Abstract

We estimated the average dust single scattering albedo (ω0) over the Sahara using four years of moderate resolution imaging spectroradiometer data. The method employed is based on the theory that the critical surface reflectance (ρc) for which the reflectance at the top-of-the-atmosphere is not influenced by the variability of dust optical thickness depends on ω0. The average dust absorption over the Sahara was estimated to be smaller than that previously reported in the literature, and this may be causing the cooling of the climate system. Our method enables one to estimate ω0 from data with a variety of aerosol optical thickness values using samples in the vicinity of ρc. Use of satellite data over large areas and for long periods enables quantification of the average dust absorption over the entire Sahara.

© 2008 Optical Society of America

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References

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  1. J.T.Houghton, L.G.Meira Filho, J.Bruce, H.Lee, B.A.Callander, E.Haites, N.Harris, and K.Maskell, eds., Climate Change, Radiative Forcing of Climate and an Evaluation of the IPCCI S92 Emission Scenarios (Cambridge University, 1994).
  2. M. O. Andreae, “World Survey of Climatology,” in Future Climates of the World, A. Henderson-Sellers, ed. (Elsevier, 1995), Vol. 16.
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    [CrossRef]
  4. I. N. Sokolik and O. B. Toon, “Direct radiative forcing by anthropogenic airborne mineral aerosols,” Nature (London) 381, 681-683 (1996).
    [CrossRef]
  5. I. N. Sokolik, D. M. Winker, G. Bergametti, D. A. Gillette, G. Carmichael, Y. J. Kaufman, L. Gomes, L. Schuetz, and J. E. Penner, “Introduction to special section: outstanding problems in quantifying the radiative impacts of mineral dust,” J. Geophys. Res. 106, 18015-18027 (2001).
    [CrossRef]
  6. J. Hansen, M. Sato, and R. Ruedy, “Radiative forcing and climate response,” J. Geophys. Res. 102, 6831-6864 (1997).
    [CrossRef]
  7. WMO Report of the Experts Meeting on Aerosols and their Climatic Effects, Rep. WCP-55, “World Climate Program,” Geneva (1983).
  8. Y. J. Kaufman, D. Tanre, O. Dubovik, A. Karnieli, and L. A. Remer, “Absorption of sunlight by dust as inferred from satellite and ground-based remote sensing,” Geophys. Res. Lett. 28, 1479-1482 (2001).
    [CrossRef]
  9. D. Tanré, Y. Y. Kaufman, B. N. Holben, B. Chatenet, A. Karnieli, F. Lavenu, L. Blarel, O. Dubovik, L. A. Remer, and A. Smirnov, “Climatology of dust aerosol size distribution and optical properties derived from remotely sensed data in the solar spectrum,” J. Geophys. Res. 106, 18205-18217 (2001).
    [CrossRef]
  10. D. Tanré, J. Haywood, J. Pelon, J. F. L´eon, B. Chatenet, P. Formenti, P. Francis, P. Goloub, E. J. Highwood, and G. Myhre, “Measurement and modeling of the Saharan dust radiative impact: overview of the Saharan Dust Experiment (SHADE),” J. Geophys. Res. 108(D18), 8574, doi:10.1029/2002JD003273 (2003).
    [CrossRef]
  11. J. Haywood, P. Francis, S. Osborne, M. Glew, N. Loeb, E. Highwood, D. Tanre, G. Myhre, P. Formenti, and E. Hirst, “Radiative properties and direct radiative effect of Saharan dust measured by the C-130 aircraft during SHADE: 1. Solar spectrum,” J. Geophys. Res. 108(D18), 8577, doi:10.1029/2002JD002687 (2003).
    [CrossRef]
