulo, and Wuhan are selected to analyze the impact of lockdown on air quality. The selected Asian cities, belong to China (Beijing and Wuhan) and India (Bengaluru, Delhi and Mumbai), falls under the list of top most polluted cities in the world. The main cause of pollution in these cities is the high population. However, due to the implemented lockdown, the human movement and outdoor activities have been significantly reduced. Few cities like London, Madrid, Moscow and Rome are the center of tourist attraction throughout the year. The main sources of air pollution in these cities are the air or road traffic, which has been halted due to shutdown of international transportation. Cities like Las Vegas, Delhi, Sao Paulo and Lima are highly industrialised cities with heavy industries, manufacturing sectors and highly polluting enterprises. The main sources of air pollution in these cities are industrial activities, including production, import/export transportation, etc. Such activities have been temporarily closed due to government guidelines during the lockdown. Besides, these cities have entirely different climatic conditions. For example, Beijing has a temperate monsoon, Delhi has semi-arid, Madrid has a mediterranean climate, Las Vegas has a subtropical hot desert climate, etc. These cities belong to those countries which are highly affected by COVID 19, and hence, a complete lockdown was imposed (Worldometer, 2020) . To analyze the global effect of restriction measures on air quality, the locations are selected from different continents/regions across the globe, as shown in Table 2 . The primary sources of air pollution in these cities are anthropogenic emissions, therefore a significant effect of lockdown on air quality is expected. Based on the above facts, considered cities provide good and diverse area for analyzing the differences in impact of lockdown on air quality of different regions. Generally, the level of air pollution in a region is measured by concentration of five major pollutants: particulate matter with diameter less than 2.5 μm and 10 μm (PM 2.5 and PM 10 ), nitrogen dioxide (NO 2 ), sulphur dioxide (SO 2 ) and ozone (O 3 ). The primary sources of PM 2.5 are emission through combustion of liquid and solid fuels in vehicles, industries, power plants, etc. It can also be produced from chemical reactions of gases such as SO 2 and nitrogen oxides (NO x ) (Singh and Chauhan, 2020) . The main emission sources of PM 10 are dust from landfills, agriculture, construction sites, wildfires, garbage burning, and dust storms. NO 2 mainly gets in the air due to the burning of fuels in vehicles, power plants, tobacco smoke, etc. Similarly, the main sources of formation of SO 2 are the combustion of fuels in power plants and industries. Other activities, such as extraction of metals from ores, volcano eruption, locomotives, ships, food and other commercial manufacturing units, etc. also generate SO 2 . At the same time, O 3 is produced when sunlight falls on few precursor chemical species emitted from primary sources in the presence of air. The presence of the abovementioned pollutants in the air is the main cause of poor air quality. Therefore, these five pollutants, including PM 2.5 , PM 10 , NO 2 , SO 2 and O 3 , are analysed to investigate the impact of COVID-19 on air quality of selected cities. The daily records of air quality data from 162 monitoring stations spread across 12 selected cities are downloaded from the World Air Quality Index portal (WAQI) (www.aqicn.org). The data at WAQI portal is collected by the stations that are managed by government institutions and departments. The retrieved dataset contains 24-h average concentration values of considered parameters (i.e. PM 2.5 , PM 10 , NO 2 , SO 2 and O 3 ). The data is collected for 23 weeks from 1st January 2020 to 10th June 2020 for each considered site. In addition, the daily average air quality data of March, April, and May of 2019 was also downloaded to present a comparison. We assume that level of air pollution in March, April, and May of 2019 was considered normal due to usual conditions, while the variation in air pollution level in March, April and May of 2020 is due to the country-specific government steps taken during the pandemic. Therefore, the monthly concentrations of five key pollutants in three months, including March, April, and May of 2019, are compared with March, April, and May of 2020. In addition, the information regarding lockdown and unlock dates are collected from the regional and global reports, as listed in Table 2 . It must be noted that lockdown refers to the minimization of vehicular and human movement, shut down of commercial and industrial activities except for essential services, which lead to a temporary reduction in air pollution. Present study follows a comparative approach to analyze the effect of lockdown or restriction measures implemented due to ongoing COVID-19 pandemic on the