Spatial Variation of Tropospheric NO 2 Concentration During First Wave CoVID-19 Induced Lockdown Estimated From Sentinel-5 Precursor

CoVID-19 pandemic has forced many countries to shut down their industries as most of these countries observed complete state-wide lockdown during the rst wave of this contagious virus. Factories and vehicles on the road are a major source of nitrogen dioxide (NO 2 ), which is also one of the major air pollutants. The emission of NO(cid:0) is considered as a good indicator of prevalant global economic activities as its predominant corollary affairs typically involve industrial oprtaions and these emissions are evidently visible from space. Sentinel-5 Precursor (launched in July 2018), equipped with spectrometer Tropomoi (Tropospheric Monitoring Instrument), is a low-earth-orbit atmosphere mission satellite dedicated towards monitoring air pollution sensing remotely. This study is focused primarily on, estimation and variation of NO 2 emission over different parts of the world, before and during the rst wave of CoVID-19 epidemic (07 weeks duration). It is found that in most of the countries during the lockdown period (7 weeks), air quality was cleaner by about 40 to 50 % over the last year, mainly due to the shutdown of industry and trac activities. The technology is quite promising and provides a tool to access if a country or place followed complete lockdown.


Introduction
The rapid increase of population in the world has led to an anthropogenic impact on the natural environment of the Earth. Urbanization can be seen to have increased in the world, rapidly (Chen et al., 2014). During the last few decades, industrialization, energy consumption, transportation, motorization, economic-growth, and power plants, etc. of the countries have proliferated with time and turn out to be the main cause of air pollution (Mayer, 1999). Growth in population has an adverse effect on the concentration of NO 2 in the atmosphere (Mayer, 1999). Particulate matter(PM)(e.g., soot, dust, smokes, fumes, mists, etc.), NO 2 , sulfur dioxide (SO 2 ), ozone (O 3 ), carbon dioxide (CO2) and carbon monoxide (CO) are major pollutants of the air and has an acute harmful effect on the health of human beings Lamptey and Serwaa, 2020; Anchordoqui, et al., 2020;Okunlola, et al., 2020). One of the major sources of NO 2 , contributing to the air, is the combustion of fossil fuels such as coal, oil, and gases due to vehicles, power stations, and heating. In any case, the role of extraterrestrial impact on the vacillation of these gases cannot be excluded (Mukherjee 2013 The CoVID-19 pandemic situation has caused lockdown in almost all the major countries of the world with the tra c and industrial activities suspended. Such a scenario creates an excellent opportunity for researchers to relatively access dynamic parameters; particularly the ones which are anthropogenic in Page 3/19 nature. Several studies regarding relative variation of air quality before and during the lockdown has been carried with an emphasis on decrease in contaminant levels, such as the extent of the reduction of major pollutants, like carbon monoxide (CO), nitrogen dioxide (NO2) and sulfur dioxide (SO2) in India and China during January to April, 2020 (Metya et al., 2020), variation of ne particulate matter (PM 2.5 ) and nitrogen dioxide (NO 2 ) in the continental United States (Berman and Ebisu, 2020), impacts on the air quality of São Paulo state, Brazil during lockdown (Nakada and Urban, 2020), affect of CoVID-19 lockdown on NO 2 , near-real-time, o ine and reprocessed data of the Sentinel-5 P. This study attempts to establish the effect of lockdown on the relative composition of NO 2 in the troposphere towards inferring signi cant improvement in air quality compared to last year using the Sentinel-5P data.

Data And Methods
The NO 2 data of the Sentinel-5 P, which was launched on 13 October 2017, were brought in to the public domain on 10 July 2018. A comparative study between the worldwide NO 2 emission in the present phase of 2020 and the same of 2019 was made using the Google Earth Engine tools. The correlation between the present pandemic and better air quality can be easily concluded from this study. It can be observed from the recently released satellite images by the European Space Agency that there has been a considerable decrease in the emission of NO 2 during this global crisis, which has led to a reduction in In order to curb the transmission rate of the deadly virus and to atten the curve of the spread of COVID-19 most countries have imposed restrictions such as shutting down o ces, factories, schools, etc. and canceling all public events so as to avoid the possibility of any mass gathering which was bound to fan the spread even more. Many countries have requested their citizens to stay at home and to implement self-imposed travel restrictions, moving out of their houses only in case of emergencies or to facilitate absolute needs. As an obvious result of such lockdowns, there has been a considerable reduction in the road and air tra c which has further led to a reduction in emission of poisonous gases in the atmosphere due to a reduction in the consumption of fossil fuel and anthropogenic activities. Thus, in the light of this deadly epidemic it can be said that although there have been colossal and long-lasting social and economic impacts, the positive impact on the environment-howsoever temporary in nature-cannot be ignored.
Readings from the Sentinel-5 P from the Satelite imagery released by the European Space Agency clearly show that over the past 7 weeks, the levels of nitrogen dioxide (NO 2 ) over cities and industrial clusters in the USA, Europe, and Asia were remarkably reduced, as evident from a comparison with last year. This

Impact on the United States of America
It is observed that NO 2 concentration has decreased by 50% in most parts of the North East, Southeast, and Midwest of the United States(US) during the studied time period (Fig. 4).From Fig. 4 it can be inferred that suspension of industry and activites during these seven weeks leadto reduction in consumption of fossil fuel and as a result it has improved the air quality of the US. Fig. 5 shows daily average NO 2 emission of 2019 and 2020 for the North East, South East and Midwest regions of the US for the studied 7 weeks duration and con rms improvement in air quality.

