Aerosol Optical Properties in the Iranian Region Obtained by Ground-Based Solar Radiation Measurements in the Summer Of 1991

Abstract

Solar radiation measurements were made using sun photometers and pyranometers during 31 May-7 June 1991 at several places in Iran and during 12 June-17 September 1991 at a fixed place, Bushehr, Iran. In the first period the aerosol optical thickness had values about 0.4 at the wavelength of 0.5 ?m in the coastal area and about 0.2 in the plateau area. The Ångström's exponent, which is the slope of optical thickness spectrum, had values around 1 for large city areas and less than 0.5 for inland arid areas. Chemical analyses of sampled air indicate an effect of fossil fuel burning from local sources. Such optical and chemical characteristics of atmospheres suggest that soil-derived coarse particles contributed considerably to the atmospheric turbidity in arid areas, whereas an active generation of aerosols was dominant near large cities. Significant rises in atmospheric turbidity were observed in the earlier part of the second period at Bushehr about once a week with a duration of about one day, which may have been caused by smoke from oil-well fires in Kuwait. The aerosol optical thickness in these events had values of about 1.5, which is equivalent to a columnar aerosol volume of 4.4 ? 10?4 cm3 cm?2. The absorption index ranged from 0.005 to 0.02 with several peaks reaching 0.1 in the second period. These peaks can be attributed to prevailing smoke particles. In spite of the large variety of optical thicknesses and absorption indices, there existed stable power-law size distributions with an exponent about 3.7.