For human health, tropospheric ozone (O3), particles (PM2.5 and PM10, particles with aerodynamic diameter <2.5 and 10 μm), and nitrogen dioxide (NO2) are the most harmful air pollutants. We have investigated the simultaneity of PM2.5, PM10, NO2, and O3 occurrence by using data from 21 ground-based monitoring stations in a megacity of the Middle East, Tehran (Iran), between 2011 and 2022. In Tehran, the daily PM2.5 (21–45 μg m−3), PM10 (52–108 μg m−3) and NO2 (75–146 μg m−3) mean concentrations have largely exceeded the 24-h limit value established by the 2021 World Health Organization for the protection of human health, i.e., 15, 45 and 25 μg m−3, respectively, in particular for NO2. The ground-level daily O3 concentrations ranged from 26 to 47 μg m−3. Changes in aerosols burden (e.g., PM2.5 and PM10) affect surface O3 through changes in aerosol chemistry and photolysis rates. Following the dust events and for the days with PM2.5 and PM10 exceeding 50 and 100 μg m−3, respectively, we observed a reduction of surface O3 levels (- 30.0% and - 13.5%), concurrently to surface NO2 (+27.9% and +16.6%) levels increases compared to the non-dusty clear-sky days over Tehran city. Surface O3 formation is suppressed under high particulate matter (PM) levels in the atmosphere, in particular PM2.5, likely due to weakened photochemical reactions (lower solar radiation and air temperature), dilution effect, and heterogeneous chemical processes occurring onto the PM2.5 surface (reactive uptake of nitrogen oxides, NOx, and hydrogen oxide radicals, HOx). To achieve O3-PM co-improvement in cities, we recommend reductions in NOx emissions concurrently with significant reductions in PM and Volatile Organic Compounds emissions, for instance by suitable greening and re-naturing programs. © 2023 Turkish National Committee for Air Pollution Research and Control

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