Analysis of optical properties of atmospheric aerosols inferred from spectral AODs and Ångström wavelength exponent G. Balakrishnaiah a , K. Raghavendra kumar a , B. Suresh Kumar Reddy a , K. Rama Gopal a , R.R. Reddy a, * , L.S.S. Reddy a , Y. Nazeer Ahammed b , K. Narasimhulu c , K. Krishna Moorthy d , S. Suresh Babu d a Aerosol & Atmospheric Research Laboratory, Department of Physics, Sri Krishnadevaraya University, Anantapur 515055, Andhra Pradesh, India b Department of Physics, Yogi Vemana University, Kadapa 516003, Andhra Pradesh, India c Department of Physics, Government First Grade College, Bellary 583101, Karnataka, India d Space Physics Laboratory, Vikram Sarabhai Space Center, Thiruvanantapuram 695 022, Kerala, India article info Article history: Received 28 May 2010 Received in revised form 29 November 2010 Accepted 1 December 2010 Keywords: Aerosols Aerosol optical depth Ångström exponent Water vapor content India abstract Aerosol measurements over a tropical semi-arid site in Anantapur (14.62  N, 77.65  E, 331 m asl), India, provide the variabilities of aerosol characteristics for the period of January 2007eDecember 2008. The mean values of aerosol optical depth at 500 nm (AOD 500 nm ) over 2-year period are found to be 0.37  0.11, 0.39  0.13 and 0.30  0.10 during winter, summer and monsoon seasons, respectively. A rather systematic variation is seen with AOD peaking in the month of March, falling off gradually to the lowest value in June and increasing slowly superposed with some modulations, which show a weak secondary peak in October. The mean and standard values of the Ångström exponent, a (turbidity coefficient, b), are found to be 1.06  0.33 (0.17  0.05) during winter, 0.97  0.35 (0.22  0.04) during summer and 0.85  0.26 (0.16  0.06) during monsoon. The frequency distribution of a is relatively broad with two modes for dusty (a < 1) and non-dusty (a > 1) situations. This frequency distribution of a reveals a great dispersion of a values in all seasons thus denoting variability in the aerosol-size distribution. In this study, spectral AOD and Ångström exponent data are analyzed to obtain information about the adequacy of the simple use of the Ångström exponent and its variation for characterizing optical properties of aerosols, and exploring possibilities for a more efficient characterization of them. Using the least-squares fit method, the Ångström parameter, a, is calculated in the spectral interval 380 e850 nm, along with coefficients a 1 and a 2 of the second-order polynomial fit to the plotted logarithm of AOD versus the logarithm of wavelength. The correlations between the coefficients a 1 and a 2 with months and a are discussed in the paper. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Aerosols, both natural and anthropogenic, play an important role in atmospheric as well as astronomical sciences. They affect the atmosphere by imparting radiative forcing and perturbing the radiative balance of the Eartheatmosphere system as well as by degrading the environment. To understand the effects of aerosols on our geo/biosphere systems, it is essential to characterize their optical, physical, and chemical properties at as many locations as possible because of the regional nature of their properties and their short lifetime (Moorthy et al., 1999; Satheesh et al., 2002). Such knowledge will help in building up a comprehensive picture of the global aerosol distribution and also their potential environmental impact. The spectral aerosol optical depth (AOD), which is an indicator of the aerosol loading in the vertical column of the atmosphere, constitutes the main parameter to assess the aerosol radiative forcing and its impact on climate. As most of the aerosol sources are of terrestrial origin, the variability of their properties is very large close to the surface. At higher altitudes above the mixing layer and in the free troposphere, the sources of aerosols are widely varied and differ on a regional basis leading to regional variations on the Earth’s radiative budget (Ramachandran and Ribu Cherian, 2008). Systematic measurements of aerosols at high altitudes are practically non-existent in India. Precise knowledge of the Earth’s atmospheric extinction above a site is essential for ground-based optical astronomical observations. Among many factors that cause extinction of light, the one due to scattering by aerosols is highly variable and controls the transparency as well as stability of the sky. Therefore the characterization of the atmospheric extinction at a site takes special importance in different atmospheric conditions like polluted or clear skies. * Corresponding author. Tel./fax: þ91 8554 255710. E-mail address: rajururreddy@gmail.com (R.R. Reddy). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.12.002 Atmospheric Environment 45 (2011) 1275e1285