air quality across the globe. Firstly, the variation in trends of the five key air pollutants has been analysed to investigate a comparison in air quality before and after the lockdown. Additionally, the concentration level of key air pollutants has also been compared during lockdown period and after the lockdown was lifted. Finally, the monthly concentrations of selected air pollutants for the lockdown period (i.e. the month of March, April and May) of 2020 have been compared with the same time frame of 2019. Since the meteorological factors such as temperature, relative humidity, etc., influences air quality. Therefore, the same time frame has been used for comparison to avoid any discrepancies caused by seasonal variations. A notable reduction in PM 2.5 concentration level was observed in all considered cities ( The concentration of PM 2.5 in Las Vegas and Moscow was increased in March 2020, which significantly reduced in April and May due to the lockdown (started in mid-March and April in the United States and Russia, respectively). As the lockdown in Rome lifted in May (18th May 2020), Rome has shown an increment in PM 2.5 concentration levels during May. Likewise, percentage change in PM 2.5 concentration in Wuhan was low in April and May as compared to March as the lockdown in Wuhan was lifted in the last week of March (24th March 2020). In contrast, the PM 2.5 concentration has been increased in Mumbai and Madrid during April and May, respectively. This increment may be ascribed to the relaxation given by their governments for essential movements. In regards to daily PM 2.5 concentration from 1st January 2020 to 10th June 2020, 10 cities including Beijing, Bengaluru, Delhi, Las Vegas, Lima, Madrid, Mumbai, Rome and Wuhan have shown significant decrement in PM 2.5 concentration accounting the lockdown implementation date (Fig. 3) . It must be noted that Beijing, Bengaluru, Delhi, Lima, Mumbai and Wuhan are highly polluted cities where anthropogenic activities are the primary source of emission. They have shown a sharp decline in PM 2.5 concentration during the lockdown period. On the contrary, Las Vegas, London, Madrid and Sao Paulo have low air pollution, i.e., their PM 2.5 concentration level is quite low as compared to heavily polluted cities due to which variation in low polluted cities was not much evident in lockdown period. (Fig. 4) . The daily time series of PM 10 from 1st January to 10th June 2020, showed that PM 10 concentrations significantly reduced in 8 cities including Beijing, Bengaluru, Delhi, Lima, Madrid, Mumbai, Rome and Wuhan while in London and Moscow, PM 10 concentration was high in initial days of lockdown which gradually decreased (Fig. 5) . (Fig. 6) . In addition, all cities except Lima have shown a significant reduction in daily NO 2 concentration after the lockdown was implemented. Lima has shown a mixed trend where NO 2 concentration declined in initial days of lockdown, which further increased. It can also be observed that Wuhan has shown a remarkable drop in NO 2 concentrations during lockdown while a significant increase is noticed as the lockdown was lifted (Fig. 7) . The comparison between monthly data of SO 2 concentration in 2019 and 2020 shows a mixed trend (Fig. 8) . Eight cities in March, nine cities in April and eight cities in May have shown a reduction in SO 2 concentration 2020 as compared to 2019. Maximum reduction was observed in Mumbai (−57.5%) followed by Madrid (−54.9%), Lima (−33.3%) and Beijing (−31%) in March 2020. In April, the maximum decline was seen in Mumbai (−77.4%), followed by Madrid (−53.3%) and Delhi (−35.3%) . Similarly in May 2020, Mumbai (−52.9%) followed by Lima (−50%), Las Vegas (−44.3%) and Madrid (−43.7%) showed maximum reduction SO 2 concentration as compared to May 2019. On the contrary, three cities (Bengaluru, London, Rome) in March 2020, 3 cities (Bengaluru, London, Wuhan) in April and May 2020 showed a higher SO 2 concentration. It must be noted that Lima followed by Mumbai and Delhi, are high SO 2 cities as compared to other selected cities (Fig. 8) . The daily concentration of SO 2 in selected cities didn't show any regular trend during lockdown period: SO 2 concentration during lockdown declined in 5 cities (Beijing, Bengaluru, Delhi, Las Vegas and London) while increased in 3 cities (Lima, Rome and Wuhan) (Fig. 9) . On comparing the available data of 11 cities, a mixed trend across cities was observed for monthly mean On the other hand, Delhi (+33.3% in April, +43.7% in May), London (+26.5% in March, +32.1% in April, +50.8% in May), Sao Paulo (+29.1% in March, +30.7% in April, +27% in May) and Wuhan (+50% in March, +30.6% in April) has shown maximum increase (Fig. 10) . In regards to the daily concentration of O 3 , mixed trends were observed during lockdown phase: 3 cities (Bengaluru, Mumbai and Sao Paulo) showed a decline while seven cities (Beijing, Delhi, Las Vegas, Madrid, Moscow, Rome and