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
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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 ) . At mo s p h e r i c r a d i a t i v e
forcing at the surface derived from aircraft
i rr ad ia nce a nd sp ec t ra l o pt ic al de pt h
me a s u r e me n t s , J o u r n a l o f G e o p h y s i c a l
R e s e a rc h , 1 1 1 ; D 1 2 2 0 7 , d o i : 1 0 . 1 0 2 9 /
20 05 J D 00 63 21
CACHORRO, V.E., VERGAZ, R. and DE
F R U TO S A . M . , ( 2 0 0 1 ) . A q u a n t i t a t i v e
c o mp ar i s o n o f An g s t r o m e x p o n en t : A
t ur b id i ty pa r ame t er re t ri e ve d i n
d iffer e nt s p ec t ra l ra nge s b a se d o n
s p e c t r o -r a d i o me t e r s o l a r r a d i a t i o n
measurements. Atmospheric Environment,
35; 5117-5124.
DUBOVIK, O., SINYUK,A., LAPYONOK,
T. ,
HOLBEN,
B .N.,
M ISHCHENKO,M ., YANG, P., ECK,
T. F. , V O LT E N , H . , O L G A M . ,
VEIHELMANN, B., VAN DER ZANDE,
W. ,
LEO N ,
J . F. ,
SO RO K IN,
M.,SLUTSKER, I. (2006). Application of
s p h e r o i d mo d e l s t o a c c o un t f o r a e r o s o l
p a r t i c l e no ns p he r i c i t y i n r e mo t e s e ns i ng
o f d e se r t d us t, J o u r n a l o f G e o p h y s ic a l
R e s e a rc h , 111 , D 11 2 0 8 , d o i : 1 0 . 1 0 2 9 /
20 05 J D 00 66 19 .
GOBBI, G.P., KAUFMAN, Y.J., KOREN, I.,
E C K , T. F. , ( 2 0 0 7 ) . C l a s s i f i c a t i o n o f
Journal of Sustainable Technology
aerosol properties derived from AERONET
direct Sun data. Atmospheric Chemistry and
Physics, 7: 453–458
HANSEL, R.A., SI-CHEE TSAY, QIANG JI,
K . N . L I O U , a n d S ZU - C H E N G O U ,
(2003). Surface aerosol radiative forcing
d e r iv e d fr o m c o ll o c a t e d gr o u n d - b a se d
radio metric ob servations during P RIDE,
SAFARI, and ACE-Asia, Applied Optics,
4 2 (2 7 ) , 5 5 3 3 -5 5 4 4
J A C O V I D E S , C . P. , K A LT S O U N I D E S ,
N.A., ASIM AK OPO ULO S, D.N.,
KA SK AO UT IS , D. G. (2 0 0 5 ) . S p e ct ra l
a er o so l o p ti c al de p th and Angst ro m
p a r a me t e r s i n t h e p o l l u t e d At h e n s
a t m o s p h e r e . T h e o re t i c a l a n d A p p l i e d
Climatology, 81; 161-167.
KALU, A.E. (1979). The African dust plume:
its characteristics and propagation across
West Africa in winter. In Morales, C (ed)
Sahara dust. Chichester, Wiley.
KING, M .D., KAUFMAN, Y. J., TANRE, D.
a n d N A K A J I M A , T. ( 1 9 9 9 ) . R e mo t e
sensing of tr o po sphe r ic ae r oso l s fro m
space: Past, present, and future, Bulletin
of American Meteorological Society, 80:
22 29 – 2 25 9.
OGUNJOBI K.O. and KIM Y.J. (2008). Aerosol
characteristics and surface radiative forcing
c o mp o n e n t s d u r i n g a d u s t o u t b r e a k i n
G wa n g j u ,
Repub lic
of
Korea.
Environmental Monitoring and Assessment
Vol. 2, No 1, (January 2011) ISSN: 2251-0680
Pp 3 2
K. O. Ogunjobi
137, 111-126, DOI 10.1007/S10661007.9733z.
PACE , G., D I SARRA , A., M ELONI, D.,
PIACENTINO, S., and CHAM ARD, P.
(2 00 5) . Aer o so l o pt ic a l pr o pe rt i es a t
Lampedusa (Central Mediterranean) – 1.
Influence of transport and identification of
d i ffe r e n t a e r o s o l t yp e s , A t m o s p h e r i c
Che mistry a nd Ph ysic s Disc ussion ,
4 92 9 –4 9 69 .
PINKER, R. T., G. PANDITHURAI, B. N.
HOLBEN, O. DUBOVIK, ARO, T.O.,
(2 00 1 ). A d ust o ut b re a k e pi sod e i n
s u b - S a h e l We s t A f r i c a , J o u r n a l o f
Geophysical Research, 106, 22,923– 22,930.
S C H A F E R , J . S . , E C K , T. F. , H O L B E N ,
B.N., ARTAXO, P., YAM ASOE, M.A.,
and PROCOPIO, A.S. (2002). Observed
r ed ucti o ns o f to tal so lar ir r ad i ance b y
biomass burning aerosols in the Brazilian
A ma z o n a n d Za mb i a n S a v a n n a .
G eo p h ys i c a l R es e a rc h Le t t er s , 2 9 ( 1 7 ) ,
1823, doi:10.1029/2001GL014309
Journal of Sustainable Technology
SMIRNOV, A., HOLBEN B.N., DUBOVIK, O.,
O’NEILL N.T., REMER L.A., ECK T.F.,
S L U T S K E R , I . , S AVO I C D . , ( 2 0 0 0 ) .
M e a s u r e me n t s o f a e r o s o l s o p t i c a l
parameters on US Atlantic coast sites, ships
and Bermuda during TARFOX, Journal of
Geophysical Research, 105,9887-9901.
TO D D , M . C . , R . WA S H I N GTO N , J . V.
M A R T I N S , O . D U B O V I K , G.
L IZC A N O ,
S.
M ’ B A I N AY E L ,
ENGELSTAEDTER, S. (2007). Mineral
dust emission from the Bodélé Depression,
nor ther n Cha d, during BoDE x 20 05.
Jo u r n a l o f G eo p h y sic a l Re sea rc h , 11 2 ,
D06207, doi:10.1029/2006JD007170.
WA S H I N G T O N , R . , M . C . T O D D , S .
ENGELSTAEDTER, S.M B AINAYEL,
M I T C H E L L F. , ( 2 0 0 6 ) . 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
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