Corona! Corona! And Corona! A menacing word has been repeated and heard more than any other word since its outbreak in Wuhan, China, in late 2019. The Covid-19 pandemic has changed human activities on Earth, including social distancing, telecommuting, online shopping, and closure of public places, such as markets, parks, schools, businesses (Berman & Ebisu, 2020; Gautam, 2020). The direct association of Covid-19 with several factors, such as air pollution, the body immune system, and environmental impacts, is an example of current knowledge from the study of Covid-19 (Contini & Costabile, 2020).
The first cases of the infection with Coronavirus in Iran were reported on February 19, 2020 (Razavi-Termeh et al., 2021). Outbreaks emerged in Asia, Europe, and other countries, and by the end of March 2020, Covid-19 turned into a global pandemic (Muhammad et al., 2020). As quarantine and lockdown started in many countries, a substantial decline in air pollution was reported due to the closure of factories and businesses, halting industrial sectors, and reducing transportation and traffic. These consequently resulted in a considerable drop in oil and fuel usage (Berman & Ebisu, 2020; Muhammad et al., 2020). On the other hand, Covid-19 mainly affects the human respiratory system causing lethal lung injury and death. Studies conducted on Covid-19 (Allu et al., 2021; Berman & Ebisu, 2020; Burnett et al., 2018; Faustini et al., 2014; He et al., 2020; Muhammad et al., 2020; Ryan et al., 2020) showed that exposure to air pollution could cause severe cardiovascular and respiratory problems. Therefore, there should be a potential relation between air quality and Covid-19 (Conticini et al., 2020; Ryan et al., 2020; Travaglio et al., 2021; Zhang et al., 2021; Zhu et al., 2020).
The lockdown was imposed in Iran from March 21 to April 21 in 2020 (Broomandi et al., 2020; Ravindran et al., 2021). During this period, the country witnessed a massive decline in transportation, industrial, and business activities. The traffic was reduced by 73%, which resulted in a 50% reduction in gas consumption in Tehran, the capital city of Iran (Rad et al., 2020). Other industrial metropolises in Iran were faced with Covid-19 restrictions, resulting in lower pollutants and cleaner air.
Extensive studies have been conducted worldwide to analyze the impact of Coronavirus spread on air pollution. For example, the effects of pollutants change due to decline in anthropogenic activities as a result of Covid-19 have been investigated in U.S. (Berman & Ebisu, 2020; Naeger & Murphy, 2020; Wu et al., 2020), England (Travaglio et al., 2021; Wyche et al., 2021), Italy (Fattorini & Regoli, 2020; Filippini et al., 2021), Spain (Briz-Redón et al., 2021; Cárcel-Carrasco et al., 2021; Querol et al., 2021; Tobías et al., 2020), India (Allu et al., 2021; Karuppasamy et al., 2020; Naqvi et al., 2021; Shehzad et al., 2020; Vadrevu et al., 2020), South Korea (Ju et al., 2021), China (Chen et al., 2020; Fan et al., 2020; Zhang et al., 2021; Zhu et al., 2020), Netherlands (Cole et al., 2020), Canada (Adams, 2020), South Africa (Ryan et al., 2020), France (Magazzino et al., 2020), Brazil (Debone et al., 2020; Nakada & Urban, 2020), Asia (Ghahremanloo et al., 2021; Gupta et al., 2021), Europe (Ogen, 2020; Skirienė & Stasiškienė, 2021), and worldwide (Albayati et al., 2021; Gautam, 2020; Muhammad et al., 2020; Pozzer et al., 2020; Singh et al., 2021; Zambrano-Monserrate et al., 2020). A few studies have also assessed the impact of the Covid-19 pandemic on air pollution in Iran. For example, Kaviani Rad et al. (Rad et al., 2020) studied air pollutants in the first and second waves of the disease in Tehran over 40 days from March 1 to April 9, 2020. Their results showed that the amount of Carbon Monoxide (CO), Nitrogen Dioxide (NO2), PM10, and PM2.5 pollutants decreased, and SO2 and O3 pollutants increased from March 1 to April 9, 2020, compared to the same period in 2019 (Rad et al., 2020). In another study, Broomandi et al. (Broomandi et al., 2020) identified environmental pollutants (e.g., O3, NO2, SO2, CO PM10, and PM2.5) using data from 12 ground stations from March 21 to April 21 in 2019 and 2020. The results showed that, except for O3 and PM2.5, the average concentrations of pollutants were decreased. Similarly, Borhani et al. (Borhani et al., 2021) investigated atmospheric pollutants, including O3, NO2, SO2, CO PM10, and PM2.5, using 21 stations in three time periods (February 23, 2020, to March 15, 2020, March 18, 2020, to April 3, 2020, and April 5, 2020, to April 17, 2020) for Tehran during the pandemic.
Although a few studies have been conducted in Iran to investigate the effects of Covid-19 spread on air pollution, to the best of our knowledge, they mainly focused on the capital of Iran (i.e., Tehran) using only ground station data, which are not available in most cities of Iran. Therefore, in this study, the temporal and spatial distributions of CO and NO2 pollutants produced by vehicles and urban traffic were investigated using the Sentinel-5P, TROPOspheric Monitoring Instrument (TROPOMI) data. The accuracy of the results obtained from the TROPOMI sensor has been confirmed in several recent studies. For example, in a study in Helsinki, Finland (Ialongo et al., 2020), a correlation coefficient of 0.68 was obtained between the results of satellite-based TROPOMI NO2 and ground observations. Correlation coefficients of 0.44 and 0.75 for Frankfurt and Barcelona were also obtained in another research (Vîrghileanu et al., 2020). This indicates that this sensor has a high potential for accurate estimation of air pollutants, especially large-scale NO2 concentration. The analyses were conducted in Iran (mainly four cities of Tehran, Tabriz, Isfahan, and Mashhad) in three years of 2019 (a year before the outbreak), 2020 (start of the outbreak), and 2021 (a year after the outbreak).
The remainder of the paper is as follows. After the introduction, the methods and materials, study area, and methodology is discussed in part 2. Then, the output of the NO2 and CO and results are presented. In the next part, the results are evaluated and compared with daily new deaths due to Covid-19. Finally, the paper is concluded in part 4.