*Article* **Spatiotemporal Variations of Air Pollution during the COVID-19 Pandemic across Tehran, Iran: Commonalities with and Differences from Global Trends**

**Mohsen Maghrebi 1,\* , Ali Danandeh Mehr 2,3,\* , Seyed Mohsen Karrabi <sup>4</sup> , Mojtaba Sadegh <sup>5</sup> , Sadegh Partani <sup>6</sup> , Behzad Ghiasi <sup>1</sup> and Vahid Nourani 3,7**


**Abstract:** The COVID-19 pandemic has induced changes in global air quality, mostly short-term improvements, through worldwide lockdowns and restrictions on human mobility and industrial enterprises. In this study, we explored the air pollution status in Tehran metropolitan, the capital city of Iran, during the COVID-19 outbreak. To this end, ambient air quality data (CO, NO<sup>2</sup> , O<sup>3</sup> , PM10, SO<sup>2</sup> , and AQI) from 14 monitoring stations across the city, together with global COVID-19-related records, were utilized. The results showed that only the annual mean concentration of SO<sup>2</sup> increased during the COVID-19 pandemic, mainly due to burning fuel oil in power plants. The findings also demonstrated that the number of days with a good AQI has significantly decreased during the pandemic, despite the positive trend in the global AQI. Based on the spatial variation of the air quality data across the city, the results revealed that increasing pollution levels were more pronounced in low-income regions.

**Keywords:** COVID-19; air pollution; Tehran; AQI

#### **1. Introduction**

Urban air pollution is known as a major human health challenge [1,2]. According to the World Health Organization (WHO), approximately seven million premature deaths occur annually across the globe due to air pollution [3]. While air quality has improved substantially in the US and many developed countries, unhealthy levels of air pollution remain a daunting challenge in many developing countries and are expected to worsen in some regions owing to a variety of natural and anthropogenic sources [4–7]. The lack of a comprehensive decision-making system, old public transportation fleets, inefficient planning and management in urban environments in the face of high population density, and inadequate financial means are the main barriers to achieving or maintaining clean air in developing countries [8–10]. Iran, rich in oil and gas resources, suffers greatly from the complex challenges caused by air pollution [4,11,12] and endures more than 49,000 air pollution-related deaths annually [13]. The air pollution status in densely populated areas, especially the capital city of Tehran, is at a critically concerning level. The main driving factors for the excessive air pollution in this metropolitan city are (i) special topography that allows the confinement of pollutants over the city: Tehran is engulfed from three sides by mountains i.e., Shemiran hills in the north, Damavand hills in the east, and Karaj hills in

**Citation:** Maghrebi, M.; Danandeh Mehr, A.; Karrabi, S.M.; Sadegh, M.; Partani, S.; Ghiasi, B.; Nourani, V. Spatiotemporal Variations of Air Pollution during the COVID-19 Pandemic across Tehran, Iran: Commonalities with and Differences from Global Trends. *Sustainability* **2022**, *14*, 16313. https://doi.org/ 10.3390/su142316313

Academic Editors: José Carlos Magalhães Pires and Álvaro Gómez-Losada

Received: 14 October 2022 Accepted: 4 December 2022 Published: 6 December 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the west [14]; (ii) dry climatic and stagnant air conditions: there is no noticeable rainfall during half of a year, and winds usually do not have the necessary power to move pollution out of the urban area, with 70% of the winds having a speed of less than 3 m/s [15]; (iii) high population density: the population density in Tehran, 11,969 people per square kilometer, is more than 180 times higher than Iran's average [16]; (iv) excessive daily trip frequency: more than 20 million daily trips occur in the city and there are currently more than 4 million vehicles and 3 million motorcycles in use, twice of its ecological capacity [17]; and (v) old and inefficient vehicles: 71% of Tehran's pollution is due to mobile pollution sources. This undesirable air condition is the main cause of 4800 annual deaths and health costs exceeding 2 billion U.S. dollars per year [1,4].

The extreme outbreak of COVID-19 occurred in the presence of significant air quality challenges in Tehran. Lockdown policy and urban activity restrictions have the potential to reduce urban transportation and ultimately help improve air quality [18–22]. Different nations, however, with different economic and social conditions responded differently to the COVID-19 pandemic, which translated to various levels of change in the urban air quality status, with significant implications for environmental justice issues [23–28]. Therefore, despite the national stay-at-home orders during the COVID-19 outbreak, albeit infrequent, some residents of Tehran had to continue their daily activities to survive. To understand how different factors affected Tehran's air quality during the COVID-19 pandemic, this study investigates Tehran's air pollution status using daily records from 14 monitoring stations across this metropolitan area and compares it with global COVID-19-related records. The results are of paramount importance to show how different socioeconomic factors affect urban air quality.