  12. B. N. Holben, T. F. Eck, I. Slutsker, D. Tanré, J. P. Buis, A. Setzer, E. Vermote, J. A. Reagan, Y. Kaufman, T. Nakajima, F. Lavenu, I. Jankowiak, and A. Smirnov, “A federated instrument network and data archive for aerosol characterization,” Remote Sens. Environ. 66, 1-16 (1998).
    [CrossRef]
  13. Y. J. Kaufman, “Satellite sensing of aerosol absorption,” J. Geophys. Res. 92, 4307-4317 (1987).
    [CrossRef]
  14. S. A. Ackerman, K. I. Strabala, W. P. Menzel, R. A. Frey, C. C. Moeller, and L. E. Gumley, “Discriminating clear sky from clouds with MODIS,” J. Geophys. Res. 103, 32141-32157(1998).
    [CrossRef]
  15. J. R. Herman, P. K. Bhartia, O. Torres, C. Hsu, C. Seftor, and E. Celarier, “Global distribution of UV-absorbing aerosols from Nimbus 7/TOMS data,” J. Geophys. Res. 102, 16911-16922(1997).
    [CrossRef]
  16. T. Nakajima and M. Tanaka, “Matrix formulation for the transfer of solar radiation in a plane-parallel scattering atmosphere,” J. Quant. Spectrosc. Radiat. Transfer 35, 13-21(1986).
    [CrossRef]
  17. T. Nakajima and M. Tanaka, “Algorithms for radiative intensity calculations in moderately thick atmospheres using a truncation approximation,” J. Quant. Spectrosc. Radiat. Transfer 40, 51-69 (1988).
    [CrossRef]
  18. K. Stamnes, S. C. Tsay, W. Wiscombe, and K. Jayaweera, “Numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media,” Appl. Opt. 27, 2502-2509 (1988).
  19. K. T. Whitby, “The physical characteristics of sulfur aerosols,” Atmos. Environ. 12, 135-159 (1978).
    [CrossRef]
  20. E. P. Shettle and R. W. Fenn, “Models for the aerosol lower atmosphere and the effects of humidity variations on their optical properties,” Rep. Tr-79-0214 (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Mass., 1979).
  21. L. A. Remer and Y. J. Kaufman, “Dynamic aerosol model: urban/industrial aerosol,” J. Geophys. Res. 103, 13859-13871(1998).
    [CrossRef]
  22. O. Dubovik and M. D. King, “A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements,” J. Geophys. Res. 105, 20673-20696(2000).
    [CrossRef]
  23. T. Nakajima, M. Tanaka, M. Yamano, M. Shiobara, K. Arao, and Y. Nakanishi, “Aerosol optical characteristics in the yellow sand events observed in May, 1982 in Nagasaki--part II model,” J. Meteorol. Soc. Jpn. 67, 279-291 (1989).
  24. J. B. Pollack and J. N. Cuzzi, “Scattering by non-spherical particles of size comparable to a wavelength: a new semi-empirical theory and its application to tropospheric aerosols,” J. Atmos. Sci. 37, 868-881 (1980).
    [CrossRef]
  25. S. A. Ackerman, “Remote sensing aerosols using satellite infrared observations,” J. Geophys. Res. 102, 17069-17079(1997).
    [CrossRef]
  26. T. N. Carlson and S. G. Benjamin, “Radiative heating rates for Saharan dust,” J. Atmos. Sci. 37, 193-213 (1980).
    [CrossRef]
  27. Y. Fouquart, B. Bonnell, M. C. Roquai, and R. Santer, “Observations of Saharan aerosols: results of ECLATS field experiment. Part I. Optical thicknesses and aerosol size distributions,” J. Clim. Appl. Meteorol. 26, 28-37 (1987).
    [CrossRef]
  28. G. A. d'Almeida, “On the variability of desert aerosol radiative characteristics,” J. Geophys. Res. 92, 3017-3026 (1987).
    [CrossRef]

2003 (2)

D. Tanré, J. Haywood, J. Pelon, J. F. L´eon, B. Chatenet, P. Formenti, P. Francis, P. Goloub, E. J. Highwood, and G. Myhre, “Measurement and modeling of the Saharan dust radiative impact: overview of the Saharan Dust Experiment (SHADE),” J. Geophys. Res. 108(D18), 8574, doi:10.1029/2002JD003273 (2003).
[CrossRef]

