IJARET: Evaluation of the Saharan Aerosol Impact on Solar Radiation Over Tamanrasset, Algeria

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN
6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 1, January - June (2012), © IAEME
ENGINEERING AND TECHNOLOGY (IJARET)

ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
IJARET
Volume 3, Issue 1, January- June (2012), pp. 25-34
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EVALUATION OF THE SAHARAN AEROSOL IMPACT
ON SOLAR RADIATION OVER THE TAMANRASSET
AREA, ALGERIA
A. FAID a,* , Y. SMARA b, V. CASELLES c, A. KHIREDDINEd
*Corresponding author: FAID Ali: Phone: +213 34 21 53 04, Fax: +213 34 21 59 86
E-mail: a_faid@yahoo.fr, or Khier_2000@yahoo.fr
a
physics department, Faculty of exact sciences ,
University of Béjaia, Algeria.
b
Image processing laboratory, Faculty of Electronic and informatic Systems, USTHB
Alger, Algeria.
c
Thermodynamics department, faculty of physics, University of Valencia, 46100,
Burjassot, Valencia, Spain.
d
Geni electric Department, Faculty Sciences and Technics, University of Bejaia,
Algeria.

Smara Youcef: +21321247912, Fax: +21321247607
E-mail: Y.Smara@lycos.com

Vicente Caselles: +34963542131, Fax: +34963543385
E-mail: vicente.caselles@uv.es

Khireddine Abdelkrim: +21334216098, Fax +21334215105
E-mail Khier_2000@yahoo.fr

ABSTRACT
We use three types of data which were measured at Tamanrasset (22.78 °N, 5.5 °E)
and Assekrem (23.26 °N, 5.64 °E): solar radiation, aerosol and atmospheric visibility.
The solar radiation is represented by the monochromatic radiation, at λ = 0.50 µm, the
direct solar radiation in the spectrum bands 0.28-0.53 µm, 0.53-0.63 µm, 0.63-0.69
µm and 0.69-4 µm and the scattered radiation. The aerosol factors are expressed by

25


the mass concentration and the particle number. We consider three short
periods (November 2001 to February 2002, May to July 2002 and October to
December 2002) and a long period (November 2003 to October 2004). From the data
of solar radiation, we calculate the atmospheric turbidity using Volz, Kasten and
Angström models. We then compare turbidities, solar radiation, aerosol factors and
visibility, using correlation and regression analysis. We note that the turbidities are
strongly related to the mineral aerosol concentration and the visibility. For example,
in the period may - July 2002, the relationship between the Kasten turbidity Tl and the
aerosol concentration C gives:
R(Tl ,C) = 0.88 , F = 682,5 , t1 = 65.4 , t2 = 26.12 , for a number of observations
equal to 207. All the results show that the Saharan aerosol has a real impact on the
solar radiation extinction. Furthermore, we note that the particle number with
intermediate sizes (0.7 – 1 µm) is strongly related to the turbidity and scattered
radiation. This result can be explained by the Mie scattering of the solar radiation.

Keywords: Aerosol, Solar radiation, Atmosphere, Turbidity, Sahara.

1. INTRODUCTION
Atmospheric aerosol plays an important role in radiative processes. The balance
between aerosol absorption and scattering (Fraser and Kaufman, 1985) determines its
ability to counteract greenhouse warming and to affect atmospheric heating rates
(Carlson and Benjamin, 1980; Alpert et al., 1998). As aerosol particles interact with
solar and terrestrial radiation, they perturb the radiative balance (Liou et al., 1978;
Coakley, 1983).
The Sahara is a major source of dust aerosols (Prospero, 1990). This aerosol has an
important climatic impact (Tegen and Lacis, 1996; Moulin et al., 1997). The optical
depth of Saharan aerosol was determined by Tanré et al. (1988a, b) and Haywood et
al. (2001).
The focus of this paper is to estimate the extinction of solar radiation in the presence
of Saharan aerosol. Two measuring sites are considered: Tamanrasset and Assekrem.
The two sites are selected by the W.M.O. within framework of Global Atmospheric
Wash (GAW) program. The choice of these sites was motivated by the fact that the
anthropogenic constituents in the Hoggar area are negligible. Indeed, the sites of

26


Tamanrasset and Assekrem are far from industrial areas. Furthermore, the
Tamanrasset region is an important source of Saharan aerosol and its soil is very
susceptible to wind erosion. Indeed, it was shown that the absence of nonerodible
elements is very favorable for the dust production. To our knowledge, the
Tamanrasset soil is naked.
Three types of data are used: solar radiation, visibility and aerosol. Firstly, we
describe the data measurements. Then, we present the formulas used to compute the
Volz and Kasten turbidities. Finally, we search the probable relationships between the
Saharan aerosol and the solar radiation extinction, using the statistical methods.

