OGUNJOBI KEHINDE OLUFUNSO
Publication
The Sahelian West Africa (Long 20W:20E, Lat 0:30N) by its climatological and geographical conditions is a key region for the characterization of global atmospheric aerosol optical properties. This study evaluates the spatial and temporal variation of the Aerosol Optical Depth (AOD440nm), aerosol particle size characterization (Angstrom exponent (α440-675nm) at four locations (Agoufou, Banizoumbou, Cape Verde and Ilorin) over a period of January 2005 to December 2009. Results of the day-to-day AOD440nm variations as well as the seasonal and annual variations are presented in order to establish the aerosol climatology in the region. We compared satellite derived data of Total Ozone Mapping Spectrometer - Aerosol Index (TOMSAI), MODIS (Terra and Aqua) with those of ground-based Sunphotometer AERONET measurements. In general, there exits good relationship between MODIS (Terra and Aqua) and the ground-based AERONET measurements with correlation coefficients, R2 › 0.8 reported in all stations. However low coefficients (as low as 0.40) were obtained in all the stations for regressions between TOMS AI and ground-based Sunphotometer AERONET data.
Aerosol index (AI) observations from the Total Ozone Mapping Spectrometers (TOMS) on board the Nimbus-7 (1979–1992) and Earth Probe (1997–2004) 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 (1979–2007) 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 (R2) 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.
Understanding of the influence of climate on agricultural production is needed to cope with expected changes in precipitation and temperature, and an increasing number of undernourished people in food unsecure regions. Climate and crop productivity are characterized by inter-annual variability. The common understanding is that the co-variation is indicative of a simple cause and effect relationship. This present study was to investigate variability of temperature and rainfall in Nigeria and to determine the overall impact on food crop production. The impacts of both annual and seasonal means of rainfall and temperature (1971 2005) were examined in all ecological zones of Nigeria using statistical methods. The crops yield studied were Maize, Cowpea and Rice during the period 1986 2005, bivariate correlation and multiple regression models were used in describing the relationship between crop yield, precipitation, and temperatures in each zone. The results show a significant low trend in both the seasonal and annual means in temperatures. Total and seasonal rainfall shows a significant increase trend from Sahel to Rainforest zones. The predictive models generated for the crops in each zone were significant with α ≤ 0.05 expect for rice yield in Rainforest zone. The results also show that June rainfall (Jun RR) and September rainfall (Sept RR) of the growing season were among the more powerful predictor of crop yield. In general, better results were achieved by adopting monthly rainfall and average temperature of growing seasons as independent variables, which reflect in multiplicity of impacts on crop growth, development, flowering, seed production and maturity.
Aerosol optical properties over Ilorin (80 19’N, 40 20’E ), Nigeria an Urban city in sub Sahel West Africa is presented in this study during the period January 2005 – December 2007. The aerosol direct radiative forcing (ADRF) at the bottom and the top of the atmosphere 􀀀F, and forcing efficiency 􀀀F eff evaluated from the Sun-photometer measurements during the harmattan period (often associated with desert dust + biomass burning) is reported for this region for the first time. Measurements show pronounced seasonal influence with maximum dust loading during harmattan seasons (November to March) of every year. L arge values of aerosol optical depth (AOD), 􀀀 500nm > 1.50 combined with low values of Ångström exponent 􀀀 < 0.2 were mainly observed during the dust periods. The aerosol volume size distributions show distinct fine (geometric mean radii of 0.18 ±0.13 μm) and coarse modes (geometric mean radii of 4.71±4.55 μm) with a remarkable increase in the volume concentration during the harmattan dust episodes of November-March. The aerosol forcing efficiency during the harmattan season is estimated to be -119.63±13.62 Wm -2 / 􀀀 500nm , -113.63 ±13.38 Wm -2 / 􀀀 500nm and 82.94 ±4.21 Wm -2 / 􀀀 500nm for 2005, 2006 and 2007 respectively at the bottom of the atmosphere (BOA). At the top of the atmosphere (TOA), the forcing efficiency were estimated to be -38.10 ±4.77 Wm -2 / 􀀀 500nm, -45.26 ±5.00 Wm -2 / 􀀀 500nm, and -41.26±1.49 Wm -2 / 􀀀 500nm for same years respectively.
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This article presents a detailed analysis of seasonal and interannual variability of total ozone content (TOC) at 16 different stations in Africa using Nimbus-7 Total Ozone Mapping Spectrometer (TOMS) data for a period of 14 years (January 1979–December 1992). The analysis provides not only an estimate of the long-term annual and seasonal trends but also statistics of means and variability of ozone on temporal and spatial scales. For example, high negative deviations were observed for stations in Northern Africa in spring (March–May), with as much as –20 DU in Alexandria. A comparison of total ozone column data retrieved from the TOMS satellite with measurements obtained from the Dobson ground-based network is further presented for Cairo, Irene, Nairobi and Springbok. Estimates of the percentage seasonal difference between TOMS satellite and Dobson ground-based measurements reveal that the ground measurements were higher in magnitude at all stations with the exception of Nairobi. To verify the level of correlation between the ground-based and satellite observations, rank correlation coefficients were determined for all stations using daily and monthly observations. The results show that there is good correlation between the compared data sets, with daily coefficient of determination (r2) values of 0.87, 0.76, 0.58 and 0.87 for Cairo, Irene, Nairobi and Springbok, respectively.
