DE NT AN D Centre for Research and Development (CERAD) FOR RESE C EN T E CH AR R VE LO PM E The Federal University of Technology, Akure, Nigeria JoST 2011 Seasonal variability of aerosol optical parameters and radiative forcing over sub- Sahel West Africa O G UNJ O B I K . O . Department of Meteorology, The Federal University of Technology, Akure, Nigeria. ABSTRACT: 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 , a nd forcing eff iciency �F eff evaluated from the Sun-photometer measurements during the harmattan period (often a s s o c i a t e d w i t h d e s e r t d u s t + b i o ma s s b u r n i n g ) i s r e p o r t e d f o r t h i s r e g i o n f o r t h e f i r s t t i me . Measurements show pronounced seasonal influence with ma ximum dus t loa ding dur ing ha r ma tta n > 1 .5 0 s ea sons (N ovember to M a rch) of ever y yea r. L a r ge va lues of a er osol optica l depth (A O D), � 50 0n m combined w ith low values of Ångström ex ponent �< 0 .2 wer e ma inly obs er ved dur ing the dus t per iods . T he a er os ol 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 .5 5 µm) w ith 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 / � , -113.63 ±13.38 Wm -2 / 50 0n m -2 � a n d 8 2 . 9 4 ± 4 . 2 1 W m / � f o r 2 0 0 5 , 2 0 0 6 a n d 2 0 0 7 r e s p e c t i v e l y a t t h e b ottom of the atmosphere 50 0n m 5 0 0 nm (BOA). At the top of the atmosphere (TOA), the forcing efficiency were estimated to be -38.10 ±4.77 Wm -2 / � , -45.26 ±5.00 Wm -2 / � , and -41.26±1.49 Wm -2 / � for same years respectively 50 0n m 50 0n m 50 0n m Keywords: Aerosol optical depth; harmattan period; Angstrom exponent; radiative forcing JoST. 2011. 2(1): 22-33. Accepted for Publication, November 19, 2010 INTRODUCTION Anthropogenic and natural aerosols are noted t o b e ma j o r a t mo s p h e r i c c o n s t i t u e n t s t h a t contribute significantly to climate change apart f r o m c a r b o n d i o x i d e . F u n d a m e n t a l l y, t h e scientific community does not know the current total aerosol loading in the atmosphere; thus we have no definitive measure of change for future assessment (And reae, 1 9 9 6). Aero so l optical depth is used in local investigations to c h a r a c t e r i z e a e r o s o l s , a s s e s s a t mo s p h e r i c pollution and make atmospheric corrections to satellite remotely sensed data. The AERONET ( A E r o s o l R O b o t i c N E T wo r k ) p r o g r a m i s a f e d e r a t i o n o f g r o u nd - b a se d r e mo t e s e ns i n g a e r o s o l n e t wo r k s e s t a b l i s h e d b y N a t i o n a l Aer o na uti c s a nd S pa c e Admi nist ra t io n (NASA). The program provides a long-term, c ont inuo us a nd re a di ly ac c es s ib l e pub li c d oma in da t ab a se of a er o so l o p ti c al , mi c r o p h ys i c a l a n d r a d i a t i v e p r o p e r t i e s f o r a er o so l r ese ar c h a nd cha ra c te ri z at i on, validation of satellite retrievals, and synergism with other databases. The automated robotic sun a n d s k y s c a n n i n g me a s u r e me n t p r o g r a m ( AE R ON E T ) ha s g r o wn r a p id l y t hr o ug h international federation since 1993 to over 600 s t a t i o n s wo r l d wi d e p r i m a r i l y i n E u r o p e a n d We st Afr ica , AE RO CAN i n Ca na d a (h tt p : // aeronet.gsfc.nasa.gov). O b s e r v a t i o n s s h o ws t h a t e a c h y e a r b e t we e n No vemb er and March large quantity o f dust particles otherwise know as the “Harmattan” dust haze are transported from the Sahara desert t o wa r d s t h e G u l f o f G u i n e a a c r o s s N i g e r i a ( P i n k e r e t a l . , 2 0 0 1 ) . I t i s k n o wn t h a t t h e harmattan dust affect greater part of West Africa in