On December 31, 2019, China alerted the World Health Organization (WHO) of several cases of unusual pneumonia in Wuhan, a city in the central Hubei Province. On January 7, 2020, the identification of a new virus, named SARS-CoV-2, was announced (WHO, 2020a). On January 30, WHO declared worldwide public health emergency. In February, outbreaks begin in Iran, Italy and other countries around the globe. Subsequently, the epidemic turns into pandemic and by end of March half of the world population was under some form of lockdown (Tosepu et al., 2020) . The recent outbreak of coronavirus disease, termed as COVID-19, has raised global concerns and led to total lockdown in many countries (WHO 2020a; Gautam and Trivedi 2020; Bherwani et al., 2020b; Wang et al., 2020a) . The disease is caused by severe acute respiratory syndrome coronavirus2 (SARS-CoV-2) (Gautam and Hens 2020b) . This fatal and novel coronavirus is likely to spread rapidly in humans with close contact to already infected people (Cascella et al., 2020; Bherwani et al. 2020b) . The spread of the virus may be contained by maintaining proper social distance, personal hygiene, avoiding gatherings, and visiting places like hospitals, meetings, and public transportations, which have a high risk of such virus contamination (WHO 2020b; Bherwani et al. 2020b; Gautam 2020b) . Preliminary investigations on the origin of COVID-19 caused by the SARS-CoV-2 coronavirus suggests a zoonotic origin (Lu et al., 2020) because other coronavirus-related diseases, such as Middle East respiratory syndrome (MERS) and severe acute respiratory syndrome (SARS) created outbreaks due to human-animal interactions.
COVID-19 is an acute respiratory disease which may lead to pneumonia with symptoms such as fever, cough and dyspnea (Jiang et al., 2020) and has an approximate fatality rate of 2-3% (Rodriguez-Morales et al., 2020). As of July 24, 2020, there have been more than 15 million confirmed cases and around 628,000 deaths reported globally (www.covid19.who.int, 2020). As the cases spread, most of the countries adopted restrictions to the transportation, commerce and cultural activities, schools and universities were closed and exams were cancelled, and social distancing was imposed (Dantas et al., 2020) . The first confirmed case of Virus COVID-19 in Iraq appeared on February 24, 2020 in Najaf, southern Baghdad (WHO, situation report-36). To control the spread of the COVID-19 in Iraq, the Iraqi government announced a series of partial and total lockdown measures from 1 March, which included closing schools, universities, restricting transportation and the movement of people between provinces (Jebril, 2020) . The total number of confirmed cases in Iraq was 104,711 and 4212 deaths from February 24 to July 24, 2020 (Ministry of Health, 2020).
The COVID-19 affects the world economy negatively, due to a sharp increase in the doubt of economic development, and due to the stop of economic activities as a result of restriction movement and transportation to control the spread of the pandemic. It is well-known that air pollution reduces with decreasing economic activities (Wang and Su, 2020b) .
The lockdown measures have shut down industries, halted vehicular traffic and have had a huge impact on the daily routine of the people. Due to this, considerable improvement in air quality levels of countries such as Spain (Tobias et al., 2020) , India (Gautam 2020b) , Brazil (Nakada and Urban 2020) , and China (Sharma et al., 2020) has been reported. The plummet in pollutant concentration is obvious, due to the restriction in anthropogenic activities.
The air is a vital element for the survival of all living beings; hence, it is necessary to keep it clean and safe. The anthropogenic activities are a major cause of ambient air pollution, due to emission of many harmful pollutants in high concentration, which are health damaging (Gautam and Hens 2020a; Ghorani-Azam et al., 2016) . The main causes of air pollution include economic development, urbanization, energy consumption, transportation and motorization, as well as the rapid increase of urban population (Kaplan et al., 2019) . The biggest air pollutants encountered in our daily life are particulate matter (PM), sulfur dioxide (SO 2 ), nitrogen dioxide (NO 2 ), ozone (O 3 ), carbon monoxide (CO), and carbon dioxide (CO 2 ) (Chen et al., 2007) . NO 2 is an important component of urban air pollution and a precursor to ground level ozone, particulate matter, and acid rain (Bechle et al., 2013) . The main sources of NO 2 in the ambient atmosphere are the burning of fossil fuels such as coal, oil and gas. NO 2 is a highly reactive pollutant and emitted, especially from the combustion of fossil fuels. Transportation is considered as the major source of NO 2 emissions (Muhammad et al., 2020) .