Aerosol optical properties including Aerosol Optical Depth (AOD, τaλ), Angstrom exponent (α), size distribution, single scattering albedo (wo) were examined using long-term ground-based radiometric measurements at five West African sites; Dakar (14°42N 17°29′W), Banizoumbou (13°45′N, 02°39′E), Agoufou (15°21N,1°29′W), Ilorin (08°32′N, 04°34′E), and Ouagadougou (12°22′N, 1°31′W). Also included were observations made at the Cape Verde Islands (16° 45′N, 22° 57′W) to investigate the aerosol transport over the Atlantic Ocean. Results from multiyear observations show that monthly average AOD generally exhibits primary and secondary maximum in March and June respectively except Ilorin that shows peak value in January. The seasonal cycle of Angström exponent (α440–870 nm) shows that minimum values are often associated with dust storm in all sites however, the monthly mean α440–870 nm yields high values at Ilorin, even for peak dust seasons. It suggests that fine mode aerosols dominate the daily aerosol optical influence even during the ‘harmattan’ season when biomass/urban pollution aerosol mixes with coarse mode dust originating aerosols at the sites. The retrieved aerosol size distributions for predominately fine mode particles (cases with α440–870 nmN0.5) at Ilorin and Cape Verde show an obvious steady increase in particle size as τa440 nm increased in both the modal maximum value and expansion of the fine mode boundary towards larger radius. The volume median radius of the size distribution of the coarse mode desert dust at Ilorin ranged from ∼1.9 to 3.2 μm with geometric standard deviation of 1.61 to 1.83. The coarse particle mode at Cape Verde shows very little dynamic changes as a function of aerosol optical depth. Results from estimated single scattering albedo (wo) show wide differences in the observed magnitudes of the fine/coarse mode particle absorption at the sites under study. The differences in dust absorption may be adduced to the possible interactions between dust and fine mode pollution aerosols and variability of the dust optical properties from different source regions in West Africa. Air mass backward trajectories based on multiyear data were developed to explain the transport and identification of different aerosol types in this region of study.
The synoptic and optical characteristic of days during severe dust spells of December 1998, January 2000, February 2001, March 2003 and January 2005 at Ilorin (8_190N, 4_200E), Nigeria are investigated in this study. For example, during the severe dust storm of 5 January 2005 a strong synoptic-scale ridge developed which caused a shift in the position of the anticyclone over the desert region. These conditions led to a higher pressure gradient, and in turn resulted in the generation of dust at the 900 hPa level and wind speeds of up to _13m=s over the source region between Bilma (18_120N, 12_550E), Faya Largeau (17_550N, 19_070E) and Maiduguri (11_500N, 13_090E) during the dust spells. Spectral aerosol optical depth _a_, Angstr€om parameters (_440-500 nm and _) derived from sunphotometer measurements and horizontal visibility (km) were used to study aerosol optical characteristics during and after the winter ‘‘harmattan’’ dust days. It was found that the daily mean _a500 nm during the dust episodes ranged between 0.31 and 3.28, with an average of 0.98 _ 0.50. _a_
showed a pronounced temporal trend, with maximum dust loading during intense dust haze as _a500 nm increased from a daily average value of 0.68 _ 0.48 to values >2.0 during severe dust spells. Daily mean spectral variations of the Angstrom exponent _ show striking changes especially at high _a_, when coarse mode aerosol dominate the accumulation mode aerosol. During the dust spells of 1998–2005 average horizontal visibility was reduced to as low as 1.66 _ 0.34 km at Ilorin with an increase in average _a500 nm and _ to _2.06 _ 0.48 and 1.87 _ 0.49, respectively. However, for cases with strong biomass burning aerosol _a500 nm¼0.67 with _ and _ ¼1.02 _ 0.08 and 0.31 _ 0.06, respectively. In some cases, smoke particles mix with desert dust to produce intermediate values of _. A trajectory analysis at three different levels (200, 500 and 1000 magl) during a severe dust and biomass burning episode were further used to explain the transport of aerosols to Ilorin, Nigeria.
Atmospheric surface aerosol radiative forcing (SARF) ΔF, forcing efficiency ΔFe and fractional forcing efficiencyΔFFe evaluated from cloud-screened narrowband spectral and thermal-offset-corrected radiometric observations during the Asia dust outbreak episodes in Gwangju, Republic of Korea are reported in this study. Columnar aerosol optical properties (aerosol optical depth (AOD), tal, Angstrom exponent !, mass concentration of fine and coarse mode particles) were also reported for the station between January 2000 and May 2001 consisting of 211cloudfree days. Results indicate that majority of the AOD were within the range 0.25–0.45 while some high aerosol events in which AODs≥0.6 were observed during the severe dust episodes. For example, AOD increases from annual average value of 0.34±0.13 at 501 nm to values >0.60 during the major dust events of March 27–30 and April 7–9, 2000, respectively. The !501–870 nm which is often used as a qualitative indicator of aerosol particle size had values ranging from 0.01 to 1.77. The diurnal forcing efficiency ΔDFe at Gwangju was estimated to be −81.10±5.14 W m −2/ t501 nm and −47.09±2.20 W m −2/t501 nm for the total solar broadband and visible band pass, respectively while the fractional diurnal forcing efficiency ΔFDFe were −15.8±0.64%/t501 nm and −22.87±1.13%/t501 nm for the same band passes. Analyses of the 5-day airmass back trajectories were further developed for Gwangju in order to classify the air-mass and types of aerosol reaching the site during the Asia dust episodes.