winter (November-March), particularly the Nigerian zone comes mainly from Northeastern Saha ra usual l y a lo ng t he Bo d él é d ust depression (170N, 180E), plain of Bilma (180N, C o r r e s p o n d e n c e t o : O g u n j o b i , K . O . : j o b i k 2 0 0 0 @ ya h o o . c o m Journal of Sustainable Technology Vol. 2, No 1, (January 2011) ISSN: 2251-0680 Pp 2 2 K. O. Ogunjobi 120E) , and Faya Largeau (180N, 190E) off the slope of Tibesti Massif (Kalu 1979; Washington et al., 2006). The radiative effects of dust aerosols at the TOA have been estimated in previous studies from sa t e l l i t e o b s e r va t i o ns a nd r a d i a t i ve t r a ns fe r mo dels (Hansel et a l., 2 00 3). Schafer et al. (2002) showed that by use of the sunphotometer to retrieve aerosol optical depths along with observed surface flux data from field campaigns i n B r a z i l a n d s o ut h c e nt r a l Af r i c a a e r o s o l r a d i a t i v e f o r c i n g e ff i c i e n c i e s d u e t o s mo k e particulates were determined to be -145 W/m2 / �a and -210 W/m2 / �a, respectively. T h i s p a p e r e x a mi n e s t h e a e r o s o l o p t i c a l properties over Ilorin, Nigeria an Urban city in sub-Sahel West Africa using an extensive daily d at a c over i ng t he pe r io d J anuar y 2 00 5 – December 2007. The study further presen ts f o r th e f i rs t ti m e th e a e ro s o l d i re c t ra d i a ti v e f o rc i n g (A D R F ) a t th e b o tto m (B OA ) a n d th e to p (T OA ) o f th e a tm o s p h e re �F, a n d f o rc i n g e f f i c i e n c y �F e ff e v a l u a t e d f r o m t h e S u n - p h o t o m e t e r me a s u r e me n t s d u r i n g t h e h a r ma t t a n p e r i o d ( o f t e n a s s o c i a t e d wi t h s a n d d u s t + b i o m a s s burning episodes) for a guinea savanna region of Nigeria. STUDY SITE, INSTRUMENTATION AND METHODS The observation site is located on the rooftop of the Department of Physics at the University o f Il o r i n , N i g e r i a ( 0 8 o 1 9 ’ N , 0 4 o 2 0 ’ E , 3 5 0 masl). Ilorin is an urban city with minimum level of urban pollution situated at the northern b o u n d a r y o f t h e G u i n e a s a v a nn a h r e g i o n o f Nigeria. During the wet season moisture laden winds that originate from the Atlantic Ocean lead to thunderstorms, rainfall and squall-line activities however in the dry winter season the prevailing north-easterly winds locally known as the harmattan often bring dust-laden air from the desert. A large amount of dust particles are transported downward from the source region in a plume, mainly at the 900 hPa level north of the Inter Tropical Discontinuity (ITD) with a south-westerly trajectory over Nigeria. All of the measurements reported in this paper were made with CIMEL sun/sky radiometers, wh i c h a r e p a r t o f t h e A E R O N E T g l o b a l n et wo r k . T h e Aer o so l R o b o t i c N et wo r k (AERONET) is an optical ground-based aerosol monitoring network and data achieves initiated by NASA’s Earth Observing system (EOS) and expanded by Federation with many non- NASA institutions. The automatic tracking Sun and s k y s c a n n i n g r a d i o me t e r s ma d e d i r e c t S u n measurements with a 1.20 full view every 15 mi n a t 3 4 0 , 3 8 0 , 4 4 0 , 5 0 0 , 6 7 5 , 8 7 0 , 9 4 0 a n d 1 0 2 0 n m. O ne s e t o f me a s u r e me n t s r e q u i r e s about 8 – 10s, and measurements are taken in Journal of Sustainable Technology triplicate at 30s intervals. The s e s o l a r e x ti n c ti o n m e a s u re m e n ts a re th e n u s e d to c o m p u te a e ro s o l at each wavelength except for o p ti c a l d e p th � � the 940 nm channel, which is used to retrieve precipitable water vapor (PWV) in