Investigation of Radiative Characteristics of the Kenyan Atmosphere due to Aerosols Using Sun Spectrophotometry Measurements and the COART Model

Abstract

Variations in the radiative characteristics of aerosols can be used to quantify their effects on climate. This study evaluated the temporal-spatial variability of aerosol radiative characteristics at λ = 440 nm, λ = 675 nm, λ = 870 nm and λ = 1020 nm over the Nairobi-1°S, 36°E, Mbita-0°S, 34°E and Malindi-2°S, 40°E sites of Kenya. Aerosol optical properties from AERONET were used as inputs in the Coupled Ocean Atmosphere Radiative Transfer (COART) code to model aerosol radiative effects. The results over Nairobi showed an increase in reflectance of 2.6%, 6.7%, 7.2% and 2.4% for 2006-2007 at the specified wavelengths, respectively. Drops of 2.7%, 12.2%, 50.6% and 25.6% were noted in the same wavelengths for the 2007-2008 period. The reflectance over Mbita (0.2284) was higher than that over Nairobi (0.1396) at λ = 675 nm for 2007, due to biomass burning at site. Maritime conditions and aerosols coupled with long range transport of monsoon winds explain the higher reflectance observed over Malindi when compared to Nairobi, except for λ = 440 nm in 2008. This is as a result of aerosols from vehicular and industrial emissions that dominate the λ = 440 nm over Nairobi. The variability of downward and upward spectral irradiance measured at the surface and 12 km levels depended on the wavelength of measurement, but was temporally invariant. Upward irradiance decreased with increasing Solar Zenith Angles (SZAs) due to strong Fresnel reflection at large angles. The equality in the upwelling irradiances at the two atmospheric levels at all sites for λ = 870 nm and λ = 1020 nm was due to the near IR absorption by aerosols. The radiant flux lost in the spectral range 440-1020 nm remained relatively constant over the study sites, and thus the influence of aerosols on radiative characteristics was independent of both site and period of study.

Trend Analysis of Aerosol Optical Depth and &#197ngstr&#246m Exponent Anomaly over East Africa

“…+0.22% yr It is evident that accurate diagnosis of the regional aerosol influence on climate models requires long-term and accurate identification of aerosol sources and sinks. For example, over East Africa, aerosol source regions are greatly influenced either anthropogenically or naturally [19]. In the previous case, fine mode aerosols over urban, industrialized and densely populated regions (Nairobi and Kampala) are mainly due to gas-to particle conversion mechanism of aerosols (fossil fuel, industrial-vehicular emissions, biomass and refuse burning) [20].…”

Optical Characterization of Atmospheric Aerosols via Airborne Spectral Imaging and Self-Organizing Map for Climate Change Diagnostics

“…land clearance, deforestation activities and biomass burning that dominate the study sites [21] [26]. This conclusion is arrived at since from Figure 3 the two sites experience distinct precipitation rates during the study period.…”

“…This implies the fact that AOD characteristics over the study site are highly variable. The two clusters in the AOD over each study site are attributed the two dry and wet seasons experienced over the region [21]. Additionally, the East Africa region's aerosol characteristics are modulated by the Monsoon precipitation [22].…”

“…On the contrary, Kampala is dominated by a single cluster in AOD during the study period; this may be attributed to the significant vehicular emissions [23]. Likewise, Mbita is dominated by a single AOD cluster over the study period that may be as a result of biomass burning and land preparation for agricultural use [21]. SOM AOD map for each study site is illustrated in Figure 1.…”

Retrieval of Aerosol Optical Depth Over Ecologically Stressed East African Land Surfaces from Top of Atmosphere Radiance Utilizing the Aerosol Free Vegetation Index

“…It is 120 kilometers northeast of Mombasa City with a population of 1.1 million.The site is dominated by maritime conditions associated with sea salt and sea spray aerosols that are known to dominate the λ = 0.44 μm atmospheric window. Unique to the site is the significance of long distance transport of coarse aerosols from the Arabian Peninsula desert via Monsoon winds (Makokha and Angeyo, 2013).…”