*3.4. Ozone (O3)*

and October (*p*‐value < 0.05).

*3.4. Ozone (O3)* Tehran's usual O3 concentration is 68.8 ppb [1]. The mean O3 concentration in 2020 compared to 2019 has increased from 42.4 ppb to 45.7 ppb, which is not a significant change (*p*‐value> 0.05). The maximum O3 concentration in 2020 (2019) was 235 ppb (204 ppb) and seen at station 2 (7) in July (June) (Figure 6). The minimum O3 level is 2 ppb in both 2019 and 2020 and can be seen in several stations during November and December. Maximum and minimum O3 concentrations are expected to occur in summer and winter due to temperature and solar radiation levels in these seasons (Figure S5). Therefore, it Tehran's usual O<sup>3</sup> concentration is 68.8 ppb [1]. The mean O<sup>3</sup> concentration in 2020 compared to 2019 has increased from 42.4 ppb to 45.7 ppb, which is not a significant change (*p*-value > 0.05). The maximum O<sup>3</sup> concentration in 2020 (2019) was 235 ppb (204 ppb) and seen at station 2 (7) in July (June) (Figure 6). The minimum O<sup>3</sup> level is 2 ppb in both 2019 and 2020 and can be seen in several stations during November and December. Maximum and minimum O<sup>3</sup> concentrations are expected to occur in summer and winter due to temperature and solar radiation levels in these seasons (Figure S5). Therefore, it can be concluded that O<sup>3</sup> concentration changed significantly in June, August, September, and October (*p*-value < 0.05).

can be concluded that O3 concentration changed significantly in June, August, September,

(**a**)

**Figure 6.** *Cont*.

**Figure 6.** (**a**) Distribution map of mean monthly O3 concentration at each station and (**b**) spatial distribution of variations in the mean monthly O3 concentration in 2020 compared to 2019. **Figure 6.** (**a**) Distribution map of mean monthly O<sup>3</sup> concentration at each station and (**b**) spatial distribution of variations in the mean monthly O<sup>3</sup> concentration in 2020 compared to 2019.

#### *3.5. Sulfur Dioxide (SO2) 3.5. Sulfur Dioxide (SO2)*

Tehran's usual SO2 concentration is 8.9 ppb, and its highest levels are usually seen in winter [1]. The mean annual SO2 concentration increased from 7.4 ppb to 17.5 ppb from 2019 to 2020 (*p*‐value< 0.05). The maximum SO2 concentration in 2020 (2019) was 143 ppb (34 ppb) at station 7 (7) and in November (January) (Figure 7). The minimum SO2 level ranges between 2 ppb to 3 ppb in 2019 and 2020 between January and March. In all sta‐ tions, the maximum and mean monthly concentration increased in 2020 compared to 2019 (Figure 7). A major change in the source of fuel in Tehran's power plants is the main cul‐ prit of this pollution increase. In the cold winter of 2020, due to the need to maintain nat‐ ural gas in home networks, the fuel of power plants was changed from natural gas to fuel oil. This decision, which made its way to the press a few months later, became a source of public controversy, and the media called it a wrong decision and a threat to public health. Tehran's usual SO<sup>2</sup> concentration is 8.9 ppb, and its highest levels are usually seen in winter [1]. The mean annual SO<sup>2</sup> concentration increased from 7.4 ppb to 17.5 ppb from 2019 to 2020 (*p*-value < 0.05). The maximum SO<sup>2</sup> concentration in 2020 (2019) was 143 ppb (34 ppb) at station 7 (7) and in November (January) (Figure 7). The minimum SO<sup>2</sup> level ranges between 2 ppb to 3 ppb in 2019 and 2020 between January and March. In all stations, the maximum and mean monthly concentration increased in 2020 compared to 2019 (Figure 7). A major change in the source of fuel in Tehran's power plants is the main culprit of this pollution increase. In the cold winter of 2020, due to the need to maintain natural gas in home networks, the fuel of power plants was changed from natural gas to fuel oil. This decision, which made its way to the press a few months later, became a source of public controversy, and the media called it a wrong decision and a threat to public health. It is noteworthy that, in the northern region of the city, the SO<sup>2</sup> concentration decreased in all months after the COVID-19 outbreak. The wealthiest population of Tehran

