TITLE:
Seasonal Variations of Aerosol Optical Properties and Identification of Different Aerosol Types Based on AERONET Data over Sub-Sahara West-Africa
AUTHORS:
Mukhtar Balarabe, Khiruddin Abdullah, Mohd Nawawi
KEYWORDS:
Aerosol, Angstrom Exponent, Harmattan, Optical Depth, Summer
JOURNAL NAME:
Atmospheric and Climate Sciences,
Vol.6 No.1,
January
8,
2016
ABSTRACT: The uncertainty in the quantification of aerosol properties
such as concentration, size, and composition, spatially and temporally makes regional
studies important. Therefore, this study presents seasonal variations of aerosol
optical properties over Ilorin (8䓠'N, 4䓢'E), Nigeria. Long-term (1998-2013)
records of aerosol optical depth (AOD) and angstrom exponent α, from ground-based Aerosol Robotic Network
(AERONET) are used to study the seasonal variability, characteristics and types
of aerosol. The study showed that seasonal variations (Harmattan and Summer) result
in different aerosol concentration, characteristics, and types. The magnitude and
sensitivity of AOD to wavelength are found low in Summer with significant increase
during Saharan dust transport season (Harmattan). The average mean AODs are 0.73
± 0.50, 0.97 ± 0.52 and 0.46 ± 0.29 with corresponding mean angstrom of 0.66 ± 0.36,
0.68 ± 0.34, and 0.64 ± 0.37 for the entire period, Harmattan and Summer seasons.
High frequency of occurrence of angstrom exponent below 1 (78% and 81%) which were
observed during Harmattan and Summer indicates that the particles are generally
coarse in mode. The results revealed that for both Harmattan and Summer seasons,
the dominant aerosol was dust (DA) with frequency of occurrence of 82% and 79%.
However, mixed aerosol (MIXA) (14.4%) is the second dominant case during Harmattan
while in Summer maritime aerosol (MA) (9.1%) associated with transport due to southwesterly
trade wind is the second dominant aerosol. This conclusion is supported by size
distribution data for the study site which showed that large volume of aerosol particle
size are enclosed in largely coarse mode range in all seasons. A 7-day back trajectory
seasonal frequency plot sourced from the Hysplit Single Particles Lagrangian Integrated
Trajectory model (Hysplit_4 model) shows that dust are transported from the Sahara
during north-easterly trade wind flow while the observed marine aerosols are conveyed
by the southwesterly trade wind influences to the study site.