Wuhan) showed a sharp increment in daily O 3 concentrations in lockdown period (Fig. 11) . Too much exposure to poor air quality may harm the humans and ecosystem beyond expectations. Air pollution has become a serious and global topic of concern. COVID-19 has presented an opportunity to study the emission sources of several air pollutants and different ways in which their concentration can be reduced. The air quality of cities considered in present study has been analysed under the influence of COVID-19 lockdown. The selected locations are greatly affected by the COVID-19 pandemic, which accounts for more than 60% of the total COVID-19 cases and deaths worldwide (Worldometer, 2020) . These countries have implemented strict lockdown to stop the spread of the disease, leading to a significant reduction in air pollution levels in selected cities. The above discussed results demonstrate that the concentration of particulate matters (PM 2.5 and PM 10 ) and NO 2 have significantly reduced in March, April and May of 2020 compared to the same period of 2019. This reduction is observed for all selected cities, except Las Vegas, Moscow and Rome. The main reason for this discrepancy is the timings of lockdown implementation. In few cities, the lockdown was imposed in late March, due to which these cities might have kept the monthly mean values prior to lockdown, which resulted in an overall high concentration of pollutants in March 2020. Moreover, in few cities such as Beijing, Delhi, London, Rome and Wuhan, the lockdown was lifted in April or May due to which things turned back to normal as people started traveling, industrial and business activities, etc. restarted following the government issued guideline. This might be the reason that these cities have shown higher or comparable monthly mean concentrations in 2020 as compared to 2019. Further, a detailed trend analysis is carried out to represent the variation in daily concentration of these air pollutants. The daily PM 2.5 , PM 10 and NO 2 concentrations showed a significant decline after the enforcement of lockdown. The primary reason for the reduction in their concentrations is the halt on anthropogenic activities such as transportation, traveling, industrial activities, which are the primary source of such pollutants (Sharma et al., 2020) . According to the discussed results, SO 2 and O 3 have shown a mixed trends (i.e., SO 2 and O 3 concentrations reduced for some cities while remained unchanged for some cities) in March, April and May of 2020 as compared to the same period of 2019. The primary reasons for the observed mixed trends of SO 2 are following. First, the major emission sources of SO 2 are coal-based power plants that might have been operational at many places during the lockdown, due to which SO 2 concentrations did not reduce in some cities (Kumari and Toshniwal, 2020) . Second, the seasonal forest fires and sand storms in few regions have influenced the level of these pollutants in few cities (Sicard et al., 2020) . Third, the local meteorological conditions such as temperature, rainfall, wind speed, solar radiation, etc. are few factors that highly affect the SO 2 concentration levels (Lokhandwala and Gautam, 2020) . Due to these reasons, the SO 2 concentration remained steady or increased for a few cities like Lima, Madrid, Moscow, Rome, Sao Paulo and Wuhan, during lockdown. Moreover, there is no evident reduction in SO 2 concentration in 2020 as compared to 2019 for these locations. is a secondary pollutant, which depends on the local availability of its precursors (i.e., NOx and volatile organic compounds (VOCs)). The reduction in emission of its precursors has increased the O 3 concentration in the atmosphere. Secondly, the nitrogen oxide (NO) consumes O 3 under the titration process: NO + O 3 = NO 2 + O 2 . However, the O 3 consumption by NO has been reduced due to a decrement in NO concentration during lockdown compared to pre-lockdown period, subsequently, the amount of O 3 in the atmosphere has been increased. Thirdly, the local meteorology (e.g., sunlight, temperature, etc.) also impacts the construction and destruction of O 3 . For example, the minimum and maximum temperature in May 2020 has been comparatively higher than that of March 2020 (Kumari and Toshniwal, 2020) . However, the O 3 concentration decreased in cities like Bengaluru and Mumbai for the following reasons: In these cities, the temperature remained constant, specifically low during these months, due to which O 3 concentrations couldn't increase. Moreover, Bengaluru and Mumbai have witnessed heavy rains during the considered time frame. Generally, O 3 production is limited during cool or rainy weather conditions (Kwak et al., 2017) . The performed analysis also shows that the concentration level of pollutants such as PM 2.5 and PM 10 , NO 2 , SO 2 and O 3 has increased after the lockdown is lifted. It can be observed from the above-mentioned results