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Abstract
We are reporting the results of ground-based spectroradiometric measurements on aerosols and water vapor in the atmosphere of Zanjan since October 2006 to September 2008 using a Cimel CE318-2 sun-photometer. Zanjan is a city in Northwest Iran, located at 36.7N, 48.6E and altitude of 1800 m above mean sea level. The spectral aerosol optical depth, Angstrom exponent and columnar water vapour have been calculated using the data recorded by the sun-photometer on direct-beam irradiance measurements of the sunlight. The mean value of aerosol optical depth (at 440 nm) and columnar water vapour during the mentioned period are measured as, 0.26 and 0.51 cm respectively. The maximum (minimum) of aerosol optical depth was recorded in May 2007 (January 2007), and the maximum (minimum) of columnar water vapour happened in July 2007 (January 2007). Using the least-squares method, the angstrom exponent, α , was calculated in the spectral interval 440-870 nm along with the coefficients, α_1 and, α_2, of the second order polynomial fit to the plotted logarithm of aerosol optical depth versus the logarithm of wavelength. The coefficient, α_2, shows that the most part of aerosols in Zanjan area have dimensions lager than 1 micron. The values calculated for, α_2 - α_1, show that 70% of the aerosols are in the coarse mode and 30% of them are in the fine mode. Comparison of, α_2 - α_1, for Zanjan atmosphere with other regions indicates dust and anthropogenic aerosols are the most dominant aerosols in the region.