2. EXPERIMENTAL PROGRAM
A large-scale aerosol and solar radiation program is carried out as a part of the
Global Atmospheric Watch program (GAW). One of the principal objectives was to
assess the impacts of desert dust storms on long-range aerosol transport and the
increases in atmospheric turbidity over the region.
The measurement stations of Tamanrasset (22.78 °N, 5.5 °E, and height1377 m)
and also Assekrem (23.26 °N, 5.64 °E, and height 2710m) are about 70km apart.
Assekrem has the advantage of being at a higher elevation. In all measurements, the
meteorological parameters were considered.
2.1. Solar radiation measurements
Two parameters of solar radiation were measured: the monochromatic and the
direct solar radiation. The monochromatic radiation was measured with a sun-
photometer at the green channel (λ=0.50 µm). The direct radiation was measured with
a pyrheliometer in the following spectrum bands: 0.28 - 4 µm, 0.53 - 4 µm, 0.63 -
4µm and 0.695 - 4 µm.
The measurements of the monochromatic radiation were made three times per day: 09
h, 12 h and 15 h, from January 2001 to December 2002. The measurements of the
direct radiation were made at 10 h, 12 h and 14 h. The measurements were not made
when the sun was obscured by clouds and dust of exceptional intensity.
2.2. Aerosol measurements
Two types of measurements were carried out in Tamanrasset and Assekrem : the
number of particles per granulometric class and the mass concentration of aerosol.
The number of particles was measured with a Laser Particle Counter (Laser Particle

27


Counter model 237). The counter is ordered with six particles size channels: >0.3 µm,
>0.5 µm, >0.7 µm, >1 µm, >2 µm, >5 µm. The data are recorded three times per day:
09 h, 12 h and 15 h. From November 2001 to February 2002, the measurements were
made at Tamanrasset. But, after March 2002, the equipment was removed to
Assekrem. In Assekrem, we use only the data of May to July 2002 and October to
December 2002.
The aerosol concentration is obtained by sampling the atmospheric air with a flow
rate of 3 l/mn through a tube into an instrument which contains a precision balance
and a filter. The equipment was installed at Assekrem. We began the measurements
on October 2002. In this work, we use only the data from October to December 2002.

3. DATA PROCESSING

3.1. Volz turbidity

Computations of Volz turbidity are made according to the usual Bouguer-Lambert-
Beer law expressing the measured intensity at wavelength λ :
 P R OZ a 
 (1)
I λ = I 0 λ exp  − m  τ λ + τ λ + τ λ  
  P0 
Where:
I0 = extraterrestrial intensity which depends of the day j of the year as:
I 0 = 1367 1 + 0 ⋅ 034 × cos 0.01746 ( 0 ⋅ 986 j − 2 )  
  
(2)

And where
τλR is the Rayleigh scattering coefficient for air molecules at the wavelength λ,
τλoz is the absorption coefficient for ozone at λ,
τλa is the extinction coefficient for aerosol at λ, it is the Volz turbidity,
P is the station pressure,
P0 is the standard pressure at sea level = 1013.2 hPa, and
m is the optical air mass.
The parameter m can be calculated using the following expression (De
Brichambaut and Vauge, 1982):
1 − 0 ⋅1× z (3)
m= −1⋅253
sin ( h) + 0 ⋅15 ( h + 3 ⋅ 885 )
where z is the station altitude in kilometers and h the solar height. The height h is
given by the expression:
sin (h) = sin (θ ) ×sin (δ ) + cos (θ ) ×cos (δ ) ×cos (ω) , (4)
where:
θ is the station latitude ,
δ is the solar declination : sin(δ) = 0.4 sin(0.986 j - 80) ,
ω is the solar horary angle, it is expressed as:
 ϕ ∆t  π In radians, (5)
ω =  TU + + − 12
 15 60  12
where:
ϕ is the station longitude,

28


∆t = 9⋅ 9×sin 2 ( 0⋅ 986× j +100)  − 7⋅ 7×sin
  ( 0⋅ 986× j − 2 ) (6)
TU is the universal time.
τλR and τλoz are calculated using respectively the following expressions (Orgeret,
1985):

τ λ = 8 ⋅ 79 × 10−3 × λ −4
R
(7)