It is noteworthy that, in the northern region of the city, the SO2 concentration decreased in

ran—where the power plants are located—we see an increase in SO2 concentration (Figure S6). These regions house working‐class and low‐income communities, which suffered in‐ creasing air pollution during the pandemic when most of the global population benefited

from cleaner air due to stay‐at‐home orders.

inhabits this region and it is not close to any power plants. In parts of the west and south of Tehran—where the power plants are located—we see an increase in SO<sup>2</sup> concentration (Figure S6). These regions house working-class and low-income communities, which suffered increasing air pollution during the pandemic when most of the global population benefited from cleaner air due to stay-at-home orders. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 14 of 23

**Figure 7.** *Cont*.

**Figure 7.** (**a**) Distribution map of mean monthly SO2 concentration at each station and (**b**) spatial distribution of variations in the mean monthly SO2 concentration in 2020 compared to 2019. **Figure 7.** (**a**) Distribution map of mean monthly SO<sup>2</sup> concentration at each station and (**b**) spatial distribution of variations in the mean monthly SO<sup>2</sup> concentration in 2020 compared to 2019.

### *3.6. Air Quality Index—AQI*

*3.6. Air Quality Index—AQI* Figure 8 compares Tehran's AQI among 14 stations in 2019 and 2020. For example, station 1 witnessed 60 days of good AQI in both years. In 2020 (2019), the maximum AQI index was 235 (204), which occurred at station 2 (12) and was observed in May (June). In addition, its minimum was equal to 15 (6) and was observed at station 4 (11) and in Feb‐ ruary (January) (Figure S6). In all stations, the mean and maximum AQI significantly in‐ creased in 2020 compared to 2019 (*p*‐value < 0.05) (except for stations 7, 9, 12, and 13, whose maximum decreased in 2020). This trend is contrary to the globally observed trends that showed improved air quality in 2020 [57]. In the rich northern region of Tehran, in all months the AQI decreased, and the air condition improved, a trend that is comparable to the rest of the globe. In the northeastern parts, however, the AQI increased and the air quality deteriorated. In almost all stations, we see an increase in the AQI in the intervals Figure 8 compares Tehran's AQI among 14 stations in 2019 and 2020. For example, station 1 witnessed 60 days of good AQI in both years. In 2020 (2019), the maximum AQI index was 235 (204), which occurred at station 2 (12) and was observed in May (June). In addition, its minimum was equal to 15 (6) and was observed at station 4 (11) and in February (January) (Figure S6). In all stations, the mean and maximum AQI significantly increased in 2020 compared to 2019 (*p*-value < 0.05) (except for stations 7, 9, 12, and 13, whose maximum decreased in 2020). This trend is contrary to the globally observed trends that showed improved air quality in 2020 [57]. In the rich northern region of Tehran, in all months the AQI decreased, and the air condition improved, a trend that is comparable to the rest of the globe. In the northeastern parts, however, the AQI increased and the air quality deteriorated. In almost all stations, we see an increase in the AQI in the intervals between stay-at-home orders. This can be attributed to the accumulation of transportation needs after periods of quarantine (Figure 9). While in 2019 the number of good days was

between stay‐at‐home orders. This can be attributed to the accumulation of transportation

marginally increased from 202 in 2019 to 214 in 2020 (Figure 8).

58, it was reduced to 30 days in 2020. On the other hand, the number of moderate days marginally increased from 202 in 2019 to 214 in 2020 (Figure 8).

*Sustainability* **2022**, *14*, x FOR PEER REVIEW 16 of 23

**Figure Figure 8. 8.** AQI AQI index among investigated stations in 2019 and 2020. index among investigated stations in 2019 and 2020.

**Figure 9.** *Cont*.

**Figure 9.** (**a**) Distribution map of the mean monthly AQI concentration at each station and (**b**) spatial distribution of variations in the mean monthly AQI concentration in 2020 compared to 2019. **Figure 9.** (**a**) Distribution map of the mean monthly AQI concentration at each station and (**b**) spatial distribution of variations in the mean monthly AQI concentration in 2020 compared to 2019.
