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Home  >  Volume 22 (2012)

Radiative Forcing of Saharan Aerosols in the Near Infrared Regions by B. I. Tijjani, Volume 22 (November, 2012), pp 73 – 84
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We study the radiative forcing of Saharan aerosols at the near infrared region (1.0 μm to 4.0μm) using data from the Optical Properties of Aerosol and Clouds software . Radiative forcing at different wavelengths and Relative humidities are calculated and analysed. We also analysed the effective refractive indicies, optical depths, single scattering albedo, and extinction, scattering and absorption coefficients able to enable us to understand the nature of the particles and their size distributions. From the analysis, it was discovered that radiative forcing (in a form of cooling) increases with the increase in relative humidities (RH) and is dependent on wavelengths and the types of particle size distributions. The value of Angstrom constant at 0% RH reflects the dominance of large particles. But as the RH increases the parameter continues to increase indicating the increase in the concentrations of smaller particles due to the sedimentation of larger particles. This is because as the RH increases, large particles continue to sediment which results in the increase in the concentrations of smaller particles that causes decrease in effective radii which results  in the increase in Angstrom constant and increase in radiative cooling. The analysis of the Angstrom coefficients and the nature of the increase in radiative cooling showed the dominance of large particles. The analysis of optical depth  with wavelengths together with the comparison with scattering and absorption coefficients, hygsroscopicity factor  and humidification factor it is discovered that the particles have bimodal type of size distributions with majority satisfying Junge type of distribution with a small component of larger particles satisfying lognormal.