Aerosol optical depth (AOD) provides a useful characterization of the total absorption and scattering effect of particles in direct or scattered sunlight, and can be derived from sun spectra measured directly by sun photometers. In this paper, atmospheric optical properties (e.g. AOD(440-1020) (nm), alpha and beta, the coefficients in Angstrom formula) and meteorological conditions are presented for: summer (July-August-September) and winter (December-January-February-March) of 2009-2010 over Zanjan (36.41 degrees N, 48.29 degrees E) in northwestern Iran. The diurnal variation of AOD in Zanjan is approximately 15%. An exponential dependence of alpha on AOD in winter indicates that dust aerosols are major contributions of atmospheric turbidity in this region. AOD regressed against PM10 to establish prediction models. The role of three meteorological parameters on the correlation of AOD and PM10 are analyzed. Results show that there is a high correlation between AOD(440) and PM10 in wintertime, and p is a better indicator of air quality in winter than in summer for the study region considered here. Hourly analysis shows that this correlation is highest in the afternoon when the atmospheric mixed layer is at its highest thickness. A similar behavior for AOD-PM10 and a correlation between optical properties with NO2 and PM10 are detected. A sensitivity study was designed to quantify the role of meteorological properties, such as relative humidity, wind speed, and temperature, on the correlation between AOD and PM10 concentration. (C) 2013 Elsevier Ltd. All rights reserved.