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Evaluation of the saharan aerosol impact on solar radiation over the tamanrasset area, algeria
Evaluation of the saharan aerosol impact on solar radiation over the tamanrasset area, algeria
Evaluation of the saharan aerosol impact on solar radiation over the tamanrasset area, algeria
Evaluation of the saharan aerosol impact on solar radiation over the tamanrasset area, algeria
Evaluation of the saharan aerosol impact on solar radiation over the tamanrasset area, algeria
Evaluation of the saharan aerosol impact on solar radiation over the tamanrasset area, algeria
Evaluation of the saharan aerosol impact on solar radiation over the tamanrasset area, algeria
Evaluation of the saharan aerosol impact on solar radiation over the tamanrasset area, algeria
Evaluation of the saharan aerosol impact on solar radiation over the tamanrasset area, algeria
Evaluation of the saharan aerosol impact on solar radiation over the tamanrasset area, algeria
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Evaluation of the saharan aerosol impact on solar radiation over the tamanrasset area, algeria

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  • 1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN6480(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) IJARETVolume 3, Issue 1, January- June (2012), pp. 25-34© IAEME: www.iaeme.com/ijaret.html ©IAEMEJournal Impact Factor (2011): 0.7315 (Calculated by GISI)www.jifactor.com 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.frABSTRACTWe 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, thedirect solar radiation in the spectrum bands 0.28-0.53 µm, 0.53-0.63 µm, 0.63-0.69µm and 0.69-4 µm and the scattered radiation. The aerosol factors are expressed by 25
  • 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 1, January - June (2012), © IAEMEthe mass concentration and the particle number. We consider three shortperiods (November 2001 to February 2002, May to July 2002 and October toDecember 2002) and a long period (November 2003 to October 2004). From the dataof solar radiation, we calculate the atmospheric turbidity using Volz, Kasten andAngström models. We then compare turbidities, solar radiation, aerosol factors andvisibility, using correlation and regression analysis. We note that the turbidities arestrongly 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 theaerosol concentration C gives:R(Tl ,C) = 0.88 , F = 682,5 , t1 = 65.4 , t2 = 26.12 , for a number of observationsequal to 207. All the results show that the Saharan aerosol has a real impact on thesolar radiation extinction. Furthermore, we note that the particle number withintermediate sizes (0.7 – 1 µm) is strongly related to the turbidity and scatteredradiation. This result can be explained by the Mie scattering of the solar radiation.Keywords: Aerosol, Solar radiation, Atmosphere, Turbidity, Sahara.1. INTRODUCTIONAtmospheric aerosol plays an important role in radiative processes. The balancebetween aerosol absorption and scattering (Fraser and Kaufman, 1985) determines itsability to counteract greenhouse warming and to affect atmospheric heating rates(Carlson and Benjamin, 1980; Alpert et al., 1998). As aerosol particles interact withsolar 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 animportant climatic impact (Tegen and Lacis, 1996; Moulin et al., 1997). The opticaldepth of Saharan aerosol was determined by Tanré et al. (1988a, b) and Haywood etal. (2001).The focus of this paper is to estimate the extinction of solar radiation in the presenceof Saharan aerosol. Two measuring sites are considered: Tamanrasset and Assekrem.The two sites are selected by the W.M.O. within framework of Global AtmosphericWash (GAW) program. The choice of these sites was motivated by the fact that theanthropogenic constituents in the Hoggar area are negligible. Indeed, the sites of 26
  • 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 1, January - June (2012), © IAEMETamanrasset and Assekrem are far from industrial areas. Furthermore, theTamanrasset region is an important source of Saharan aerosol and its soil is verysusceptible to wind erosion. Indeed, it was shown that the absence of nonerodibleelements is very favorable for the dust production. To our knowledge, theTamanrasset soil is naked.Three types of data are used: solar radiation, visibility and aerosol. Firstly, wedescribe the data measurements. Then, we present the formulas used to compute theVolz and Kasten turbidities. Finally, we search the probable relationships between theSaharan 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 theGlobal Atmospheric Watch program (GAW). One of the principal objectives was toassess the impacts of desert dust storms on long-range aerosol transport and theincreases 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, themeteorological parameters were considered.2.1. Solar radiation measurements Two parameters of solar radiation were measured: the monochromatic and thedirect solar radiation. The monochromatic radiation was measured with a sun-photometer at the green channel (λ=0.50 µm). The direct radiation was measured witha 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: 09h, 12 h and 15 h, from January 2001 to December 2002. The measurements of thedirect radiation were made at 10 h, 12 h and 14 h. The measurements were not madewhen the sun was obscured by clouds and dust of exceptional intensity.2.2. Aerosol measurementsTwo types of measurements were carried out in Tamanrasset and Assekrem : thenumber 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