### **2. Materials and Methods**

Tehran, the capital of Iran, is located at 35◦400 North and 51◦190 East with an average altitude of 1190 m above sea level. Figure 1 demonstrates the 22 municipal regions of Tehran and the location of meteorological stations distributed across the city. This city has a length of approximately 27 km from north to south and 50 km from east to west. Figure 2 illustrates the main meteorological features of the city during the COVID-19 pandemic. This vibrant metropolitan area is home to more than 8.5 million people (10% of Iran's total population and 30% of Iran's urban population). Tehran's population increases by almost three million during the day as residents from surrounding communities move to Tehran for work and personal business [16]. This mega city alone accounts for more than 20% of total energy consumption in Iran [29]. One of the most important features of this city is its self-purification capacity due to its vast green space, a capacity that is overpowered by the city's power plants and the immense number of vehicles. Tehran has 2277 orchards with a total area of 5949 hectares (Figure S1). In addition, green space along Tehran's urban roads accumulates to 8253 hectares and the green belt around the city has an area of 42,855 hectares. Around Tehran, there are five fossil fuel power plants, including the Montazer Ghaem and Tarasht power plants in the west, and the Rey, Besat, and Parand power plants in the south of the city, with a capacity of 3000 megawatts (MW). The main power generation capacity is mainly located in the south of the city [30].

**Figure 1.** Study area and location of monitoring stations. **Figure 1.** Study area and location of monitoring stations.

(**c**) (**d**) **Figure 2.** Main meteorological variables at Tehran synoptic station during the COVID-19 pandemic, (**a**) maximum and minimum mean temperature with mean temperature, (**b**) mean daylight and sunshine hours, (**c**) mean monthly rainfall and number of rainy days, and (**d**) mean wind speed.

#### *Air Quality Indices*

Ambient air quality data for critical pollutants used in this study include carbon monoxide (CO), nitrogen dioxide (NO2), coarse particulate matter (PM10), ozone (O3), sulfur dioxide (SO2), as well the as air quality index (AQI). CO is an odorless toxic pollutant in the air that is largely produced from fossil fuel combustion [31]. Long-term exposure to CO can reduce blood oxygen-carrying capacity and cause neurological and cardiovascular diseases in humans [32]. NO is a gaseous air pollutant that is formed when fossil fuels such as oil or coal are burned at high temperatures. In the atmosphere, it turns into NO<sup>2</sup> through a reaction with O3. Public and private cars are the largest source of this pollutant, followed by power plants, heavy equipment, and other mobile engines and industrial boilers [33,34]. Long exposure to NO<sup>2</sup> can cause bronchitis, asthma attacks, and phlegm [35]. Tehran's NO<sup>2</sup> usual concentration is 85 ppb [1]. PM<sup>10</sup> can cause serious environmental problems and certain types of cancer [36,37]. O<sup>3</sup> is produced by chemical reactions between nitrogen oxides and volatile organic compounds in the presence of sunlight and heat. Therefore, the possibility of increasing the O<sup>3</sup> levels to unhealthy levels on sunny and hot days is high. Ground-level O<sup>3</sup> is a grave environmental hazard [38]. People with asthma, children, older adults, and people with reduced intake of certain nutrients, such as vitamins C and E, are at greater risk from O<sup>3</sup> exposure. SO<sup>2</sup> is a colorless, bad-smelling, toxic gas that is emitted by the burning of fossils. Diesel vehicles and equipment (i.e., power plants) have long been a major source of SO<sup>2</sup> [39]. SO<sup>2</sup> can harm the human respiratory system and make breathing difficult [40]. At high concentrations, gaseous SO<sup>2</sup> can harm trees and plants by damaging foliage and decreasing growth. The AQI is a unitless measure of air quality that runs from 0 to 500. An AQI of 100 is usually associated with the regulatory pollutant level. The higher the AQI value, the greater the health threat from air pollution and the higher the environmental hazards [41]. To attain the AQI at each air quality monitoring station, first, the pollutant index (*Ip*) is calculated individually for each ambient pollutant using Equation (1) [42]. Then, the maximum acquired *Ip* is considered as the station's AQI.