Impact on European Countries
With the rapid escalation of CoVID-19 in Lombardy, Italy, the country which was the rst in Europe to become a hotspot of the disease and declared complete lockdown on March 9, 2020, which was followed by Spain on March 14, issuing general con nement and further toughening the measures on 28 March; France declared complete lockdown on March 17 and Germany, rather, opted for strict social distancing measures since March 22, while the UK announced a complete lockdown on March 23 ("Coronavirus: What are the lockdown measures across Europe?" DW; 14.04.2020; and "When did the UK lockdown start and how long will it last? -Heart"; 28.05.2020). Comparing the mean NO 2 emissions for the identical months and dates for the year 2019 and 2020 (Fig. 6 & 7

Impact on India
In India, it's evident from both the gures (Fig. 8 & 9) that a signi cant and essential drop in NO 2 emission is observed all over the country with the atypical exception in few northeastern states, where the subtle increase in the emission concentration is detected (Fig 8 & 9). The highest concentrations, during the lockdown period, are conspicuous to the coal excavated central-eastern sectors, while the industrial belt of Gujrat-Maharashtra appears to indicate reduced NO 2 emission by more than 70%. Similar depletion is also observed in the central and southern states where the blistering NO 2 concertation zones from 2019 appears mildewed or even nulli ed. The area in and around Delhi, considered as one of the worst air quality index cities by the World Meteorological Organization, has seen a signi cant drop in NO 2 release.
As such, all the hotspots of nitrogen oxides (Ghude et al., 2008, Ghudeet al., 2011 clearly show dramatic declination. The uninhabited or sparsely populated northern regions, quintessentially, show no difference. The temporal shutdown of industries and less tra c explains the considerable decrease, while for the coal-bearing zones, emission may be expected even without anthropogenic interventions. (Reference can be added)

Impact on China
The most subtle change in NO 2 concentration, across the globe, is observed in China (Fig. 10). While NO 2 emanation drastically declined in most of the industrial and populated cities across the globe, Beijing and its adjacent northern cluster of cities recorded the highest emanation in the world at a time when, almost, the whole world was under a lockdown or restricted public activity. Here, the e cacy of time series evaluation (Fig.11) plays a key role in distinguishing and elucidating the fall in emission. No perceptible variation in NO 2 emission could be traced from the uninhabited terrains. In a nutshell, the NO 2 emanating sources were actively functioning in China as usual years, as evidenced by the high emissions stemming from industrially captivated Beijing and its neighbored regions, with the remarkable exception of the Hubei province. With this premise, it may be inferred that China, well-nigh, operated its tra c and e uent emanating industries conventionally, diametric to global scenario. Incidentally, the prime reason for the eccentric pattern in global NO 2 emission stems from China (Liu et al., 2020b;Ding, et al., 2020).
Further, the Hubei province is studied with supplementary inclusion of November and December 2019 and January to April 2020.

Impact on Wuhan (China)
Even as the scheming decline in NO 2 emission is observed in China, the Hubei Province with Wuhan as its capital city, where the CoVID-19 case reportedly emerged for the rst time , however, recorded a sharp fall. Such contrasting variation within similar geographical expanses prompted a localized study of mean NO 2 emission in Hubei province. It is evident from the map that in January 2020 lowest ever emission is detected which sluggishly increased over the two months. Now, it is imperative to note that public tra c was suspended within Wuhan city on 23 January , showing stark inconsistency with the NO 2 emission ndings as shown in the map (Fig 12) and time-series variation ( Fig   13).
Ideally, the declining volume of NO 2 for a week could never average out the higher emission of the remaining three weeks to produce the lowest ever emission volume for the entire month of January 2020.
However, the emissions were typically high during November and December 2019. The discordant correlation suggests that restriction in public tra c was imposed well before the last week of January, as known widely across the globe.

Conclusion
The pandemic brought about by the novel Corona Virus-2019, known for causing one of the most contagious diseases in the recent past, has paralyzed several aspects of day-to-day life. Most importantly, from a broader perspective, it has in icted deceleration in global economic growth. While the pandemic meted out multifaceted abominate repercussions, it ironically engaged mankind to contemplate on the environmental exploitation due to industrialization with vivid and life-size examples and recognize climate change. This paper scienti cally documents one such example in the form of depletion in NO 2 emission in some of the major developed and developing countries of the world due to lockdown or restricted public activity. In presenting the moderate to the steep decline in NO 2 concentrations in major cities, the results also portray a grim picture of how most of the public activities are still dependent on processes which yield environmentally hazardous e uents even after widespread awareness on promoting a switch over to the 'clean and green' technologies.

Declarations
Acknowledgements: (To be written after the review and before the nal publication) (A) The Data Availability Statement (DAS): Data was downloaded from the sites mentioned in the text and processed. There is no data uploaded to the journal.

(B) Declaration of interest:
This is to declare by all the authors that no nancial and personal relationships directly or indirectly with other people or organizations have been taken and there is no con ict of interest directly or indirectly with other people or organizations.

(C) Funding:
This is to declare that no fund has been received by any indivisual or organization to support this research. This is not applicable in this research.