centimeters. T h e C I M E L s u n / s k y r a d i o me t e r a t I l o r i n i s recalibrated at Mauna Loa Observatory (MLO) station approximately every 6 months using the Langley plot technique with morning data only. T h e a e r o s o l d a t a p r e s e nt e d i n t hi s s t u d y a r e q ua li ty a nd c lo ud -s c r e e ne d fo ll o win g th e method of Smirnov et al. (2000). The Ångström empirical for approximating aerosol extinction is expressed as: -� � =�� � w h e re �i s e x p re s s e d i n m i c ro m e te r o f th e v a l u e s a n d �i s th e A n g s tro m c o rre s p o n d i n g � � tu rb i d i ty c o e f f i c i e n t. �i s re l a te d to th e a m o u n t o f a e ro s o l p re s e n t i n th e a tm o s p h e re a n d i s u s e d to c h a ra c te ri z e th e d e g re e o f a i r p o l l u ti o n o r tu r b i d i ty . T h e p a r ame te r ch a r a c t er i ze s th e spectral features of the aerosols and it relates to the size of the particles (King et al. 1 9 9 9 ). L a rg e v a l u e s o f �i n d i c a te a re l a ti v e l y h i g h ra ti o o f s m a l l p a rti c l e s to l a rg e p a rti c l e s . A tm o s p h e ri c ra d i a ti v e f o rc i n g �F d e f i ned as the difference between the net flux at the surface and the same quantity when there is no-aerosol present in the atmosphere can be expressed as �F = F N – FoN Vol. 2, No 1, (January 2011) ISSN: 2251-0680 Pp 2 3 Seasonal variability of aerosol optical parameters where the net flux FN is the difference between the downwelling flux at the top [bottom] of the )] and upwelling atmosphere F TOA (� ) [F BOA (� ) [F BOA (� )] at the top [bottom] of flux F TOA (� the atmosphere. The radiative forcing efficiency a t t he t o p a nd b o t t o m o f t he a t mo sp h e r e i s therefore estimated as: �Feff TOA = �FTOA/�(� =5 0 0 n m ) =5 0 0 nm) �Feff BOA = �FBOA/�(� RESULTS AND DISCUSSION Optical properties of dust aerosol T h e d a y to d a y v a ri a b i l i ty o f a e ro s o l o p ti c a l d e p th ( � ) and the precipitable water vapor � =5 0 0 n m at Ilorin is shown in Figure1 from January 2005 – D e c e mb e r 2 0 0 7 . A s t r o n g s e a s o n a l a n d i n te r-a n n u a l v a ri a b i l i ty o f a e ro s o l o p ti c a l d e p th w a s o b s e rv e d f o r 2 0 0 5 , 2 0 0 6 a n d 2 0 0 7 s h o ws r e s p e c t i v e l y . T h e v a r i a ti o n i n � 5 0 0 nm primary and secondary peaks in January and December of 2005 respectively, while the peak was noted in March of 2006 with a secondary p e ak in J a nu ary. S imila rl y h igh es t v alu e o f aerosol optical depth during the harmattan dust storms of 2007 was reordered in March with two secondary peaks in January and December. The gradual change in seasonal variability of a e r o s o l o p t i c a l d e p t h d u r i ng t h e a n a l yz e d 3 yea r s may b e d ue t o nat ura l c ha nge s o f fluctuations in transport and meteorology of the region under consideration. The average daily values of � a t I l o ri n = 0 .6 8 , 0 .7 0 a n d 0 .7 3 500nm d u ri n g 2 0 0 5 , 2 0 0 6 a n d 2 0 0 7 re s p e c ti v e l y. � 500nm varies from 0.09 to 2.47 in the year 2005 while in the years 20 0 6 and 20 0 7, aeroso l o ptical depth increases from the background harmattan average of 0.55 (for the three y e a rs ) to v a l u e s >3 .0 f o r � =5 0 0 n m . F i g u re 1 f u rth e r re v e a l s a f ro m N o v e m b e r to s h a rp i n c re a s e i n d a i l y � � M a rc h ( h a rm a tta n d u s t s to rm s e a s o n ) o f e a c h va lue of y e a r. F o r e x a m p l e a v e r a g e � 50 0nm ~ 0 . 1 6 ± 0 . 