PM is a major pollutant, emitted from vehicles, residential, energy, industrial and dust (Guo et al., 2017; Guo et al., 2019) . PM is responsible for respiratory infections, lung disease, and importantly compromised immune system (Kim et al., 2018) . PM 2.5 specifically, has an impact as it passes through the respiratory system and provides high chances of getting deposited in lungs (Kim et al., 2018; Li et al., 2018) .
O 3 is a common oxidant gas in urban air, and exposure to ozone can induce oxidative stress causing airway inflammation and increased respiratory morbidities (Adhikari and Yin 2020). Surface O 3 concentration depends on the magnitude and ratio of the emissions of precursor gases (e.g. NOx, and VOCs), photochemical reactions, atmospheric conditions (weather), removal processes at the earth's surface and hence on local, regional, seasonal factors. In most regions O 3 declines with decreasing NOx emissions; however, in some traffic-intensive urban regions dominated by high-NOx emissions, O 3 may initially increase in response to declining NOx emissions, but after transport of the urban plume to rural areas O 3 will eventually decrease (Dentener et al., 2020) .
The energy sector (crude oil production) represents the backbone of the Iraqi economy and its exports. Iraq relies heavily on the use of fossil fuels for electricity production, which has increased from recent years, due to population growth and growing electricity consumption (Hashim et al., 2020) . Electricity production, oil refining, natural gas flaring, transportation and population are among the most important sources of air pollution in Iraq (IEA, 2012) . The absence of a reliable national electrical power network has led to the spread of different kinds of generators of various sizes servicing homes, farms, factories, and various governmental and nongovernmental institutions. These sources led to the low air quality in Iraq, especially in large cities such as Baghdad and Basra (Ministry of Environment, 2016). A lot of sites in Iraq can be regarded as high density traffic sites especially in the capital Baghdad. An excess of pollution and high levels of emission of various pollutants due various reasons are produced. One of the major reasons is pollution, due to automobile fuel consumption which led to a negative impact on the economy and environment (Jassim et al., 2014) .
Air quality changes due to the COVID-19 lockdown quickly became a new topic of recent research studies. Kerimray et al. (Kerimray et al., 2020) analyze the effect of the lockdown from March 19 to April 14, 2020, on the concentrations of air pollutants in Almaty, Kazakhstan. Daily concentrations of PM 2.5 , NO 2 , SO 2 , CO and O 3 were compared between the periods before and during the lockdown. During the lockdown, the PM 2.5 concentration was reduced by 21%. There were also substantial reductions in CO and NO 2 concentrations by 49% and 35%, respectively, but an increase in O 3 levels by 15% compared to the prior 17 days before the lockdown. Otmani et al. (Otmani et al., 2020) evaluated the changes in levels of some air pollutants, PM 10 , NO 2 J o u r n a l P r e -p r o o f and SO 2 in Salé city, Morocco, during the lockdown measures. The obtained results showed that the difference between the concentrations recorded before and during the lockdown period were respectively 75%, 49% and 96% for PM 10 , SO 2 and NO 2 .
