Solar irradiance is attenuated spectrally when passing through the earth’s atmosphere and it is strongly dependent on sky conditions, cleanliness of the atmosphere, composition of aerosols and gaseous constituents. In this paper, aerosol optical properties including aerosol optical depth (AOD), Angstrom exponent (𝛼) and Angstrom turbidity coefficient (𝛽) have been investigated during December 2009 to October 2010, in a suburban area of Zanjan (36°N, 43°E, 1700 m), in the north–west of Iran, using meteorological and sun photometric data. Results show that turbidity varies on all time scales, from the seasonal to hourly, because of changes in the atmospheric meteorological parameters. The values of 𝛼 range from near zero to 1.67. The diurnal variation of AOD in Zanjan is about 15%. The diurnal variability of AOD, showed a similar variation pattern in spring (including March, April, May) and winter (December, January, February) and had a different variation pattern in summer (June, July, August) and autumn (September and October). During February, spring and early summer winds transport continental aerosols mostly from the Iraq (dust events) and cause the increase of beta and turbidity of atmosphere of Zanjan.