A long-term record of aerosol index from TOMS observations and horizontal visibility in sub-Saharan West Africa

Abstract

Aerosol index (AI) observations from the Total Ozone Mapping Spectrometers (TOMS) on board the Nimbus-7 (1979Nimbus-7 ( -1992 and Earth Probe (1997Probe ( -2004 satellites are used to investigate the annual and seasonal variations of dust haze in Sahelian West Africa. Eight synoptic stations, namely Port Harcourt (PHA), Lagos, Ilorin, Makurdi, Minna, Yola, Maiduguri and Kano, representing the four major climatic zones (tropical rainforest and Guinea, Sudan and Sahel savannahs) of Nigeria were considered in this study. Twenty-nine years of horizontal visibility data are also analysed in order to investigate the effect of dust haze on visibility in the stations under study. Results show that there are general decreasing trends in the visibility with corresponding increases in TOMS AI trends in all the climatic zones of Nigeria. This clearly suggests that there had been an increase in the integrated column dust aerosols over the years, especially during the harmattan dusty months. The average visibility was highest in Ilorin (11363.2 ± 1644.4 m) and the lowest in Maiduguri (9017.9 ± 1021.3 m) when the dust-prone months of November to March of the following year were considered. Logarithmic equations with inverse relationships are developed between TOMS AI and horizontal visibility for each station with correspondingly high coefficients of determination (R 2 ) in all the locations. It is concluded that TOMS AI can reasonably be used to reproduce visibility measurement in sub-Saharan West Africa where sparse visibility data are available for meteorological uses.

Monthly Temporal-Spatial Variability and Estimation of Absorbing Aerosol Index Using Ground-Based Meteorological Data in Nigeria

“…These higher AI values do not affect the visibility at the ground surface, which remains clear despite the presence of dust in the atmosphere. One possible explanation is that this is a transition period between moist air from the equatorial regions and dry Harmattan air from the Sahara [6]. Hence, during this period, the ICTZ begins to move northwards, so that the moisture-laden southwest trade wind moves into Nigeria and undercuts the dust-laden northeasterly trade wind.…”

“…The AI values predicted by our model have already been compared to the measured AI values in the data for subset 2 in section 3.5. Anuforom et al [3], Kehinde et al [6] and Balarabe et al [19] proposed a simple linear regression that relates the AI to the visibility data. It is important to know that the linear regressions by Anuforom et al [3] and Kehinde et al [6] were based on daily observations, including the retrieved missing TOMS AI data of [2001][2002][2003].…”

“…Anuforom et al [3], Kehinde et al [6] and Balarabe et al [19] proposed a simple linear regression that relates the AI to the visibility data. It is important to know that the linear regressions by Anuforom et al [3] and Kehinde et al [6] were based on daily observations, including the retrieved missing TOMS AI data of [2001][2002][2003]. It is also observed that the study by Anuforom et al [3] is limited to the Sahel zone of Nigeria, while Kehinde et al [6] provide linear regression models for eight individual stations in Nigeria.…”

“…The most observable of dust aerosol effect across Nigeria is visibility reduction resulting from scattering and absorption of solar radiation by suspended particles in the atmosphere. During low-visibility events, many flight operations are suspended or delayed, which leads to a significant economic loss for Nigeria [6]. It has been recognized that dust affects not only the regional but also the global climate system [3], respiratory tracts, and cloud microphysical properties [1].…”

Measurements of Aerosol Optical and Microphysical Properties in Tizi Ouzou

“…The dust particles travel long distances (Ansmann et al, 2003;De Tomasi et al, 2003), carried by the power and the direction of the wind. Several campaigns (AMMA (African Monsoon Multidisciplinary Analyses), SAMUM (SAharan Mineral dUst experiment), SAMUM-2, ACE-(Aerosol Characterization Experiment), SHADE (Saharan Dust Experiment), SHADOW (Study of SaHAran Dust Over West Africa),…) have been organized to study the optical and microphysical properties of these particles yielding many results (Vitale et al, 2000;Tanré et al, 2003;Muller et al, 2009;Cavalieri et al, 2010;Flamant et al, 2011;Formenti et al, 2011;Ogunjobi et al, 2012;Yang et al, 2013;Kocha et al, 2013;Vesselovsky et al, 2016;Mortier et al, 2016). These properties can be modified when the particles travel a long distance by interacting with other types of aerosols and, more generally, with their environment.…”