0 9 d u r i n g t h e d u s t f r e e mo n t h s o f 2005-2007 rises to 1.6 5 d u ri n g m o d e ra te d u s t e v e n t o f 6 D e c e m b e r 2 0 0 5 , w h i l e th e m a j o r d u s t e v e n ts o f 1 2 M a rc h 2 0 0 6 a n d 7 J a n u a ry 2 0 0 7 values of 3.26 and 2.85 respectively ha v e � 500nm ( F ig 1 ) . T he se o c c ur r e nc e s c a n st r o ngl y b e attributed to the rapid increase in dust loading f r o m N o v e mb e r – M a r c h . T h e d a i l y v a l u e s rep o rted in this stud y are similar to aeroso l optical depth values reported during major dust Journal of Sustainable Technology e v e n t s o f 1 0 t o 1 1 th M a r c h i n B o d é l é d u s t depression during the 2005 BoDEx experiment (Todd et al., 2007). Table 1 summarizes the aerosol optical properties calculated for Ilorin u n d e r d i ff e r e n t e n v i r o n me n t a l c o n d i t i o n s (seasons). For examp le the range o f aero so l optical depth during h a rm a tta n s e a s o n o f 2 0 0 7 <3 .2 7 (a v =1 .0 1 ± 0 .5 0 ) a s c o m p a red i s 0 .2 8 <� 500nm <0.87 (av = 0.35±0.16) for the w i th 0 .0 7 d ” � 500nm we t / m o n s o o n s e a s o n s i n d i c a t i n g r a p i d d u s t loading during the harmattan dust spells. The precipitable water vapor shows an interesting pattern over Ilorin with a gradual decrease in PWV as aerosol optical depth increases. This b e h a v i o r a l p a t t e r n ma y b e d u e t h e d r y continental air mass from Chad basin and the S a ha r a mo v i n g o v e r I l o r i n d u r i ng t h e d r y harmattan seasons. The average PWV recorded during the harmattan seasons of 2005, 2006 and 2 0 0 7 we r e 2 . 6 8 ± 0 . 8 9 , 2 . 3 7 ± 0 . 9 3 , 2 . 3 5 ± 0 . 9 0 respectively as compared to 4.31±0.32, 4.13±0.24, 4 . 0 8 ± 0 . 2 7 d u r i n g t h e we t / m o n s o o n m o n t h s (Table 1). PWV varies from 0.79 to 5.07 in the year 2005 and from 0.58 to 4.88 in 2006 while it varies from 0.81 to 4.58 in the year 2007. F i g u re s 2 a -c i l l u s tra te th e s c a tte r g ra m o f d a i l y a v e ra g e s o f � w i th �380-500nm during d ifferent � seasons under various environmental conditions while the scatter plot for the whole period o f –� m e a s u re m e n ts i s s h o w n i n F i g 2 d . T h e � � p l o t, a n d i n g e n e ra l th e c o m b i n e d u s e o f th e s e tw o p a ra m e te rs , a l l o w s th e d i s c ri m i n a ti o n o f d i f f e re n t ty p e s o f a e ro s o l ( P a c e et al. 2 0 0 5 ). � d e p e n d s m a i n l y o n s i z e d i s tri b u ti o n a n d to a l e s s e r e x t e n t o n r e f r a c ti v e i n d e x , w h i l e � � d e p e n d s m a i n l y o n a e ro s o l c o l u m n d e n s i ty . S e v e ra l a u th o rs h a v e d i s c u s s e d h o w th e s p e c tra l v a ri a ti o n s i n �c a n p ro v i d e f u rth e r i n f o rm a ti o n a b o u t a e ro s o l ty p e s a n d s i z e d i s tri b u ti o n (G o b b i e t a l . , 2 0 0 7 ) . C o r r e l a t i o n s we r e d e v e l o p e d between the two variables under the existing Vol. 2, No 1, (January 2011) ISSN: 2251-0680 Pp 2 4 K. O. Ogunjobi environmental conditions that occurred in this location. For example high negative correlation c o e ff i c i e n t s o f 0 . 6 7 , 0 . 7 5 a n d 0 . 7 4 w e r e estimated for the dusty harmattan period, the t r a n s i t i o n mo n t h s a n d t h e d u s t - f r e e s e a s o n s respectively. This indicates a strong dependence of the atmospheric turbidity on �d u ri n g v a ri o u s s e a s o n s i n th e re g i o n . F i g u re 2 d s h o w s a ra th e r w e a k c o e f f i c i e n t o f d e te rm i n a ti o n (R 2=0 .5 5 ) f o r th e e n ti r e p e ri o d o f o b s e rv a ti o n f ro m 2 0 0 5 -2 0 0 7 . T h e re g re s s i o n s b e tw e e n th e tw o a n d �) u n d e r t h e d i f f e r e n t p a ra m e te rs ( � � e n v i ro n m e n