OZ
T0 = exp(− m.τ λ ) = 1 − a − b (8)
0 ⋅ 002118 X
with: a = 1 + 0 ⋅ 0042 X + 0 ⋅ 00000323 X 2 ; (9)

0 ⋅1082 X 0 ⋅ 00658 X
b= +
1 + 13 ⋅ 86 X 1 + (10 ⋅ 36 X ) 2

and X=3.5m

3.2. Linke turbidity

The Linke turbidity Tl is calculated using the Kasten formula:
 mTl  (10)
I = I 0 exp −  0 ⋅ 9m + 9 ⋅ 4 
 
where:
I = direct solar radiation in the spectrum band 0.28 - 4 µm ,
I0 and m are defined presciently,
Tl = Linke turbidity.

3.3 Angström turbidity
It is determined from the measurements of a direct solar radiation in the large
spectrum band not affected by the water vapor absorption (0.28<λ<0.63 µm). Taking
into account the attenuation by molecular scattering and ozone absorption, and the
hypothesis that the aerosol scattering is proportional to λ-α, we have to solve the
following equation:

  p R   (11)
I s (λ ) = I 0 s (λ ) exp  − m  τ λ + τ λ + βλ −α  
oz

  p0  
where:
IS(λ) is the Direct solar spectral intensity,
IS0(λ) is the extraterrestrial solar spectral irradiance, which depends of the day j of the
year (as calculated presciently) and the spectrum band,
β is the Angstrom turbidity coefficient.
α is the Wavelength exponent, which is equal to 1.3 for aerosol.
The other parameters are defined presciently.
We calculate β in two spectrum bands: 0.28 – 0.53 µm and 0.53 – 0.63 µm.

4. STATISTICAL COMPARISON OF SOLAR AND AEROSOL DATA
Before dealing with upon the relationships between the Saharan aerosol and
the solar radiation extinction, we begin by the estimation of the maximum, minimum
and average values of the data in each period. Table 1 shows a general view of the

29


computed data of aerosol, visibility and turbidity. For the three periods, we determine
the maximum, minimum and average values of the:
- Particle number per granulometric class,
- Volz, Linke and Angstrom turbidity, and
- Visibility.
Because of the delayed installation of the aerosol captor (October 2002), the mass
concentration parameter is used only in the period October to December 2002. We
illustrate the average values of particle number for each period in the fig. 1.

Nov. 01 - Feb. 02 May - July 02 October - Dec 02
Max. Min. Mean Max. Mi Mean Max. Mi Mean
n. n.
Aerosol (µg/m3) ------ ----- ----- ----- ----- ----- 307.8 02. 75.10
7
Aerosol > 0.3 306503 5365 4335 166370 584 5802 8289 280 1875
Number µm 211655 1206 4 126223 5 1 6 20 4
> 0.5 151393 687 1327 101234 133 2761 5378 19 7863
µm 112067 466 4 80694 5 2 3 18 5695
> 0,7 30386 143 8879 23817 810 1889 4385 01 4297
µm 3935 29 6534 1935 552 1 6 00 1215
> 1 µm 1987 111 1363 3495 105
> 2 µm 319 09 5 7
> 5 µm 3268 1040
241 3
798
Volz turbidity 1.508 0.383 0.482 2.03 0.5 0.922 1.2 0.3 0.546
3 6
Linke turbidity 12.53 2.0 2.75 19.06 2.9 6.36 9.0 2.0 3.44
2 8
Angs. Turbidity β1 0.452 0.000 0.032 0.515 0.0 0.135 0.20 0.0 0.049
in 0.28-0.53 µm 1 0 0
Angs. Turbidity β2 1.580 0.090 0.261 1.174 0.1 0.505 0.58 0.2 0.305
in 0.53-0.63 µm 6 0
Visibility (in km) 55 03 47 55 0.3 28.6 70 04 44.5
0

Table 1 Maximum, minimum and average values of the aerosol and solar radiation
parameters.

30


Particles with size superior to 0.7 µm

20000

15000

particles
Number
of
10000

5000

0

Nov01 - May-july Oct - dec
feb02 02 02

Fig 1 Average values of particle number for each period.