$$I\_p = \frac{I\_{Hi} - I\_{Lo}}{BP\_{Hi} - BP\_{Lo}} \left(\mathbb{C}\_p - BP\_{Lo}\right) + I\_{Lo} \tag{1}$$

where *C<sup>p</sup>* is the truncated concentration of pollutant *p* (µg/m<sup>3</sup> ), *BPHi* is the concentration breakpoint that is greater than or equal to *Cp*, *BPLo* is the concentration breakpoint that is less than or equal to *Cp*, *IHi* is the AQI value corresponding to *BPHi*, and *ILo* is the AQI value corresponding to *BPLo*. It is required to mention that O<sup>3</sup> should be truncated to three decimal places, CO should be truncated to one decimal place, and SO2, PM10, and NO<sup>2</sup> should be truncated to an integer number. For details on pollutant-specific breakpoints/truncations and examples of the AQI calculation/classification procedure, the interested reader is referred to technical assistance on the AQI [42].

All the data were collected from 14 atmospheric monitoring stations in Tehran between 1 January 2019, and 30 December 2020, provided by the Air Quality Control Center of the Tehran municipality. COVID-19-related data was acquired from the "Our World in Data" website (https://ourworldindata.org/coronavirus/country/iran?country=~IRN, accessed on 21 April 2021). It should be noted that COVID-19 national cumulative data is published daily, and spatial classification of these data did not exist at the time of this study. To spatially interpret the data and estimate the values between measurements, the deterministic reverse distance weighting interpolation method was used. In this method, the effect of a known data point is inversely related to the distance from the location being estimated [9,42]. In this study, the most recent published values of the ambient air quality indices have been used as the baseline (usual) concentration. Finally, the well-known *t*-test was implemented to discover how significant the differences between the indices before and during COVID-19 are.

#### **3. Results and Discussion**

In Iran, the official onset date of COVID-19 was announced as February 19, 2020 (in the city of Qom near Tehran). On the same day, the Iranian government raised the COVID-19 alert to yellow [43]. The number of confirmed cases increased slightly from mid-March to early July followed by a slight decrease in August; since September, with the arrival of rapid diagnosis kits from South Korea, the number of confirmed cases escalated. To reduce the propagation of the illness, some of the restrictions imposed on daily life and social activities, such as the travel ban policy and prevention of intercity transportation, positively affected the air quality of big cities in Iran [44]. Referring to Tehran, the air pollution related to the travel ban policy was reduced significantly in 2020 [45]. A pioneering study on the effect of changes in traffic flow patterns across Iran has demonstrated that each one million intercity traffic was associated with a 3% increase in COVID-relevant mortality with a time lag of five weeks [46].

#### *3.1. Carbon Monoxide (CO)*

The concentration of CO in 2015 was considered here as a usual concentration in different districts of Tehran, which varied between 26.6 to 32.6 ppm [9]. The mean annual CO concentration in 2020 decreased to 25.2 ppm from 27.7 ppm in 2019, which is not significant (the significancy of variation is discussed later in Section 3.7), both of which are lower than the usual concentration. In 2020, the range of change in CO concentration was lower than its preceding at all stations (Figure S2). The highest inter-annual range of change in CO concentration in 2020 was 91 ppm (station 14), which was lower than that in 2019 at 157 ppm (station 13). The minimum and maximum CO concentrations in 2020 were also lower than in 2019. Minimum CO concentrations of 3 ppm and 4 ppm were measured at stations 5 and 6 in April 2020 and September 2019, respectively, whereas its maximum concentration levels were 99 ppm and 163 ppm in October 2020 and November 2019 at stations 14 and 13, respectively. The CO concentration only changed significantly in November and December (*p*-value < 0.05), and we cannot prove any significant trends in other months. Traditionally, the CO concentration is higher in winter and cold weather than in the rest of the year [47], a phenomenon that was observed in both years. The main reason for this is the confinement of pollution due to the special topography of Tehran, and temperature. Despite the 2020 average CO concentration levels across all stations being lower than that of 2019, there was significant heterogeneity among stations without a clear trend that holds for all (Figure 3). Only in November 2020 did Tehran witness a steady reduction in CO concentration at all stations, compared to the same month in 2019, which is attributed to the enforcement of strict traffic restrictions as the confirmed deaths due to COVID-19 soared. The southwestern part of the city had a higher concentration of CO from March to September 2020 compared to the previous year. A similar pattern was observed in the center and northwest of Tehran but during October and December. We highlight a decrease in CO concentration in the west and southwest of Tehran after the outbreak of COVID-19 compared to previous months.