J. Haywood, P. Francis, S. Osborne, M. Glew, N. Loeb, E. Highwood, D. Tanre, G. Myhre, P. Formenti, and E. Hirst, “Radiative properties and direct radiative effect of Saharan dust measured by the C-130 aircraft during SHADE: 1. Solar spectrum,” J. Geophys. Res. 108(D18), 8577, doi:10.1029/2002JD002687 (2003).
[CrossRef]

2001 (3)

I. N. Sokolik, D. M. Winker, G. Bergametti, D. A. Gillette, G. Carmichael, Y. J. Kaufman, L. Gomes, L. Schuetz, and J. E. Penner, “Introduction to special section: outstanding problems in quantifying the radiative impacts of mineral dust,” J. Geophys. Res. 106, 18015-18027 (2001).
[CrossRef]

Y. J. Kaufman, D. Tanre, O. Dubovik, A. Karnieli, and L. A. Remer, “Absorption of sunlight by dust as inferred from satellite and ground-based remote sensing,” Geophys. Res. Lett. 28, 1479-1482 (2001).
[CrossRef]

D. Tanré, Y. Y. Kaufman, B. N. Holben, B. Chatenet, A. Karnieli, F. Lavenu, L. Blarel, O. Dubovik, L. A. Remer, and A. Smirnov, “Climatology of dust aerosol size distribution and optical properties derived from remotely sensed data in the solar spectrum,” J. Geophys. Res. 106, 18205-18217 (2001).
[CrossRef]

2000 (1)

O. Dubovik and M. D. King, “A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements,” J. Geophys. Res. 105, 20673-20696(2000).
[CrossRef]

1998 (3)

S. A. Ackerman, K. I. Strabala, W. P. Menzel, R. A. Frey, C. C. Moeller, and L. E. Gumley, “Discriminating clear sky from clouds with MODIS,” J. Geophys. Res. 103, 32141-32157(1998).
[CrossRef]

B. N. Holben, T. F. Eck, I. Slutsker, D. Tanré, J. P. Buis, A. Setzer, E. Vermote, J. A. Reagan, Y. Kaufman, T. Nakajima, F. Lavenu, I. Jankowiak, and A. Smirnov, “A federated instrument network and data archive for aerosol characterization,” Remote Sens. Environ. 66, 1-16 (1998).
[CrossRef]

L. A. Remer and Y. J. Kaufman, “Dynamic aerosol model: urban/industrial aerosol,” J. Geophys. Res. 103, 13859-13871(1998).
[CrossRef]

1997 (3)

S. A. Ackerman, “Remote sensing aerosols using satellite infrared observations,” J. Geophys. Res. 102, 17069-17079(1997).
[CrossRef]

J. Hansen, M. Sato, and R. Ruedy, “Radiative forcing and climate response,” J. Geophys. Res. 102, 6831-6864 (1997).
[CrossRef]

J. R. Herman, P. K. Bhartia, O. Torres, C. Hsu, C. Seftor, and E. Celarier, “Global distribution of UV-absorbing aerosols from Nimbus 7/TOMS data,” J. Geophys. Res. 102, 16911-16922(1997).
[CrossRef]

1996 (2)

I. A. Tegen, A. Lacis, and I. Fung, “The influence of mineral aerosols from disturbed soils on the global radiation budget,” Nature (London) 380, 419-422 (1996).
[CrossRef]

I. N. Sokolik and O. B. Toon, “Direct radiative forcing by anthropogenic airborne mineral aerosols,” Nature (London) 381, 681-683 (1996).
[CrossRef]

1989 (1)

T. Nakajima, M. Tanaka, M. Yamano, M. Shiobara, K. Arao, and Y. Nakanishi, “Aerosol optical characteristics in the yellow sand events observed in May, 1982 in Nagasaki--part II model,” J. Meteorol. Soc. Jpn. 67, 279-291 (1989).

1988 (2)

T. Nakajima and M. Tanaka, “Algorithms for radiative intensity calculations in moderately thick atmospheres using a truncation approximation,” J. Quant. Spectrosc. Radiat. Transfer 40, 51-69 (1988).
[CrossRef]

K. Stamnes, S. C. Tsay, W. Wiscombe, and K. Jayaweera, “Numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media,” Appl. Opt. 27, 2502-2509 (1988).