ta l c o n d i tions are described by a l o g a r i t h mi c e q u a t i o n . T h e p r e s e nt f i n d i ng s agree well to those reported by Jacovides et al. ( 2 0 0 5 ) f o r p o l l u t e d A t h e n s a t mo s p h e r e a n d Cachorro et a l. (2 0 01 ) fo r the greater M ed i te r r a n ea n b a si n d e p ic t in g t h e S p a n is h e nv i r o n me n t . F ur t h e r a n a l ys i s i nd i c a t e s t ha t ae r o so l o p ti cal d ep th is r ed u ced b y ~ 4 7 % d u r i n g t h e d u s t – f r e e - we t s e a s o n w h e n compared with the average of 0.50 for the whole data set. The seasonal frequency distribution of aerosol optical depth is shown in Figure 3 with the d a i l y f re q u e n c y d i s tri b u ti o n o f �s h o w n a s i n s e rt. T h e f re q u e n c y d i s tri b u ti o n o f � 500 nm d e m o n s tra te s a w i d e ra n g e o f a e ro s o l o p ti c a l d e p th d u ri n g th e h a rm a tta n s e a s o n w i th m o re va l ue s > 1 . 0 th a n 3 0 % s k e w e d to w a rd � 500 nm ( F i g . 3 a ) wh i l e ma j o r i t y o f t h e v a l u e s (~8 0 – 9 5 % ) w e re l e s s th a n 0 .5 d u ri n g th e d u s t f r e e w e t /m o n s o o n s e a s o n ( F i g .3 c ) . T h e for f re q u e n c y o f o c c u rre n c e s h i s to g ra m o f � 500 nm the entire data record of 2005-2007 shows a skewed distribution with a peak from 0.2-0.25, at wi t h l e s s e r f r e q u e n c i e s t r a i l i n g o f � 50 0 nm h i g h e r v a l u e s (F i g u re 3 d ). T h e f re q u e n c i e s o f o c c u rre n c e h i s to g ra m o f �s h o w b ro a d p e a k f ro m ~0 .2 to 1 .0 w i th m i n i m u m n e a r z e ro a n d a m a x i m u m o f 1 .4 . T h e m a x i m u m a e r o s o l o p ti c a l d e p th d u ri n g th e h a rm a tta n s e a s o n a n d th e tra n s i ti o n m o n th s a re a s s o c i a te d w i th v e ry s m a l l �v a l u e s ( <0 .2 ) w h i c h i n d i c a t e s t h e c o n tri b u ti o n o f d u s t to th e o p ti c a l th i c k n e s s d u ri n g th e s e p e ri o d s . F igur e 1 : Da ily a vera ge va lues of long term mea surements of a erosol optica l depth (� ) and � precipitable water vapor (PWV) at Ilorin, Nigeria showing day-to-day seasonal variability. Journal of Sustainable Technology Vol. 2, No 1, (January 2011) ISSN: 2251-0680 Pp 2 5 Seasonal variability of aerosol optical parameters Table 1: Summary of aerosol optical properties retrieved for Ilorin. Number of clear sky M e a s ur e me nt Range of AOD Range of alpha Range of pwv Number of clear sky measurement Range of AOD Range of alpha Range of pwv Number of clear sky measurement Range of AOD Range of alpha Range of pwv 2 00 5 Harmattan season (November-March) 2 00 6 2 00 7 13 2 12 6 < 2 . 4 7 0 .1 9 <� <3.26 0 .1 8 <� 500nm 500nm ( av = 0 . 8 7 ± 0 . 4 1 ) ( av = 0 . 8 2 ± 0 . 4 8 ) 0 .0 3 <�380-870nm<1.38 0 .0 1 <�380-870nm <1.34 ( av = 0 . 6 0 ± 0 . 3 8 ) ( av = 0 . 6 5 ± 0 . 3 3 ) 0.79<PWV<4.33 0.58<PWV<4.35 ( av = 2 . 6 8 ± 0 . 8 9 ) ( av = 2 . 3 7 ± 0 . 9 3 ) Transition months (Apr/Oct) 2 00 5 2 00 6 14 9 0 .2 8 <� <3.27 500nm ( av = 1 . 0 1 ± 0 . 5 0 ) 0 .0 5 <�380-870nm<1.36 ( av = 0 . 5 8 ± 0 . 3 3 ) 0.81<PWV<3.96 ( av = 2 . 3 5 ± 0 . 9 0 ) 2 00 7 51 46 52 < 1 . 2 8 0 . 1 9 <� < 1 . 6 5 0 .1 2 <� <1.98 0.15<� 500nm 500nm 500nm ( av = 0 . 6 0 ± 0 . 3 1 ) ( av = 0 . 6 6 ± 0 . 3 1 ) ( av = 0 . 5 7 ± 0 . 3 7 ) 0 .0 4 <�380-870nm<1.16 0 .0 9 <�380-870nm<1.13 0 .0 8 <�380-870nm<1.12 ( av = 0 . 4 6 ± 0 . 3 0 ) ( av = 0 . 4 8 ± 0 . 2 7 ) ( av = 0 . 5 8 ± 0 . 2 8 ) 2.73<pwv<4.90 1.30<PWV<4.88 2.64<PWV<4.48 ( av = 3 . 8 9 ± 0 . 5 2 ) (av=3.11±0.85) ( av = 3 . 7 9 ± 0 . 3 7 ) Wet/monsoon season (May-Sept) 2 00 5 2 00 6 2 00 7 83 <1.07 0 .0 9 <� 500nm ( av = 0 . 4 0 ± 0 . 