| dc.description.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 and optical properties in order to characterize and model their
effects. This thesis embodies the results of the derivation of radiative characteristics of the
atmosphere over Nairobi (2006-2008), Mbita (2007) and Malindi (2008) using aerosol data
obtained from sun spectrophotometry. Aerosol optical depths (τ), single scattering albedo (ω),
angstrom exponent (α), asymmetry factor ( ), real ( ) and imaginary ( ) refractive indices at
zero Solar Zenith Angle (SZA) were derived through AErosol RObotic NETwork (AERONET)
framework. Temporal and spatial characteristics in τ and α were investigated using multivariate
techniques viz. Principal Component Analysis (PCA), Partial Least Squares Discriminant
Analysis (PLS-DA), Principal Component Regression (PCR) and Hierarchical Cluster Analysis
(HCA). Annual averages of the optical properties together with selected physico-chemical
properties i.e. aerosol number densities and extinction cross section were determined. The
Coupled Ocean and Atmosphere Radiative Transfer (COART) model was used to solve the
radiative transfer equation (RTE) for an atmosphere assumed to be purely impacted by aerosols
of different sizes and estimated their radiative impacts.
The use of multivariate chemometric techniques revealed that temporal and spatial
characteristics of both τ and α over the study sites are modulated by weekly total rainfall, relative
humidity, temperature, aerosol hygroscopic properties, aerosol burden, aerosol mode of
generation and composition, both local air circulation and urban heat island effects. There was no
significant spectral dependence in ω, and both and at zero SZA over the study sites.
Comparison of the measured τ and α from AERONET at λ = 500 nm and 440/675 nm
respectively was achieved by utilizing Moderate Resolution Imaging Spectrometer (MODIS)
data at 550 nm and 470/660 nm. The values agreed to within 12.4 % and 10.9 % levels of
accuracy respectively, showing consistency in the two aerosol remote sensing techniques. There
was a declining loss in radiant energy with increasing aerosol particle sizes over the sites of
study which is associated to increasing heating effect of the incoming solar radiation. Radiative
characteristics (spectral irradiance, integrated fluxes, reflectance) over Nairobi depicted temporal
variations as influenced by rainfall distribution. There was an increase in up/down irradiance
ratio of spectral irradiance of 2.6 %, 6.7 %, 7.2 % and 2.4 % and a drop of the ratio by 2.7 %,
12.2 %, 50.6 % and 25.6 % for 2007-2008 for λ = 440 nm, λ = 675 nm, λ = 870 nm and λ = 1020
nm respectively for 2006-2007 across all wavelength channels as impacted on by the total
amount of rainfall received. Up/down integrated flux ratio remained virtually constant for time
considered over each site. Utilizing these data, radiative forcing due to atmospheric aerosols was
estimated, and found to remain relatively constant at 0.46 for all the three sites
despite the observed differences in the various aerosol particle properties dominating the sites. | en |