We note that the aerosol number in Tamanrasset is more important than that of
Assekrem. This difference can be explained by the two following facts:
1°) the aerosol number decreases with the height (Durand and Druilhet, 1983);
2°) Tamanrasset station is much closer to the aerosol sources.
Furthermore, we note that the three turbidities are more important in the period
May-July 2002. This difference can be related to the seasonal variation of the
emissivity with maximum in summer and minimum in winter (Jaenicke, 1979).
In order to search a likely relationship between the Saharan aerosol and the solar
radiation extinction, represented by Volz, Linke and Angström turbidity, we use the
correlation and regression methods. The significance of the models is tested by an
evaluation of the correlation, Fisher and Student coefficients. The lack of data, for
solar radiation, is caused by obscuration of the sun by clouds. Therefore, the time-
series of the observed variables (xi , yi) are not in chronological order.

The correlation coefficients between aerosols and turbidities are given in the table 2.

Nov01- February May - July 02 October-December
02 02
Tl τaλ β1 Tl τaλ β1 Tl τaλ β1
> 0.3 µm 0.8 0.70 0.7 0.64 0.60 0.6 0.8 0.80 0.54
> 0.5 µm 2 0.84 4 0.73 0.67 1 2 0.82 0.64
Aerosol > 0.7 0.8 0.83 0.8 0.75 0.67 0.7 0.8 0.82 0.64
µm 8 0.83 1 0.75 0.67 0 5 0.81 0.64
number > 1 µm 0.8 0.79 0.8 0.76 0.66 0.7 0.8 0.81 0.66
> 2 µm 8 0.69 2 0.65 0.51 2 5 0.80 0.65
> 5 µm 0.8 0.8 0.7 0.8
8 1 3 5
0.8 0.7 0.7 0.8
5 9 5 6
0.7 0.6 0.6 0.8

31


3 9 6 5
Aerosol amount ---- ---- --- ---- --- --- 0.7 --- ---
6
Visibility --- - 0.69 --- --- - 0.75 --- -- - 0.83 ---

Table 2 Correlation coefficients between turbidities and aerosol parameters.

Nov 01 - May - July 02 October-
February 02 December 02
Tl τaλ β1, β2 Tl τaλ β1, β2 Tl τaλ β1, β2
Particle number 209 269 209 167 263 167 209 230 209
Mass ----- ----- ----- ----- ----- ----- 132 ----- -----
concentration
Visibility ----- 269 ----- ----- 263 ----- ----- 230 -----
Table 3 The number of observations for each correlation.

From the tables 2 and 3, we can see that all the correlations are significant. We
observe narrow relationships between aerosol parameters and solar radiation
extinction factors. The increase of the turbidity is related to the increases of aerosol
number and mass concentration, and the decrease of the visibility. The important
correlations are obtained with the Linke turbidity and in the period November 2001 to
February 2002. There could be a few reasons for this relationship:
- The pyrheliometer is perhaps more efficient than the sun photometer
- In the period November 2001 to February 2002, the aerosol number and the solar
radiation intensities were measured at the same site: Tamanrasset.
Furthermore, we note that the Angstrom turbidity, calculated in the spectrum band
0.28 – 0.53 µm, is well correlated with the Saharan aerosol number.
The regression equations, between aerosol parameters (mass concentration C and
particle number N) and turbidity (β and Tl), and the significant tests of correlation
(R), Student (t1 and t2) and Fisher (F) are given in the table 4.
We show in fig. 2 the linear regression between the number of particles, with size
superior to 0.5 µm, and the turbidity parameter of Kasten (Tl).

Period Equation R F t1 t2
Nov 01 - Feb 02 τaλ = 0.427 + 4.19.10- 0.839 631.8 104.6 25.1
6
.N0.5
May - July 02 τaλ =EXP (0.32 - - 0.774 389.5 12.9 - 19.7
1.54.Vis)
Oct 02 - Dec 02 τaλ = 1.13 – 1.31.10-5.Vis - 0.832 510.3 42.4 - 22.6
Oct 02 - Dec 02 τaλ= 0.43 + 1.41.10-5.N0.5 0.817 457.8 50.6 21.4
Nov 01 - Feb 02 Tl = 2.31 +5.10-5.N0.7 0.880 708.8 66.2 26.6
Oct 02 - Dec 02 Tl = 2.27 +3.6.10-5.N0.5 0.884 741.1 65.3 27.2
Oct 02 - Dec 02 Tl = 2.61 + 0.0134.C 0.760 175.8 24.7 13.3
Table 4 Regression equations aerosol parameters - turbidities and significant tests

32


Predicted vs. Observed Values
Dependent variable: TURB
1600

1400

1200

Observed Values
1000

800

600

400

Regression
200
300 500 700 900 1100 1300 1500 95% confid.

Predicted Values

Fig. 2 Curve regression between Kasten turbidity and aerosol number.

