Airborne particulate matter with a diameter of 2.5 microns or less (PM2.5), as well as slightly bigger particles (PM10), arrive from the westerly direction and collect in the city centre of Tehran, the capital of Iran. The statistical characteristics and daily trend of the air quality index (AQI) in Theran were studied over an 11-year period (2002- 2012). Various statistical analyses were applied including descriptive statistics, correlation analysis, trend analysis and the sequential nonparametric Mann-Kendall test. The significance of the series was investigated by regression analysis and Kriging interpolation. It was found that Tehran's daily AQI increased by 11.8% over the study period, with the frequency distribution of days with good and average air quality showing a strongly declining trend. The AQI of Tehran was shown to contain a large part of PM10 and PM2.5, the latter having the largest contribution (coefficient=0.853).