We are reporting the results of ground-based spectroradiometric measurements on aerosols and water vapor in the atmosphere of Zanjan for the period of October 2006 to September 2008 using a CIMEL CE318-2 sun-photometer. Zanjan is a city in Northwest Iran, located at 36.70 degrees N, 48.51 degrees E, and at an altitude of 1800 ma.m.s.l. (above mean sea level). The spectral aerosol optical depth, Angstrom exponent, and columnar water vapor have been calculated using the data recorded by the sun-photometer through the direct measurements on the sun radiance (sun-mode). The average values of aerosol optical depth at 440 nm, columnar water vapor, and the Angstrom exponent, alpha, during the mentioned period are measured as, 0.28 +/- 0.14, 0.57 +/- 0.37 cm and 0.73 +/- 0.30, respectively. The maximum (minimum) value of the aerosol optical depth was recorded in May 2007 (November 2007), and that of columnar water vapor, in July 2007 (January 2008). Using the least-squares method, the Angstrom exponent was calculated in the spectral interval 440-870 nm along with alpha(1) and alpha(2), the coefficients of a second order polynomial fit to the plotted logarithm of aerosol optical depth versus the logarithm of wavelength. The coefficient alpha(2) shows that most of the aerosols in the Zanjan area have dimensions larger than 1 micron. The calculated values for alpha(2) - alpha(1) indicate that 80% of the aerosols are in the coarse-mode (>1 mu m) and 20% of them are in the fine-mode (<1 mu m). Comparison of alpha(2) - alpha(1) for the atmosphere over Zanjan with other regions indicates dust particles are the most dominant aerosols in the region.