Satellite instruments afford a global view of the planet's air pollution. The Sentinel-5Precursor/ Tropospheric Monitoring Instrument (TROPOMI) sensor is a part of the European Space Agency (ESA) space Program for earth monitoring; the main objectives are to provide operational space-borne observations in support to the operational monitoring of Air Quality (Borsdorff et al., 2018) . Using spectral bands from the ultraviolet, visible and near-infrared wavelength range, TROPOMI measures O 3 , NO 2 , SO 2 , bromate (BrO3-), formaldehyde (HCHO) and water vapor (H 2 O) tropospheric columns from the ultraviolet, visible and near-infrared wavelength, and CO and methane (CH 4 ) tropospheric columns are measured from the short-wave infrared wavelength range (Veefkind et al., 2017) . The S-5P/ TROPOMI data used in several air pollution studies during outbreak of the COVID-19 pandemic around the world. Gautam, 2020 studied NO 2 column observations to compare the air quality data released by international agencies before and after the novel coronavirus pandemic over India, China, Spain, France and Italy. The significant reduction in the percentage of NO 2 in India and China by using S-5P/TROPOMI reached 70% and 20-30% NO 2 reduction in India and China, respectively. The NO 2 reduced 20-30% in European countries (i.e., Spain, Italy, and France).
Finally, in an attempt to point out the main changes in NO 2 , O 3 , PM 2.5 , PM 10 concentrations and Air Quality Index (AQI) in Baghdad, their daily averages were calculated for the periods before the lockdown from January 16 to February 29, 2020, and during four periods of partial and total lockdown from (March 1 to July 24, 2020). The objective was to evaluate the relative variation (in %) and the difference in the mean concentration (in μg/m 3 ) between the five periods (before and during the lockdown). Study of NO 2 tropospheric column data derived from TROPOMI over Iraq from January to July, 2020, and calculates reduction in the percentage of NO 2 before and during the lockdown. This study aims to assess the impacts of COVID-19 lockdown conditions on the air quality of Baghdad, which is one of the most polluted large cities in the Middle East.
Iraq is located in the eastern part of the Middle East and North African countries (MENA region). It is surrounded by Iran in the east, Turkey to the north, Syria, and Jordan to the west, Saudi Arabia, and Kuwait to the south and the Gulf to the southeast (Al-Ansari, 2013). Iraq has a narrow coastal strip on the Arabian Gulf with a length of about 58 km (El Raey, 2010). The climate in Iraq is mainly of the continental, subtropical semiarid type, with the north and north-eastern mountainous regions having a Mediterranean climate (FAO, 2003) . Iraq is shaped like a basin containing the great Mesopotamian plain of the Tigris and Euphrates rivers, as shown in Fig.1A (Al-Ansari, 2013). The current population of Iraq is about 40 million, shown in Fig.1B based on projections of the latest United Nations data. Iraq is currently growing at a rate of 2.32% per year. Nearly 70% of Iraq's population lives in urban areas, and they have several large cities that reflect that. The largest by far is the nation's capital, Baghdad, with a population of 7.5 million. The cities of Basra and Mosul both have populations exceeding 2 million (https://worldpopulationreview.com/countries/iraq-population/). Baghdad city is located in central Iraq. The borders of the municipality of Baghdad encompass fourteen administrative units, eight in Rusafa (east of Tigris River) and six in Karkh (west of Tigris River). The area of the municipality of Baghdad reached 870 km 2 . Advantages of the characteristics of study area are: essentially great extremism in temperature, little precipitation, low relative humidity and high brightness of the sun (Hashim and Sultan 2010).
Due to the technical damage of automatic monitoring stations for air pollutants of the Ministry of Environment in Baghdad during the previous period, air pollution data were collected from an online platform (https://air.plumelabs.com/en/) monitoring and analyzing the air quality. Daily concentrations of four air pollutants were measured, including NO 2 , O 3 , PM 2.5 and PM 10 for Baghdad before and during the lockdown, in addition to the air quality index (AQI) for the same time period. NO 2 , O 3 , PM 2.5 and PM 10 concentration values were not available for the previous years. In this work, used S-5P/TROPOMI global daily gridded data at 0.05 o ×0.05 o derived from the near-real-time operational product (van Geffen et al., 2019b) , obtained via the Copernicus open data access hub (https://s5phub.copernicus.eu) for NO 2 tropospheric column data. Table 1 J o u r n a l P r e -p r o o f shows dates of data acquisition of S-5P/TROPOMI that cover the period from January 02, 2020, before the lockdown in Iraq until July 24, 2020.
