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Abstract
We are reporting the calculated values of columnar aerosol size distribution function for atmosphere of Zanjan, a city in Northwest Iran (36.7 N, 48.6 E). Ground-based measurements of the total optical depth of the Zanjan atmosphere at 440 nm, 670 nm, 870 nm and 1020 nm are recorded using a Cimel CE318-2 sunphotometer during the time period of October 2006 to September 2008. The spectral aerosol optical depth has been obtained by subtraction of molecular optical depth from the total optical depth for each wavelength channel. Also the Ångström exponent, α, is determined by a logarithmic fit to the aerosol optical depth when it is plotted versus the logarithm of the wavelength. Daily average of the measured aerosol optical depth and α values has been implemented in an inversion algorithm for calculation of the columnar aerosol size distribution function. In this algorithm, the aerosols are considered as spheres of different size and refractive index of 1.45. As a result, we found that during the measurements usually more than 50% of the aerosol volume concentrations were belong to those with radius of >1 μm, 40% to those with radius of <0.5 μm and the rest 10% have radii between the mentioned sizes. Considering these results, aerosols in the coarse mode are the most dominant ones. We believe this related to Zanjan geographical location when it is located in a dry area and subjected to frequent dust winds. Also the urban anthropogenic aerosols have a considerable share in the aerosol content of the atmosphere.
Keywords: Aerosol, Columnar aerosol size distribution function, Ångström exponent, Aerosol Optical Depth, Sunphotometer