The photochemical smog is an important problem in air pollution of large cities. Photochemical smog is created by sunlight romoted reactions in the lower atmosphere, which secondary pollutant such as ozone, organic nitrate, oxidized hydrocarbons and photochemical aerosols produce. These secondary pollutants have destructive effects on human health, vegetation and materials. In this study the phenomena of photochemical smog in Tehran are investigated. Also, the average annual, monthly and daily concentrations of NO, NO2 and O3 at three stations (Mehrabad, Aghdasieh and Fatemi) are studied. The data used from Organization of Air Quality Control of Tehran. Finally, the Air Quality Index (AQI) is used to characterize the quality of the air at a given location. The results show, there are 178 unhealthy days per year.
