Abstract. Urban air pollution has become a serious challenge in all societies. Nowadays, pollutants are measured by various tools and devices, including local particulate measurement instruments and ground-based remote sensing devices. This paper aims to estimate dust downfall time in dusty days of Zanjan city using the data obtained from ground-based aerosol sampling device and sunphotometer. The particulates with the size of less than 10 micrometers (PM₁₀) were measured by ground-based measurement device in the height of 3 meters above the ground, while aerosol optical depth (AOD), which represents the amount of aerosols in vertical column of atmosphere, was measured by sunphotometer. According to correlation coefficient diagram, PM₁₀ and AOD reached a maximum of 2, 6, 3 and 3 hours in dusty days of 9, 10, 24 and 25 June 2010. In other words, dust downfall took 2, 6, 3 and 3 hours respectively in the aforementioned days. Keywords: Downfall, dust, sunphotometer, correlation