1 8 ) 0 .1 0 <�380-870nm<1.27 ( 0 . 7 2 ± 0 .3 ) 3.38<pwv<5.07 ( av = 4 . 3 1 ± 0 . 3 2 ) Journal of Sustainable Technology 99 0 .1 5 <� <1.03 500nm ( av = 0 . 4 2 ± 0 . 2 1 ) 0 .0 9 <�380-870nm<1.17 ( av = 0 . 6 5 ± 0 . 2 6 ) 3.19<pwv< 4.66 ( av = 4 . 1 3 ± 0 . 2 4 ) 96 0.07<� <0.87 500nm ( av = 0 . 3 5 ± 0 . 1 6 ) 0 .0 7 <�380-870nm<1.19 ( 0. 6 0± 0 . 30 ) 3.00<pwv<4.58 ( av = 4 . 0 8 ± 0 . 2 7 ) Vol. 2, No 1, (January 2011) ISSN: 2251-0680 Pp 2 6 K. O. Ogunjobi F igure 2 : S ca tter gra ms of the daily a tmospheric turbidity (� )and A ngstrom exponent � =1 020nm (�380-500nm) for different atmospheric conditions and seasons. Also shown are the power fits and the 95% upper and lower prediction levels. Figure 3: Frequency of occurrences of daily averaged aerosol optical depth for different seasons and the entire data set of 2005-2007. Shown as insert is the distribution for Angstrom exponent. Journal of Sustainable Technology Vol. 2, No 1, (January 2011) ISSN: 2251-0680 Pp 2 7 Seasonal variability of aerosol optical parameters Aerosol Volume Size distributions The aerosol volume size distribution (dV(R)/d (lnR)) is retrieved from the spectral Sun and sky radiance data. This study made use of the version (2) inversion algorithm as described by Dubovik et al., (2006) which removes both the artificially increased fine particle mode and the unrealistic spectral dependence in the real part of the refractive index. The volume particle size distribution dV(R)/dlnR (�m 3/�m 2) i s re tri e v e d i n 2 2 l o g a ri th m i c a l l y e q u i d i s ta n t b i n s i n th e ra n g e o f s i z e s 0 .0 5 �m < r < 1 5 �m . T h e re a l n (� ) ( 1 .3 3 < n ( � ) < 1 .6 ) a n d i m a g i n a ry k (� ) p a rt s o f the complex refractive index (0.0005 < k (� ) < 0.5) a re re tri e v e d f o r w a v e l e n g th s c o rre s p o n d i n g to s k y ra d i a n c e m e a s u r e m e n ts . I n th i s s tu d y , s e p a ra te o b s e rv a ti o n s th a t a re d o m i n a te d b y c o a rs e m o d e p a rti c l e s f o r w h i c h ra d i u s R >0 .5 �m ( d e s e rt d u s t) a n d th o s e d o m i n a te d b y f i n e / a c c u m u l a ti o n m o d e p a rti c l e s ( p re d o m i n a n tl y p o l l u ti o n , a n d s o m e ti m e b i o m a s s b u rn i n g ) a re b e i n g i n v e s ti g a te d f o r v a ri o u s ty p e s o f a e ro s o l mi xtu re i n I lo ri n . F i g ure 4 a -c s h o w s th e s ea s o n a l a eros ol s iz e d is trib uti on ov er I lorin f or 20 05, 20 06 a n d 2 0 0 7 re s p e c ti v e l y w h i l e f i g u re 4 d s h o w s th e a v e ra g e v a l u e s f o r th e e n ti re d a ta re c o rd . T h e s i z e d i s tri b u ti o n re v e a l s tw o d i s ti n c t m o d es ; f i n e (p a rti c l e s i z e <0 .3 5 �m ) a n d c o a rs e (p a rti c l e s i z e >0 .3 5 �m ) . H i g h v o l u m e d i s tri b u ti o n v a l u e s w e re n o te d i n th e a c c u m u l a ti o n ( f i n e ) m o d e a n d th e c o a rs e m o d e d u ri n g th e h a rm a tta n s e a s o n s . T h e b i m o d a l s tr u c tu r e o f th e v o l u m e d i s tri b u ti o n m a y b e a ttr i b u te d to th e h o mo g e n o u s h e te ro m o l e c u l a r n u c l ea ti o n o f n e w f i ne particles in the air or heterogeneous nucleation and growth of larger particles by condensation of gas –phase reaction products. An interesting feature in year 2007 is a shift in the peak of the distribution of the coarse mode v o l u m e r a d i u s f r o m ~2 .2 4 �m d u r i n g t h e h a r m a t t a n s e a s o n ( N o v e m b e r- M a r c h ) t o ~2 .9 4 �m i n t h e t r a n s i t i o n m o n t h ( A p r i l / Oc to b e r) . H i g h v o l u m e c o n c e n tra ti o n a t th e c o a rs e m o d e a n d l