Sun-photometric study and multivariate analysis ofaerosol optical depth variability over some representative sites of the Kenyan atmosphere

Abstract

The goal of this study was to explore the temporal-spatial characteristics of aerosol optical depth (τ)over the Kenyan urban (Nairobi-1°S, 36°E), rural (Mbita-0°S, 34°E) and maritime (Malindi-2°S, 40°E) atmospheres using sun spectrophotometric measurements obtained from Aerosol Robotic Network (AERONET).AERONET measurements have been taken in Kenya since 2006 and are aimed at assessing aerosol effects on climate and improving the aerosol data base in the region. The multivariate nature of environmental measurements however allows only a limited understanding of atmospheric aerosol characteristics when univariate analysis technique is used. Temporal-spatial characteristics of atmospheric aerosol optical depth can be understood comprehensively if it is appropriately retrieved from ground-based spectrophotometric measurements and then decoupled and analyzed using multivariate analysis techniques since they can explore groups of variables simultaneously, thus providing a more meaningful insight into the temporal-spatial variability of τ is inevitable. The influence of rain and dry spells and temperature on τ at wavelengths, λ = 440 nm and λ = 1020 nm as quantified by Principal Component Analysis (PCA) ranged between 76-83 % and 7-14 %and 4-7 % respectively for all the sites. It was found out that urban heat island (over Nairobi) and local air circulation effects (over Mbita and Malindi) modulate the characteristics of aerosol optical depth over the studied sites. Spatial variability in τ as shown by Hierarchical Cluster Analysis (HCA) is independent of measurement wavelength but dependent on aerosol burden in the atmosphere for each site. The individual and coupled influence of weather parameters on atmospheric aerosols has been\ isolated and quantified and found to be site dependent.