Trends in Air Quality Index (AQI) in Tehran, Iran: A Comprehensive Analysis Before, During, and After COVID-19 Pandemic (2018–2024)

Background:The largest capital of the Middle East, with 8000 Km 2 and a population of 13 million people, has about 40% of Iran's industrial units in its suburbs.The purpose of this study was to investigate the trend of air quality in Tehran between 2018 and 2024.Methods: Daily air quality index (AQI) data were collected from 16 monitoring stations in Tehran province over three time periods: 2 years before, 2 years during, and 2 years after the COVID-19 pandemic.Descriptive statistics were calculated for each pollutant during these periods, and box plots were used to visualize AQI distributions.The Kruskal-Wallis test was applied to assess significant differences in AQI across the three time periods.Results: PM2.5 was the dominant pollutant across all seasons, peaking at 99.4% in autumn and remaining above 90% in winter.The AQI for PM2.5 increased from 109 prepandemic to 143 post-pandemic.Seasonal trends revealed significant increases in pollutants such as O3 and PM2.5 in spring, summer, and autumn, while NO₂ levels fluctuated.The Kruskal-Wallis test indicated significant seasonal variations, with PM2.5 showing the most substantial increase (H = 262.43,p < 0.001).O3 and PM10 also showed significant post-pandemic growth, whereas SO₂ and NO₂ had smaller but significant fluctuations (p < 0.05). Conclusion:The results show that reducing traffic and preventing the use of Mazut fuel in power plants have had significant effects on reducing air pollutants, and the sharp decrease in precipitation in recent years has increased the dispersion and emission of particles.