To understand the overall improvement in air quality, AQI was detailed. AQI uses NO 2 , O 3 , PM 2.5 and PM 10 , of which minimum concentrations three pollutants should be available. The concentrations are converted to a number on a scale of 0-500. The sub index AQI (AQIi) for each pollutant (i) is calculated using Eq. (1) (Sharma et al., 2020) .
Lockdown due to COVID-19 reduced transportation activities, which results in less energy consumption and lower oil demand. These changes in transport activities and oil demand exert a significant impact on the environmental quality. Fig.2 represents the NO 2 concentrations (µg/m 3 ) in Baghdad, before and during the lockdown from January 16 to July 24, 2020. The results showed that average of NO 2 concentration before lockdown from January 16 to February 29 was 91 µg/m 3 . During the 1 st partial and total lockdown from March 1 to April 21; daily NO 2 concentrations reduced in Baghdad, due to the slowdown in transportation activity and social distancing measures, and average of NO 2 of this period was 86 µg/m 3 . The NO 2 average continued to decline during the period of April 22 to May 23, during the 2 nd partial lockdown, reached 85 µg/m 3 .
As a result of high number of confirmed cases of Covid-19 in Iraq, the total lockdown was imposed again from May 24 to June 13. The average of NO 2 concentration in Baghdad during this period reached 84 µg/m 3 . The concentration of NO 2 continued to fluctuate with a tendency to decrease in Baghdad from June 14 to July 24 during the 3 rd partial lockdown. The average of NO 2 concentration during this period reached 73 µg/m 3 , that is confirms decrease NO 2 concentration in Baghdad during the lockdown, relative to same levels in January 2020. From Fig.2 , noted the daily concentrations and averages of NO 2 did not exceed the WHO limit 200 µg/m 3 , before and after lockdown. In mid-July, the daily O 3 concentration exceeded WHO limit 100 µg/m 3 for the first time in 2020, due to high temperatures and low NO 2 concentrations. The average of O 3 from June 14 to July 24 reached 50µg/m 3 . The increase in O 3 concentrations in Baghdad was associated with a decrease in NO 2 concentrations, due to the inverse relationship between them. Fig.4 represents the daily concentrations (µg/m 3 ) of PM 2.5 in Baghdad from January 16 to July 24, 2020. The results showed that the average of PM 2.5 concentration before the pandemic, from January 16 to February 29, J o u r n a l P r e -p r o o f was 40 µg/m 3 . During the 1 st partial and total lockdown from March 1 to April 21, the average of PM 2.5 concentration dropped to 37 µg/m 3 . Daily concentrations of PM 2.5 increased at the end of April and beginning of May, reaching a maximum value of 117 µg/m 3 . The average of PM 2.5 from April 22 to May 23, reached 42 µg/m 3 . During the 2 nd total lockdown from May 25 to June 13, the average of PM 2.5 decreased slightly to 40 µg/m 3 . From June 14 to July 24, the period characterized by fluctuating daily PM 2.5 concentrations during the 3 nd partial lockdown, bringing the average of PM 2.5 to 39 µg/m 3 . Although daily concentrations of PM 2.5 in Baghdad recorded few values less than the WHO limit 25 µg/m 3 , but during the study period most of the PM 2.5 concentration exceeded this limit, especially in May and June, due to drought and high temperature. Fig.5 represents the daily concentrations of PM 10 (µg/m 3 ) in Baghdad from January 16 to July 24, 2020. The results showed that the average of PM 10 concentration before the pandemic, from January 16 to February 29, was 119 µg/m 3 . During the 1 st partial and total lockdown from March 1 to April 21, the average of PM 10 concentration dropped to 101 µg/m3. Daily concentrations of PM 10 increased at the end of April and beginning of May, reaching a maximum value of 541 µg/m 3 . The average of PM 10 from April 22 to May 23, reached 160 µg/m3. During the 2 st total lockdown from May 25 to June 13, the average of PM 10 increased to 185 µg/m3. From June 14 to July 24, the period characterized by fluctuating daily of PM 10 concentrations during the 3 nd partial lockdown, the average of PM10 increased slightly to 186 µg/m 3 . The summer in Iraq lead to activity in the movement of wind, which raises dust, which increases the concentration of PM in Baghdad; in addition to the influence of transportation and fuel combustion activities. Almost all daily recorded concentrations of PM 10 in Baghdad exceeded the WHO limit 50 µg/m 3 , during the study period.