CONCLUSION
We note that the particle number with intermediate sizes (N0.5 and N0.7) is strongly
related to the two turbidities (β and Tl). This result can be explained by the Mie
scattering of the solar radiation. However, the mass concentration C is proportional to
the Linke turbidity. Almost all relationships are linear. However, in the period May to
July, the relationship between the Voltz turbidity and the horizontal visibility is
exponential. In this period, the dust frequency is more important and all the
significant relationships (we have not written down. All the significant regression
equations) obey to the multiplicative or exponential models.

ACKNOWLEDGEMENTS
Prs. A. Khireddine, Y.Smara and Vicente Caselles are thankful to University of Bejaia
for financial support and the people in charge of Laboratorio Teledeteccion II of
University of Valencia (Spain) for their welcome and help. We would like to thank
also the persons responsible of ONM Tamanrasset where experiments were
performed.
REFERENCES

1. Alpert, P., Kaufman, Y., Shay El, Y., Tanré, D., da Silva, A., Schubert, S., and
Joseph, Y.H., 1998. Quantification of dust-forced heating of the lower
troposphere, Nature, 395, 367-370.

2. Carlson, T.N., and Benjamin, S.G., 1980. Radiative heating rates of Saharian
dust, J. Atmos. Sci., 37, 193-213.

33


3. Coakley, J.A., Cess, R.D., and Yurevich, F.B., 1983. The effect of
tropospheric aerosol on the earth’s radiation budget: a parameterization for
climate models, J. Atmos. Sci., 40, 116-138.

4. De Brichambaut, C.P., and Vauge, C., 1982. Le gisement solaire, Ed.
Lavoisier TEC&DOC.

5. Durand, P., and A. Druilhet , 1983. Contribution à l’étude de la structure
turbulente de la couche limite convective sahélienne en présence de brume
sèche, La Météorologie, 29, 213-226.

6. Fraser, R.S., and Kaufman, Y.J., 1985, The relative importance of aerosol
scattering and absorption in remote sensing, IEEE J. Geosc. Rem. Sens., GE-
23, 525-633.

7. Haywood, J.M., Francis, P.N., Geoogdzhayev,I., Mishchenko, M., and Frey,
R., 2001. Comparison of Saharian dust aerosol optical depths retrieved using
aircraft mounted pyranometers and 2-channel AVHRR algorithms, Geophys.
Res. Lett., 28, 2393-2396.

8. Jaenicke, R., 1979. Monitoring and critical review of the estimated source
strength of mineral dust from the Sahara, in Saharian Dust : Mobilisation,
Transport, Deposition , edited by C. Morales, , SCOPE Rep. 14, John Wiley,
New York, 233 - 242.

9. Liou, K.N., Freeman, K.P., and Sasamori, T., 1978. Cloud and aerosols effects
on the solar heating rate of the atmosphere, Tellus, 30, 62-70.

10. Moulin, C., Guillard, F., Dulac, F., Lambert, C.E., Chazette, P.,Jankowiak, I.,
Chatenet, B., and Lavenu, F., 1997. Long-term daily monitoring of Saharian
dust load over ocean using Meteosat ISCCP B2 data : 2. Accuracy of the
method and validation using sun photometer measurements, J. Geaophys.
Res., 102, 16,959-16, 969.
11. Orgeret, M., Les piles solaires, 1985. Ed. Masson, 1-24.
12. Prospero, J.M., 1990. Mineral-aerosol transport to the North Atlantic Ocean
Pacific: the impact of Africa and Asian sources in the long-range atmospheric
transport of natural and contaminant substances, Ed. A.H. Knap and
13. Norwell (Kluiwer Acad.), 59-86.
14. Tanré, D., Devaux, C., Herman, M., and Santer, R., 1988a. Radiative
properties of desert aerosols by optical ground-based measuremants at solar
wavelengths, J. Geophys. Res., 93, D11, 14,223-14,231.
15. Tanré, D., Deschamps, P.Y., Devaux, C., and Herman, M., 1988b, Estimation
of Saharian aerosol optical thickness from blurring effects in thematic mapper
data, J. Geophys. Res., 93, D12, 15,955-15,964.
16. Tegen, I., and Lacis, A.A., 1996. Modeling of particle size distribution and its
influence on the radiative properties of mineral dust aerosol, J.Geophys.Res.,
101, 19237-19244.

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IJARET: Evaluation of the Saharan Aerosol Impact on Solar Radiation Over Tamanrasset, Algeria

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