Seasonal Variations of Aerosol Optical Properties and Identification of Different Aerosol Types Based on AERONET Data over Sub-Sahara West-Africa

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˚32'N, 4˚34'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 Saha-* Corresponding author. ra during north-easterly trade wind flow while the observed marine aerosols are conveyed by the southwesterly trade wind influences to the study site.

Mass concentration, optical depth and carbon composition of particulate matter in the major southern West African cities of Cotonou (Benin) and Abidjan (Côte d'Ivoire)

“…The contribution of advected mineral dust and biomass burning aerosol by northerly winds is also high during the dry season (Balarabe et al, 2016). Following Kaufman et al (2000), large particles (dust or maritime aerosols) are dominant in the size distribution when the Ångström coefficient is below 0.7, although Toledano et al (2007) have reported a threshold of 1.05.…”

“…Following Kaufman et al (2000), large particles (dust or maritime aerosols) are dominant in the size distribution when the Ångström coefficient is below 0.7, although Toledano et al (2007) have reported a threshold of 1.05. A low value, typically below 0.8, of the Ångström exponent associated with high AOD, typically above 0.6 (Verma et al, 2015), indicates a significant contribution of coarse dust-like particles to the AOD (Balarabe et al, 2016).…”

Comparison of Seasonal Cycles of Phytoplankton Chlorophyll, Aerosols, Winds and Sea-Surface Temperature off Somalia

“…The AOT values are lower for fine aerosols and higher for coarse aerosols. In the literature, different criteria have been proposed to identify dust aerosols at different locations: for example, α < 0.6 (Dubovik et al, 2002;Brindley et al, 2015), α < 0.8 (Eck et al, 2005;Che et al, 2013), α < 1 (Eck et al, 1999;Schuster et al, 2006;Papaynannis et al, 2007;Yoon et al, 2012;Valenzuela et al, 2014;Zu et al, 2014;Pakszys et al, 2015) and α < 1.4 (Gobbi et al, 2007;Pereira et al, 2011;Shinozuka et al, 2011); similarly, AOT > 0.11 (Toledano et al, 2007;Balarabe et al, 2016), AOT > 0.2 (Salinas et al, 2009;Pakszys et al, 2015) and AOT > 0.25 (Guleria et al, 2012) have been recommended to identify dust aerosols. After considering all these studies, we have adopted the ranges of AOT and AE for off Somalia as follows: AE less than 1, and AOT at 440 nm (τ 440 ) greater than 0.2 (i.e., α < 1 and τ 440 > 0.2) are designated as DOT or dust aerosols.…”

Classifying Aerosols Based on Fuzzy Clustering and Their Optical and Microphysical Properties Study in Beijing, China

“…To better depict aerosol properties, many studies were carried out in the last decades [10][11][12][13][14][15][16][17]. Some of the studies [10][11][12][13] classify aerosol types by thresholds of aerosol properties, such as aerosol optical depth (AOD),Ångström exponent (AE), fine-mode fraction (FMF), single scattering albedo (SSA), or their combinations.…”

“…Some of the studies [10][11][12][13] classify aerosol types by thresholds of aerosol properties, such as aerosol optical depth (AOD),Ångström exponent (AE), fine-mode fraction (FMF), single scattering albedo (SSA), or their combinations. For example, to discriminate aerosol types of biomass-urban, desert dust, clean maritime, and mixed-type, appropriate thresholds for AOD and AE are applied [10].…”