For the purpose of studying the air quality of Baghdad before and during the lockdown, according to the recorded values of air pollutants, Fig.6 represents the AQI for Baghdad from January 16 to July 24, 2020. Before the pandemic, from January 16 to February 29, AQI values ranged between 50-300, with an average of 120; meaning that AQI located within the third level (unhealthy for sensitive groups). From March 1 to April 21, AQI values improved in Baghdad, and the average for this period reached 105.
The reduction average of AQI before and during this period reached around 13 %. AQI of Baghdad declined during the 2 nd partial lockdown from April 22 to May 23, with highest daily value reached 512. The average for this period increased to 151. AQI of Baghdad continued to record poor numbers. The average for the total and partial lockdown periods from May 24 to July 24, 2020 recorded 161 and 167, respectively. The above results confirm that the AQI for Baghdad recorded the lowest valuesduring the 1 st partial and total lockdown period in March and April, 2020. This period represented the lowest daily concentrations of air pollutants recorded in the current study. It also recorded the lowest average of pollutants compared to the pre-pandemic and post-pandemic period, due to citizen's commitment to lockdown measures, slow transportation, closures of universities and schools, and restrictions on the movement of employees. These measures contributed to a large extent to the decrease in the air pollutants emission in Baghdad during the 1 st partial and total lockdown.
The concentrations of NO 2 , O 3 , PM 2.5 and PM 10 were not available from the used sources for the previous years; therefore, the values were compared between the periods before the lockdown (January 16, 2020) and during the lockdown (March 1 to July 24, 2020). There was a reduction average concentrations of PM 10 , PM 2.5 and NO 2 by 15%, 8% and 6%, respectively, compared to the period before the lockdown, as shown in Table 2 . There was a substantial reduction in the average concentrations of NO 2 in lockdown periods by 7%, 8% and 20%, respectively, compared to the period before the lockdown. There was a reduction average concentrations PM 2.5 by 2.5% during June 14 to July 24, compared to the period before the lockdown. There was an increase in PM 10 by 34%, 55% and 56%, respectively, during the periods of lockdown. The natural sources of suspended particles in Iraq are dust and suspended in the air. This phenomenon reaches its top activity in spring and summer, due to the arrival of depressions coming from north of the Arabian Gulf and from central Asia causing northwesterly winds with a varying severity according to the depression's severity (1 st National Communication, 2016). As for the industrial sources, represented by power plants and transportation, as well as the spread of different kinds of generators of various sizes servicing homes, farms, factories, and various governmental and non-governmental. This leads to increased air pollutants in the atmosphere, and environmental J o u r n a l P r e -p r o o f pollution (Hashim et al, 2020) . On the other hand, there was an increase in O 3 by 13%, 75%, 225% and 525%, between the periods before the lockdown (January 16 to February 29, 2020) and during the lockdown (March 1 to July 24, 2020). That's can be explained by the higher levels of solar activity during the period of the lockdown, especially in June and July. The AQI improved in Baghdad during the 1 st lockdown, compared to its pre-lockdown average of -13%. However, this temporary improvement diminished in the subsequent periods, reaching 39% during the June and July. The poor air quality of Baghdad is due to the presence of most industrial and commercial activities there, in addition to high population density and traffic pollution. Also, the climate of Iraq in general is dry and hot in summer, which led to increase the concentration of pollutants and deteriorate the air quality (Ministry of Environment, 2016). Fig.7 presents the correlation coefficients between air pollutants (NO 2 , O 3 , PM 2.5 and PM 10 ) in air of Baghdad