Estimation of Radiative Forcing due to Aerosols over Selected Sites in Kenya J. W. Makokha, J. N. Kimani and H. K. Angeyo Department of Physics, University of Nairobi CORRESPONDING AUTHOR J . W. Makokha Department of Physics, University of Nairobi P.O. Box 30197-00100, Nairobi, Kenya (Manuscript received 19 October 2010, in final form 17 January 2011) ABSTRACT Atmospheric aerosols modulate the radiative budget and ambient air quality of the atmosphere, thus, there is a need to develop both analytical and computational methodological techniques that determine their physical, chemical, optical and radiative properties in order to characterize and model their environmental effects. This paper embodies the results of the derivation of radiative characteristics of the atmosphere over Nairobi, Mbita and Malindi using aerosol data obtained from sun spectrophotometry from 2006-2008. Aerosol optical depths (τ), single scattering albedo (ω), angstrom exponent (α), asymmetry factor (g) at zero Solar Zenith Angle (SZA) were derived through AErosol RObotic NETwork (AERONET) framework. The Coupled Ocean and Atmosphere Radiative Transfer (COART) model was then used to solve a radiative transfer equation (RTE) for an atmosphere modulated by aerosols of different particle sizes. Utilizing the integrated fluxes, radiative forcing due to atmospheric aerosols was estimated, and found to remain relatively constant at 0.46 K/(W/m 2 ) for all the three sites despite the observed differences in the various aerosol particle properties that is physical, mode of gen- eration, chemical and number densities dominating the sites. This value was slightly lower as compared to the combined global anthropogenic radiative forcing estimated to be +1.6 [-1.0, +0.8] W/m 2 . Keywords: Radiative forcing; integrated fluxes; radiative characteristics, Kenyan atmosphere MARCH 2012 J. Met eor ol . Rel. Sci ., 6, 3 –13 (2012) 3 1. Introduction Aerosols directly affect the radiation budget of the atmosphere by absorbing and scattering solar radiation. Scattering of incom- ing solar radiation by aerosols yields a cooling effect on the Earth’s atmosphere, while its ab- sorption yields a warming effect. These effects are highly dependent on the coupled influences of the physical, chemical and optical properties of the predominant atmospheric aerosol types over a given region. Aerosols also indirectly modulate the radiative budget of the Earth by modifying the properties of clouds. They do this by acting as Cloud Condensation Nuclei (CCN), which aid in the formation of cloud droplets (Alpert et al ., 1998). A constant liquid content acts as an enhancement in the cloud dr- oplet number through CCN which leads to an increase in cloud albedo. This indirect effect is also known as Twomey effect (Twomey, 1991). Radiative forcing is the change in net (down minus up) irradiance (solar plus long- wave; in W/ m 2 ) at the top of the Earth’s atmos- phere or surface, due to secular changes in the atmospheric concentration of radiative active species e.g. aerosols. Global mean radiative forc- ing due to indirect aerosol effects ranges from 0 to -2 W/ m 2 inclusive of ice and mixed phase clouds, though the magnitude of any indirect effect associated with ice phase is not known (IPCC, 2001). The total spectral irradiance (TSI) at the Earth’s orbit can be calculated knowing the sun’s radius, the photospheric temperature