Retrieval of atmospheric optical parameters from ground-based sun-photometer measurements for Zanjan, Iran

Abstract. 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° N, 48.51° E, and at an altitude of 1800 m a.m.s.l. (above mean sea level). The spectral aerosol optical depth, Ångström 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 Ångström exponent, α, 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 Ångström exponent was calculated in the spectral interval 440–870 nm along with α₁ and α₂, the coefficients of a second order polynomial fit to the plotted logarithm of aerosol optical depth versus the logarithm of wavelength. The coefficient α₂ shows that most of the aerosols in the Zanjan area have dimensions larger than 1 micron. The calculated values for α₂ − α₁ indicate that 80 % of the aerosols are in the coarse-mode (>1 μm) and 20 % of them are in the fine-mode (<1 μm). Comparison of α₂ − α₁ for the atmosphere over Zanjan with other regions indicates dust particles are the most dominant aerosols in the region.