a rg e r p a rti c l e ra d i u s ( �m ) n o t e d i n 2 0 0 7 s u g g e s t l a rg e r d u s t p a r t i c l e s l o a d i n g a s re f l e c te d i n th e � range in Table 1. � The dominance of large particle is the principal feature differentiating the optical properties of Journal of Sustainable Technology dust fro m the fine mod e d ominated biomass burning and urban aerosols. The radiative forcing efficiencies I n o r d e r to und e r sta n d the a e r o so l fo r c ing efficiency trends, it is important to have a range of high and low aerosol optical depths as low a e r o s o l c o n d i t i o ns a r e p r e s ume d t o b e mo s t u s e f u l i n v a l i d a t i n g t h e c o mp o n e nt s us e d i n c a l c u l a t i n g t h e r a d i a t i v e fo r c i n g . T h e r a t e a t is which the atmosphere is forced per unit � � called the forcing efficiency. Figures 5 and 6 s h o w �F p l o tte d a g a i n s t th e a e ro s o l o p ti c a l d e p th f o r th e h a rm a tta n s e a s o n s d u s t h a z e d a y s o f 2 0 0 5 , 2 0 0 6 a n d 2 0 0 7 a t I l o ri n a t th e to p o f th e a tm o s p h e r e ( T O A ) a n d s u r f a c e ( B O A ) re s p e c ti v e l y. T h e s l o p e s o f th e l i n e a r f i t o f th e ra d i a ti v e f o rc i n g g i v e th e f o rc i n g e f f i c i e n c y �F eff which corresponds to the aerosol present in a localized region, defined as the rate at which the atmosphere is forced per unit optical depth. For the top and bottom of the atmosphere, the radiative forcing efficiencies at Ilorin during t h e h a r ma t t a n s e a s o n we r e c o mp u t e d a s -38.10±4.77 Wm-2/� and -119.63±13.63 Wm500nm 2 /� respectively during year 2005. In similar 500nm manner the estimated TOA and BOA forcing and efficiencies were -45.26± 5.00 Wm -2 /� 500nm r e s p e c t i v e l y i n 2 0 06 -113.63±13.38 Wm-2/� 500nm a n d 8 2 . 9 4 ± 4 . 21 and -41.26±1.49 Wm -2 /� 500nm Wm-2 / � i n y e a r 2 0 0 7 . T h e s c a t t e r p l o t s h o w s 500nm a definite dependence of atmosphere radiative forcing on the magnitude of the aerosol optical depth which is evident from the high coefficient of determination R2 > 0 .6 f o r v a ri o u s p e ri o d s o f m e a s u re me n t. C o n s i d e ri ng th e a v e ra g e ra d i a ti v e f o rc i n g d u ri n g th e e n ti re m e a s u re m e n ts p e ri o d , t h e e s t i m a t e d �F e ff w e r e - 3 9 . 7 8 ± 1 . 3 5 a n d -89.42±3.72 Wm-2/� and corresponding R2 500nm of 0.83 and 0.76 for TOA and BOA respectively. I n o t h e r r e l a t e d s t u d y p e r f o r me d d u r i n g t h e Indian Ocean Experiment (INDOEX) Bush et al., (2006) reported forcing efficiencies ranging for irradiances at from -74.0 to -120 Wm-2 / � 500nm t h e s u r f a c e a s we l l a s s p e c t r a l ( i n a l l s e v e n channels spanning 400-700nm) and broadband aerosol optical depths. During the Asian Pacific Regional Characterization Experiment (ACE- Vol. 2, No 1, (January 2011) ISSN: 2251-0680 Pp 2 8 K. O. Ogunjobi Asia), the aerosol forcing efficiency at the surface are slightly lower than those during INDOEX in .The visible the range ~ -54 to -110 Wm-2 / � 500nm forcing efficiency values at the surface of the atmosphere reported by Bush et al., (2006) are consistent with that reported at Ilorin during the harmattan dust haze season. Similar work by O gu nj o b i a n d K i m ( 2 0 0 8 ) r e p o r t e d d i u r n a l f o rc ing ef f i c i enc y �D F e -81 .1 0±5.14 Wm-2 / � 500nm and -47.09±2.20 Wm-2 / � for the total solar 500nm broadband and visible band pass, respectively at Gwangju, South Korea. Figure 4: Seasonal aerosol volume size distribution over Ilorin, Nigeria for 2005, 2006, 2007 and average of the