1987 (3)

Y. Fouquart, B. Bonnell, M. C. Roquai, and R. Santer, “Observations of Saharan aerosols: results of ECLATS field experiment. Part I. Optical thicknesses and aerosol size distributions,” J. Clim. Appl. Meteorol. 26, 28-37 (1987).
[CrossRef]

G. A. d'Almeida, “On the variability of desert aerosol radiative characteristics,” J. Geophys. Res. 92, 3017-3026 (1987).
[CrossRef]

Y. J. Kaufman, “Satellite sensing of aerosol absorption,” J. Geophys. Res. 92, 4307-4317 (1987).
[CrossRef]

1986 (1)

T. Nakajima and M. Tanaka, “Matrix formulation for the transfer of solar radiation in a plane-parallel scattering atmosphere,” J. Quant. Spectrosc. Radiat. Transfer 35, 13-21(1986).
[CrossRef]

1980 (2)

J. B. Pollack and J. N. Cuzzi, “Scattering by non-spherical particles of size comparable to a wavelength: a new semi-empirical theory and its application to tropospheric aerosols,” J. Atmos. Sci. 37, 868-881 (1980).
[CrossRef]

T. N. Carlson and S. G. Benjamin, “Radiative heating rates for Saharan dust,” J. Atmos. Sci. 37, 193-213 (1980).
[CrossRef]

1978 (1)

K. T. Whitby, “The physical characteristics of sulfur aerosols,” Atmos. Environ. 12, 135-159 (1978).
[CrossRef]

Appl. Opt. (1)

Atmos. Environ. (1)

K. T. Whitby, “The physical characteristics of sulfur aerosols,” Atmos. Environ. 12, 135-159 (1978).
[CrossRef]

Geophys. Res. Lett. (1)

Y. J. Kaufman, D. Tanre, O. Dubovik, A. Karnieli, and L. A. Remer, “Absorption of sunlight by dust as inferred from satellite and ground-based remote sensing,” Geophys. Res. Lett. 28, 1479-1482 (2001).
[CrossRef]

J. Atmos. Sci. (2)

T. N. Carlson and S. G. Benjamin, “Radiative heating rates for Saharan dust,” J. Atmos. Sci. 37, 193-213 (1980).
[CrossRef]

J. B. Pollack and J. N. Cuzzi, “Scattering by non-spherical particles of size comparable to a wavelength: a new semi-empirical theory and its application to tropospheric aerosols,” J. Atmos. Sci. 37, 868-881 (1980).
[CrossRef]

J. Clim. Appl. Meteorol. (1)

Y. Fouquart, B. Bonnell, M. C. Roquai, and R. Santer, “Observations of Saharan aerosols: results of ECLATS field experiment. Part I. Optical thicknesses and aerosol size distributions,” J. Clim. Appl. Meteorol. 26, 28-37 (1987).
[CrossRef]

J. Geophys. Res. (12)

G. A. d'Almeida, “On the variability of desert aerosol radiative characteristics,” J. Geophys. Res. 92, 3017-3026 (1987).
[CrossRef]

S. A. Ackerman, “Remote sensing aerosols using satellite infrared observations,” J. Geophys. Res. 102, 17069-17079(1997).
[CrossRef]

L. A. Remer and Y. J. Kaufman, “Dynamic aerosol model: urban/industrial aerosol,” J. Geophys. Res. 103, 13859-13871(1998).
[CrossRef]

O. Dubovik and M. D. King, “A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements,” J. Geophys. Res. 105, 20673-20696(2000).
[CrossRef]

D. Tanré, Y. Y. Kaufman, B. N. Holben, B. Chatenet, A. Karnieli, F. Lavenu, L. Blarel, O. Dubovik, L. A. Remer, and A. Smirnov, “Climatology of dust aerosol size distribution and optical properties derived from remotely sensed data in the solar spectrum,” J. Geophys. Res. 106, 18205-18217 (2001).
[CrossRef]