before and after the lockdown. Air pollutants had significant correlations with each other. The PM 2.5 and PM 10 concentrations correlated well (R 2 = 0.6174), which could indicate that PM 2.5 and PM 10 originated from common sources. On the other hand, NO 2 had negatively correlated with O 3 and PM 10 . The concentration of surface O 3 decreases substantially with increased concentration of NO 2 . When the nitric oxide (NO) emissions are sufficiently large, NO released in the atmosphere converts a large fraction of O 3 into NO2 (Monks et al., 2015) . Fig.8 represents NO 2 emission in Iraq before, and during the COVID-19, based on dates of data acquisition derived from S-5P/TROPOMI, as shown in Table 1 . According to these data, the NO 2 emissions reduced up to 35 to 40% across Iraq due to lockdown, between January and July, 2020, especially across the major cities such as Baghdad, Basra, Najaf, and Erbil. Fig.8 showed that Baghdad recorded the highest NO 2 emission before the lockdown, compared to other cities, due to high population density, traffic pollution and industrial activities. The region located southern Baghdad, such as Babylon, Najaf and Karbala, contained high NO 2 emissions, due to industrial activities and power plants. Basra in southern Iraq has hotspots of NO 2 , because it is the largest city in the south of Iraq contains oil fields and associated gas burning activities. The effect of the lockdown is apparent on the reduction of NO 2 emission in Baghdad, Basra and Erbil. Transportation restriction, industry emission slowdown, led to a clear decrease in NO 2 emission, compared to previous periods, before and during the COVID-19 lockdown. Due to the high incidence of confirmed cases of COVID-19, the Iraqi government announced the 2 nd total lockdown again on May 24, 2020, to control the spread of the pandemic. As a result, NO 2 emission decline almost to the lowest levels on June 5, 2020 in all parts of Iraq. On June 14, up to July 24, 2020 Iraq has returned and announced the partial lockdown for the third time. NO 2 emission decline slightly, especially in Baghdad, Basra and Erbil, compared to the levels in the beginning of April. The results showed that the NO 2 tropospheric column over Iraq revealed a major hotspot of high concentration, especially in the Baghdad and Basra before the lockdown, and these hotspots receded significantly during the lockdown on April, June and July, 2020.
In this study, the influences of emission reductions due to reduced anthropogenic activities, mainly on transportation and industry during the COVID-19 lockdown in Iraq on air pollution were investigated. There was a substantial reduction in the NO 2 concentration by 6, 7, 8 and 20% in Baghdad during four periods of partial and total lockdown, compared to the period before the lockdown. There was 8 and 15% reduction in PM 2.5 and PM 10 concentrations, respectively, in Baghdad, during the 1 st partial and total lockdown. Even under the lowtraffic pollution in Baghdad, the PM 2.5 and PM 10 concentrations before and during the lockdown period exceeded the WHO daily limit values, providing evidence of the high contribution from non-traffic related sources, such as natural sources and local dust. On the other hand, there was an increase in O 3 by 13%, 75%, 225% and 525%, between the periods before the lockdown (January 16 to February 29, 2020) and during the lockdown (March 1 to July 24, 2020), which could be due to the decrease in NO 2 and increase PM 10 concentrations. The AQI improved in Baghdad during the 1 st lockdown, average of 13%, compared to its pre-lockdown. However, this temporary improvement diminished in the subsequent periods, during the June and July. Data of NO 2 tropospheric column derived from Sentinel-5P/TROPOMI shown clearly reduction up to 35 to 40% across Iraq, during the lockdown; due to transportation restriction and industry emission slowdown. Transportation and industrial activities have significantly been affected due to COVID-19 lockdowns, as there are less energy consumption and less oil demand. The outcomes of the lockdown can be easily identified as an improvement of - 