entire data set. Journal of Sustainable Technology Vol. 2, No 1, (January 2011) ISSN: 2251-0680 Pp 2 9 Seasonal variability of aerosol optical parameters Figure 5: The top of atmosphere (TOA) radiative forcing as a function of aerosol optical ) for cloud free atmosphere at Ilorin during the harmattan dust days. The depth (� 500nm dashed blue lines represent the 95% upper and lower prediction levels for the data set. Journal of Sustainable Technology Vol. 2, No 1, (January 2011) ISSN: 2251-0680 Pp 3 0 K. O. Ogunjobi Figure 6: Same as in Figure 5 above but for surface/bottom of the atmosphere (BOA) with the dashed blue lines represent the 95% upper and lower prediction levels for the data set. CO NC LU SI O N This work presents the seasonal and interannual v a r i a b il i t y o f a e r o so l o p t i c a l a nd r a d i a ti v e properties in Ilorin (80 19’N, 40 20’E) an Urban city in sub Sahel West Africa. The aerosol optical properties were found to show strong seasonal e f f e c t wi t h m a x i m u m v a r i a b i l i t y d u r i n g t h e harmattan (November -March) and transition months (April/October). The observed range of ae r o so l o p ti cal d ep t h ind ic ate d r ap id d u st loading during the harmattan dust spells of 2005200 7. T he regi on is charac teriz ed by the transport of dust particle downwind from the Journal of Sustainable Technology B o d é l é d u s t d e p r e s s i o n t o wa r d s t h e G u l f o f Guinea by strong north easterly winds mainly at 900 mb and 80 0mb levels north o f the InterT r opi cal Di sco nti nui ty (I T D) during t he intensive harmattan dust season. The average PWV recorded during the harmattan seasons of 2005, 2006 and 2007 were 2.68±0.89, 2.37±0.93, 2.40±0.90 respectively as compared to 4.31±0.32, 4.13±0.24, 4.08±0.27 during the wet/monsoon mo nths which sho ws the dry winter weather d u r i n g t h e d u s t y h a r ma t t a n s ea s o n s . T h e a nd relationships between the two paramete rs (� � Vol. 2, No 1, (January 2011) ISSN: 2251-0680 Pp 3 1 Seasonal variability of aerosol optical parameters �) u n d e r th e d i f f e re n t e n v i ro n m e n ta l c o n d i ti o n s a re d e s c ri b e d b y a l o g a r i th m i c e q u a ti o n i n d i c a ti n g a s tr o n g d e p e n d e n c e o f th e a t m o s p h e ri c t u rb i d i ty o n �d u ri n g v a ri o u s s e a s o n s i n th e re g i o n . T h e s i z e d i s tri b u ti o n o v e r I l o ri n re v e a l s tw o d i s ti n c t m o d e s : f i n e (p a rti c l e s iz e <0 .3 5�m) a nd c o a rs e (p a rti c le s i z e >0.3 5 �m) w h i c h s u g g e s t th a t th e a e r o s o l ty p e s i n th i s re g i o n a re o f te n a s s o c i a te d m a i n l y w i th d e s e rt d u s t a n d b i o m a s s b u rn i n g . T h e v a ri a ti o n o f th e f o rc i n g e f f i c i e n c y a t th e T OA a n d B OA i n d i c ate a corresponding variation in the aerosol amount, type, composition and characteristics from 20052007. ACK NO W LE DGM E NTS The author wishes to acknowledge the critical roles of the Principal Investigator and the Project M a n ag e r f o r t h e ir e f f o r t i n e s t a b li s h i n g a n d maintaining the AERONET site at Ilorin. REF ERENCE ANDREAE, M.O., (1996). Raising dust in the greenhouse. Nature, 380, 389-390. BUSH, B. C., VALERO, F.P.J. and POPE S . K . , ( 2 0 0 6 ) . 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D u s t a n d t h e l o w- l e v e l c i r c u l a t i o n o v e r t h e B o d e l e Dep res sio n, Chad : Ob servatio n s fro m B o DE x 20 05 , J ou rn a l o f Ge o ph y si ca l R e s e a r c h , 11 1 ; D 0 3 2 0 1 , d o i : 1 0 . 1 0 2 9 / 20 05 J D 00 65 02 . Vol. 2, No 1, (January 2011) ISSN: 2251-0680 Pp 3 3