D. Tanré, J. Haywood, J. Pelon, J. F. L´eon, B. Chatenet, P. Formenti, P. Francis, P. Goloub, E. J. Highwood, and G. Myhre, “Measurement and modeling of the Saharan dust radiative impact: overview of the Saharan Dust Experiment (SHADE),” J. Geophys. Res. 108(D18), 8574, doi:10.1029/2002JD003273 (2003).
[CrossRef]

J. Haywood, P. Francis, S. Osborne, M. Glew, N. Loeb, E. Highwood, D. Tanre, G. Myhre, P. Formenti, and E. Hirst, “Radiative properties and direct radiative effect of Saharan dust measured by the C-130 aircraft during SHADE: 1. Solar spectrum,” J. Geophys. Res. 108(D18), 8577, doi:10.1029/2002JD002687 (2003).
[CrossRef]

I. N. Sokolik, D. M. Winker, G. Bergametti, D. A. Gillette, G. Carmichael, Y. J. Kaufman, L. Gomes, L. Schuetz, and J. E. Penner, “Introduction to special section: outstanding problems in quantifying the radiative impacts of mineral dust,” J. Geophys. Res. 106, 18015-18027 (2001).
[CrossRef]

J. Hansen, M. Sato, and R. Ruedy, “Radiative forcing and climate response,” J. Geophys. Res. 102, 6831-6864 (1997).
[CrossRef]

Y. J. Kaufman, “Satellite sensing of aerosol absorption,” J. Geophys. Res. 92, 4307-4317 (1987).
[CrossRef]

S. A. Ackerman, K. I. Strabala, W. P. Menzel, R. A. Frey, C. C. Moeller, and L. E. Gumley, “Discriminating clear sky from clouds with MODIS,” J. Geophys. Res. 103, 32141-32157(1998).
[CrossRef]

J. R. Herman, P. K. Bhartia, O. Torres, C. Hsu, C. Seftor, and E. Celarier, “Global distribution of UV-absorbing aerosols from Nimbus 7/TOMS data,” J. Geophys. Res. 102, 16911-16922(1997).
[CrossRef]

J. Meteorol. Soc. Jpn. (1)

T. Nakajima, M. Tanaka, M. Yamano, M. Shiobara, K. Arao, and Y. Nakanishi, “Aerosol optical characteristics in the yellow sand events observed in May, 1982 in Nagasaki--part II model,” J. Meteorol. Soc. Jpn. 67, 279-291 (1989).

J. Quant. Spectrosc. Radiat. Transfer (2)

T. Nakajima and M. Tanaka, “Matrix formulation for the transfer of solar radiation in a plane-parallel scattering atmosphere,” J. Quant. Spectrosc. Radiat. Transfer 35, 13-21(1986).
[CrossRef]

T. Nakajima and M. Tanaka, “Algorithms for radiative intensity calculations in moderately thick atmospheres using a truncation approximation,” J. Quant. Spectrosc. Radiat. Transfer 40, 51-69 (1988).
[CrossRef]

Nature (London) (2)

I. A. Tegen, A. Lacis, and I. Fung, “The influence of mineral aerosols from disturbed soils on the global radiation budget,” Nature (London) 380, 419-422 (1996).
[CrossRef]

I. N. Sokolik and O. B. Toon, “Direct radiative forcing by anthropogenic airborne mineral aerosols,” Nature (London) 381, 681-683 (1996).
[CrossRef]

Remote Sens. Environ. (1)

B. N. Holben, T. F. Eck, I. Slutsker, D. Tanré, J. P. Buis, A. Setzer, E. Vermote, J. A. Reagan, Y. Kaufman, T. Nakajima, F. Lavenu, I. Jankowiak, and A. Smirnov, “A federated instrument network and data archive for aerosol characterization,” Remote Sens. Environ. 66, 1-16 (1998).
[CrossRef]

Other (4)

J.T.Houghton, L.G.Meira Filho, J.Bruce, H.Lee, B.A.Callander, E.Haites, N.Harris, and K.Maskell, eds., Climate Change, Radiative Forcing of Climate and an Evaluation of the IPCCI S92 Emission Scenarios (Cambridge University, 1994).