  • 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 1, January - June (2012), © IAEMECounter 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 weremade at Tamanrasset. But, after March 2002, the equipment was removed toAssekrem. In Assekrem, we use only the data of May to July 2002 and October toDecember 2002.The aerosol concentration is obtained by sampling the atmospheric air with a flowrate of 3 l/mn through a tube into an instrument which contains a precision balanceand a filter. The equipment was installed at Assekrem. We began the measurementson October 2002. In this work, we use only the data from October to December 2002.3. DATA PROCESSING3.1. 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 whereτλR 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, andm is the optical air mass. The parameter m can be calculated using the following expression (DeBrichambaut 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 isgiven 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
  • 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 1, January - June (2012), © IAEME∆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) OZT0 = exp(− m.τ λ ) = 1 − a − b (8) 0 ⋅ 002118 Xwith: 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 ) 2and X=3.5m3.2. 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 largespectrum band not affected by the water vapor absorption (0.28<λ<0.63 µm). Takinginto account the attenuation by molecular scattering and ozone absorption, and thehypothesis that the aerosol scattering is proportional to λ-α, we have to solve thefollowing 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 theyear (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 andthe solar radiation extinction, we begin by the estimation of the maximum, minimumand average values of the data in each period. Table 1 shows a general view of the 29
  • 6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 1, January - June (2012), © IAEMEcomputed data of aerosol, visibility and turbidity. For the three periods, we determinethe 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 massconcentration parameter is used only in the period October to December 2002. Weillustrate 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 7Aerosol > 0.3 306503 5365 4335 166370 584 5802 8289 280 1875Number 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 798Volz turbidity 1.508 0.383 0.482 2.03 0.5 0.922 1.2 0.3 0.546 3 6Linke turbidity 12.53 2.0 2.75 19.06 2.9 6.36 9.0 2.0 3.44 2 8Angs. Turbidity β1 0.452 0.000 0.032 0.515 0.0 0.135 0.20 0.0 0.049in 0.28-0.53 µm 1 0 0Angs. Turbidity β2 1.580 0.090 0.261 1.174 0.1 0.505 0.58 0.2 0.305in 0.53-0.63 µm 6 0Visibility (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
  • 7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 1, January - June (2012), © IAEME 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 ofAssekrem. 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 periodMay-July 2002. This difference can be related to the seasonal variation of theemissivity with maximum in summer and minimum in winter (Jaenicke, 1979). In order to search a likely relationship between the Saharan aerosol and the solarradiation extinction, represented by Volz, Linke and Angström turbidity, we use thecorrelation and regression methods. The significance of the models is tested by anevaluation of the correlation, Fisher and Student coefficients. The lack of data, forsolar 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
  • 8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 1, January - June (2012), © IAEME 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. Weobserve narrow relationships between aerosol parameters and solar radiationextinction factors. The increase of the turbidity is related to the increases of aerosolnumber and mass concentration, and the decrease of the visibility. The importantcorrelations are obtained with the Linke turbidity and in the period November 2001 toFebruary 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 solarradiation intensities were measured at the same site: Tamanrasset.Furthermore, we note that the Angstrom turbidity, calculated in the spectrum band0.28 – 0.53 µm, is well correlated with the Saharan aerosol number.The regression equations, between aerosol parameters (mass concentration C andparticle 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 sizesuperior 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
  • 9. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 1, January - June (2012), © IAEME 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.CONCLUSIONWe note that the particle number with intermediate sizes (N0.5 and N0.7) is stronglyrelated to the two turbidities (β and Tl). This result can be explained by the Miescattering of the solar radiation. However, the mass concentration C is proportional tothe Linke turbidity. Almost all relationships are linear. However, in the period May toJuly, the relationship between the Voltz turbidity and the horizontal visibility isexponential. In this period, the dust frequency is more important and all thesignificant relationships (we have not written down. All the significant regressionequations) obey to the multiplicative or exponential models.ACKNOWLEDGEMENTSPrs. A. Khireddine, Y.Smara and Vicente Caselles are thankful to University of Bejaiafor financial support and the people in charge of Laboratorio Teledeteccion II ofUniversity of Valencia (Spain) for their welcome and help. We would like to thankalso the persons responsible of ONM Tamanrasset where experiments wereperformed.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
  • 10. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 1, January - June (2012), © IAEME 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. 34

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