**Figure 3.** *Cont*.

**Figure 3.** (**a**) Distribution map of maximum, minimum and mean of monthly CO concentration at each station and (**b**) spatial distribution of variations in the mean monthly CO concentration in 2020 compared to 2019. **Figure 3.** (**a**) Distribution map of maximum, minimum and mean of monthly CO concentration at each station and (**b**) spatial distribution of variations in the mean monthly CO concentration in 2020 compared to 2019.

#### *3.2. Nitrogen Dixide (NO2) 3.2. Nitrogen Dixide (NO2)*

The mean annual NO2 concentrations in 2020 and 2019 were 71.7 ppb and 71.8 ppb, respectively (Figure S3), which although being lower than the usual concentration level, are about 1.7 times higherthan the standard threshold recommended by the World Health Organization (WHO). The mean monthly concentration decreased significantly only in March, April, and December (*p*‐value < 0.05). The maximum NO2 level in 2020 (2019) was 114 ppb (142 ppb), which was observed at station 8 (6) in December (November). The minimum NO2 concentration in 2020 (2019) was 13 ppb (5 ppb), which was observed at station 12 (3) in August (March). The average NO2 concentration was 85.9 ppb in 2020 as compared to 88.1 ppb in 2019 (Figure 4). While other studies show a sharp decrease in NO2 concentrations globally due to the COVID‐19 outbreak [48–51], a significant decrease in NO2was not observed in Tehran. Furthermore, changes in NO2 concentrations follow a The mean annual NO<sup>2</sup> concentrations in 2020 and 2019 were 71.7 ppb and 71.8 ppb, respectively (Figure S3), which although being lower than the usual concentration level, are about 1.7 times higher than the standard threshold recommended by the World Health Organization (WHO). The mean monthly concentration decreased significantly only in March, April, and December (*p*-value < 0.05). The maximum NO<sup>2</sup> level in 2020 (2019) was 114 ppb (142 ppb), which was observed at station 8 (6) in December (November). The minimum NO<sup>2</sup> concentration in 2020 (2019) was 13 ppb (5 ppb), which was observed at station 12 (3) in August (March). The average NO<sup>2</sup> concentration was 85.9 ppb in 2020 as compared to 88.1 ppb in 2019 (Figure 4). While other studies show a sharp decrease in NO<sup>2</sup> concentrations globally due to the COVID-19 outbreak [48–51], a significant decrease in NO<sup>2</sup> was not observed in Tehran. Furthermore, changes in NO<sup>2</sup> concentrations follow a non–uniform pattern. Most increases in NO<sup>2</sup> concentrations from 2019 to 2020 were seen in the west, south, and southwest of Tehran, where the main out-of-town passenger

non–uniform pattern. Most increases in NO2 concentrations from 2019 to 2020 were seen

August and September when the highest intercity travels traditionally occur. The maxi‐ mum increase in NO2 levels occurring in these months could be a sign of a change in travel patterns and a higher rate of intercity public transport use—such as buses—in the shadow

centers and main city terminals are located. The highest monthly increase was observed in August and September when the highest intercity travels traditionally occur. The maximum increase in NO<sup>2</sup> levels occurring in these months could be a sign of a change in travel patterns and a higher rate of intercity public transport use—such as buses—in the shadow of the COVID-19 outbreak when the government prohibited the entrance of cars with out-of-province license plates into Tehran. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 8 of 23 of the COVID‐19 outbreak when the government prohibited the entrance of cars with out‐ of‐province license plates into Tehran.

**Figure 4.** *Cont*.

**Figure 4.** (**a**) Distribution map of mean monthly NO2 concentration at each station and (**b**) spatial distribution of variations in the mean monthly NO2 concentration in 2020 compared to 2019. **Figure 4.** (**a**) Distribution map of mean monthly NO<sup>2</sup> concentration at each station and (**b**) spatial distribution of variations in the mean monthly NO<sup>2</sup> concentration in 2020 compared to 2019.