M. O. Andreae, “World Survey of Climatology,” in Future Climates of the World, A. Henderson-Sellers, ed. (Elsevier, 1995), Vol. 16.

WMO Report of the Experts Meeting on Aerosols and their Climatic Effects, Rep. WCP-55, “World Climate Program,” Geneva (1983).

E. P. Shettle and R. W. Fenn, “Models for the aerosol lower atmosphere and the effects of humidity variations on their optical properties,” Rep. Tr-79-0214 (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Mass., 1979).

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Figures (9)

Fig. 1
Fig. 1

Simulated TOA reflectance as a function of surface reflectance with imaginary refractive indices 0.001 (dashed line) and 0.004 (solid line) at 0.469 μm . Results with aerosol optical thickness 0 (correspond to Rayleigh scattering), 0.182, 1.059, 1.674, and 2.290 are shown as colors. The TOA reflectance is simulated under the condition of solar zenith angle 17.2 ° , satellite zenith angle 15 ° , and relative azimuth angle 171.3 ° . The computations of the TOA reflectance are performed using RSTAR [16, 17, 18] with varying the surface reflectance from 0 to 0.4 at 0.01 intervals.

Fig. 2
Fig. 2

Observed TOA reflectance averaged every 16 days for clear conditions over the Sahara at 0.469 μm starting on 16 May 2003.

Fig. 3
Fig. 3

(a) Observed TOA reflectance during hazy conditions. (b) Difference of the TOA reflectance between hazy and average clear conditions. (c) Difference in BT11 and BT12. (d) TOMS aerosol index on 8 July 2005.

Fig. 4
Fig. 4

Scatter diagram of Δ ρ t and ρ clear t for observations with sensor zenith angle between 5 ° and 25 ° at 0.469 nm in (a) all target area, (b) area I, (c) area II, and (d) area III in Fig. 8. The mean (cross) and standard deviation (vertical bar) of Δ ρ t at each bin are displayed. The derived ρ c t (diamond) with the range (horizontal bar) is also shown. ρ c t using the value of ω 0 by [8] is shown in asterisk.

Fig. 5
Fig. 5

Simulated critical TOA reflectance at 0.469 μm for varying ω 0 with the aerosol optical thickness in the clear and hazy conditions of 0.182 and 1.674, respectively (solid line). The assumption errors of aerosol optical thickness are simulated using the aerosol optical thickness in clear conditions of 0.125 (dotted line) and 0.239 (dashed line), and the aerosol optical thickness in hazy conditions of 1.059 (dash–dot line) and 2.290 (dash–dot–dot line). The geometry of the simulation is sensor zenith of 15 ° , Sun zenith angle of 17.2 ° , and relative azimuth angle between Sun and sensor of 171.3 ° . The ρ c t (vertical solid line) and the error value (vertical dotted lines) derived from observation are also shown.

Fig. 6
Fig. 6

Estimated ω 0 of dust over the Sahara from 0.412 to 1.24 μm with the total rms error estimated in Fig. 7. Our results are compared to those of previous studies [7, 8, 11, 27, 26, 28].

Fig. 7
Fig. 7

Error in the estimated ω 0 due to error sources from (1)–(10), and the total rms error as a function of wavelength from 0.412 to 1.24 μm . The numbers in parentheses show the degree of error or the value set for the sensitivity test.

Fig. 8
Fig. 8

Spatial distribution of the samples used for the analysis. Black marks in areas I, II, and III show the locations of the samples in scatter diagrams in Figs. 4b, 4c, 4d, respectively. Background color indicates average ρ clear t with the sensor zenith angle between 5 ° and 25 ° .

Fig. 9
Fig. 9

Daily averaged radiative forcing of dust at shortwave from 0.412 to 2.13 μm using the ω 0 of (a) our results and (b) Carlson and Benjamin [26].

Equations (1)

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d V ( r ) d ln r = i = 1 2 C v , i 2 π ln σ i exp [ ( ln r - ln r v , i ) 2 2 ln 2 σ i ] ,

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