*3.7. Significance of Variations*

**Index**

**Month**

*3.7. Significance of Variations* As previously mentioned, a correlated pairs t‐test was implemented to determine if the differences (results) were statistically significant. The null hypothesis is that the true difference in the mean value of each index before and during COVID‐19 is zero. Table 1 summarizes the *p*‐values achieved for the indices. Considering the 95% confidence inter‐ val (i.e., *p*‐value ≤ 0.05), the null hypothesis is rejected in this study. Accordingly, signifi‐ cant change in SO2 (AQI) was observed in all (almost all) months. Regarding CO and NO2, significant variation in air quality was seen in two (December and November) and three As previously mentioned, a correlated pairs *t*-test was implemented to determine if the differences (results) were statistically significant. The null hypothesis is that the true difference in the mean value of each index before and during COVID-19 is zero. Table 1 summarizes the *p*-values achieved for the indices. Considering the 95% confidence interval (i.e., *p*-value ≤ 0.05), the null hypothesis is rejected in this study. Accordingly, significant change in SO<sup>2</sup> (AQI) was observed in all (almost all) months. Regarding CO and NO2, significant variation in air quality was seen in two (December and November) and three (March, April, and December) months, respectively. The monthly significant changes in PM<sup>10</sup> and O<sup>3</sup> indices were seen in four months during the pandemic.

(March, April, and December) months, respectively. The monthly significant changes in

February <0.05 0.62 0.43 0.54 0.98 <0.05 March <0.05 0.23 <.05 <0.05 0.40 <0.05 April <0.05 0.57 <.05 0.20 0.70 <0.05 May <0.05 0.52 0.13 0.30 0.18 0.65

**SO2 CO NO2 PM10 O3 AQI**

**Table 1.** The attained *p*‐value on air quality indices across Tehran.

PM10 and O3 indices were seen in four months during the pandemic.


**Table 1.** The attained *p*-value on air quality indices across Tehran.

Reductions in primary air pollutants during the COVID-19 outbreak were reported in several studies. Table 2 compares our findings with those attained in different regions.



#### **4. Conclusions**

In this article, using daily data from 14 air quality monitoring stations across Tehran metropolitan city, air quality dynamics before and during the COVID-19 pandemic and its driving factors were examined. The results of this study compare monthly and mean annual pollution levels in 2019, 2020, and usual concentration for CO (20.17 ppm, 25.2 ppm, 26.6—32.1 ppm), NO<sup>2</sup> (71.8, 71.7, 85) ppb, PM<sup>10</sup> (56.7, 55.8, 99.5) µg/m<sup>3</sup> , O<sup>3</sup> (42.4, 45.7,68.8) ppb, SO<sup>2</sup> (7.4, 17.5, 8.9) ppb, respectively. According to the results, any significant reduction in annual concentration of CO, NO2, PM<sup>10</sup> and O<sup>3</sup> was not observed. However, the SO<sup>2</sup> concentrations increased significantly during the pandemic. These trends are

attributed to the change in the main fuel source from natural gas to fuel oil in Tehran power plants. Political sanctions and undue economic pressure inhibited the maintenance and upgrade of Iran's infrastructure, which forced the decision makers to choose poor air quality (i.e., use of fuel oil in power plants) over cutting natural gas for urban consumers in the cold winter days. Spatial analysis shows that air pollution indices have drastic heterogeneity in Tehran, which are attributed to topography, population density and land use patterns. This heterogeneity leaves the poor with higher pollutant levels than the wealthy, with significant implications for environmental justice issues. Furthermore, changes in the pattern of inter-city travel, from personal travel to public bus travel due to the COVID-19 outbreak, have increased and concentrated pollution around suburban terminals. The AQI index analysis shows the days with a good quality index (AQI ≤ 50) in 2020 decreased when compared to 2019, whereas the number of moderate days (50 < AQI ≤ 100) increased. This trend is also in contrast to a global improvement in air quality, highlighting that economic restraints limited the effects of social activity reduction in Tehran. Finally, in the intervals between the lockdown periods, we see an increase in the AQI index. This could be due to the accumulation of social transportation needs after periods of quarantine.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/su142316313/s1, Figure S1: Green space and green area per capita across Tehran; Figure S2: Temporal distribution of CO content at each station, gray rectangular shows strict lockdown period. Figure S3: Temporal distribution of NO<sup>2</sup> content in all investigated station, gray rectangular shows strict lockdown period; Figure S4: Temporal distribution of PM<sup>10</sup> content in all investigated station, gray rectangular shows strict lockdown period; Figure S5: Temporal distribution of O3 content in all investigated station, gray rectangular shows strict lockdown period; Figure S6: Temporal distribution of SO<sup>2</sup> content in all investigated station, gray rectangular shows strict lockdown period; Figure S7: Temporal distribution of the AQI indices in all investigated stations, the gray rectangles show strict lockdown periods.

**Author Contributions:** Conceptualization, M.M. and A.D.M.; methodology, M.M. and S.M.K.; software, M.M. and S.P.; validation, A.D.M., M.S. and V.N.; formal analysis, M.M. and B.G.; investigation, M.M. and A.D.M.; resources, B.G.; data curation, M.M. and S.P.; writing—original draft preparation, M.M., S.M.K. and A.D.M.; writing—review and editing, M.M., S.M.K. and A.D.M.; visualization, M.M. and A.D.M.; supervision, M.S. and V.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data used in this study available from corresponding author upon a reasonable request.

**Acknowledgments:** The authors are thankful to three anonymous reviewers for their constructive comments.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