that in Beijing, Bengaluru, Delhi, Mumbai, Rome and Wuhan, the concentration level of considered pollutants substantially go up once the lockdown is revoked in these cities. This observation leads to an important finding that the air quality improvement achieved during lockdown is very short-term and temporary. Overall, in European cities, including London, Madrid and Rome, the major sources of pollution are residential buildings, small scale industrial and commercial activities which might have continued in the lockdown period. Therefore, pollutants in these cities have not shown much reduction in lockdown phase. However, in highly polluted cities like Beijing, Bengaluru, Delhi, Lima, Mumbai, Rome and Wuhan, the impact of lockdown on pollution reduction is highly observable as the primary emission sources of pollution in these cities are anthropogenic activities, which drastically reduced in lockdown phase. Based on the studied ground-level station data, it can be concluded that three major air pollutants PM 2.5 , PM 10 and NO 2 are highly reduced across the world due to the concurrent implementation of lockdown in the ongoing COVID-19 pandemic. The COVID-19 pandemic has emerged as a serious threat to the entire humankind in many ways. However, the pandemic triggered restriction measurements and the lockdown has come out to be a "blessing in disguise" as the earth is reviving itself. This study investigates the impact of lockdown, which was imposed as a precautionary measure to control the spread of COVID-19, on the air quality of different cities across the globe. To conduct the investigation, the ground-based station air pollution data from 162 monitoring station in 12 cities from the countries which are highly affected by COVID-19 has been analysed. The variation in concentration of five key air pollutants, including PM 2.5 and PM 10 , NO 2 , SO 2 and O 3 , has been studied. In addition, the monthly average concentration of these pollutants during March, April and May of 2020 is compared with the same time frame of 2019. The significant findings of the study are as follows: • The major pollutants, including PM 2.5 , PM 10 and NO 2 , have shown a remarkable reduction in post-lockdown period compared to pre-lockdown period, for all selected locations due to restricted emissions during the lockdown phase. Similarly, the concentration level of these pollutants has been highly reduced in March, April and May of 2020 compared to those of 2019. • The highly polluted cities like Beijing, Bengaluru, Delhi, Lima, Mumbai, Rome and Wuhan have witnessed a notable decline in air pollution in the lockdown period of 2020 compared to the same time of 2019. The PM 2.5 concentration levels showed a drop in the range of 20.2% to 34.3%. Similarly, PM 10 concentration was reduced by 23.7% to 47.3%. However, a higher decline, between 31.6% and 64.5%, has been observed in the case of NO 2 . • SO 2 concentration level showed a mixed trend during the lockdown phase. For few cities like Lima, Madrid, Moscow, Rome, Sao Paulo and Wuhan, SO 2 concentration remained unchanged in lockdown phase as the main emission source of SO 2 is power plants, which remained operational at most of the location. O 3 concentration level demonstrated an increment in the lockdown phase at many locations, which might be due to the reduction of O 3 consumption in the titration process and change in meteorological conditions over time. • Notably, the observed variations and improvements in air quality are temporary as the concentration level of studied pollutants has gone up again in cities where lockdown was lifted. To sum up, the findings of this study suggest that the lockdown has led to a significant reduction in air pollution across the globe, irrespective of the geographical and climatic condition of the considered locations. Although the present environmental changes are temporary, this short-lived enhancement in air quality during lockdown may be an encouraging signal for governing authorities and policymakers to achieve better air quality through planned and strict restrictions on emission sources. The governments may implement a strategic lockdown at the pollution hotspots for a period of time to control the pollution levels in their regions with minimal economic loss. However, present work has certain limitations that can be worked upon in future studies. Firstly, the variation of remaining parameters such as carbon monoxide, benzene, black carbon and air quality index can be studied in further research. Secondly, this study is conducted for 12 cities that are highly affected by COVID-19; future studies can incorporate a larger number of locations irrespective of the number of COVID-19 cases. Thirdly, a more detailed analysis of variation in O 3 concentration levels can be carried out to understand the mixed trend. 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. 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