Aerosol columnar characterization in Morocco: ELT prospect
Introduction
Extremely large telescopes are considered worldwide as one of the highest priorities in ground based Astronomy. They will advance astrophysical knowledge and could completely change our understanding of the universe and may answer fundamental questions about exo-planets, dark matter and energy… At the present time, many countries are involved in the ELT project prospect, from site testing and selection to instrumentation. Morocco is part in the extremely large telescope (ELT) project prospect. A team of Moroccan astronomers is busy working on site qualification and testing. Since the performance of large telescope at visible and infrared wavelengths is critically dependant on sky transparency and then on atmospheric aerosol cover (Muñoz-Tuñon et al., 2004, Sarazin, 2006), a quantitative survey of the aerosol loadings and their microphysical and optical properties is an essential part of the site selection process.
The study of sky transparency has been initiated by astronomers, who were carrying about photometric characterization of their observatories. Stellar photometric measurements are carried out in most observatories as routine astronomic observations, since 1950 (1970) for La Silla and (1980) for La Palma. Before daytime photometry being popular, atmospheric scientists took expertise from astronomers (Laulainen, 1977, Laulainen et al., 1977. Formenti et al. (2002)) have used a record of atmospheric nighttime turbidity data from astronomical stellar radiation measurements, in South Africa, to study changes in aerosol concentrations over the past three decades prior to the popularization of sun photometry. Astronomical light extinction coefficient can be converted to aerosol optical depth and vice versa. Since the dilemma concerning the heating of the Earth, the new field of aerosols; their optical and microphysical properties with sun photometry, LIDAR techniques and satellite remote sensing, is emerging and growing very fast. Now astronomers can take advantage of these well developed tools to characterize sky transparency with even more deeper insight.
The astronomical light extinction (A) is determined by comparing the apparent brightness, B, of the standard stars with there intrinsic luminosity at different air masses, through the Beer’s law. The Langley technique relying on the plot of ln(B) versus air masses is used to extract the astronomical light extinction which is the slope of the linear regression. Daylight photometry is based on the same physical principles: Iλ = Iλ0 ∗ exp(τtot ∗ m); Iλ is the direct solar irradiance, Iλ0 is the extraterrestrial irradiance, λ is the wavelength, τtot is the total atmospheric optical depth; the sum of the optical depths of molecules (Rayleigh scattering), gazes (absorption) and aerosols (Mie scattering and absorption), m is the inverse of the cosine of the zenith angle θ. As for nighttime photometry, the linear regression Langley procedure is applied to extract the atmospheric total optical depth. The aerosol optical depth is then calculated by subtracting to τtot the aerosol optical depth of Rayleigh scattering τR and the Ozone absorption optical depth in the Chappuis bands, and the absorptions of water vapor, carbon dioxide, methane and nitroxyde, depending on the wavelength used. The astronomical light extinction coefficient A is related to the total optical depth: A = 1.086 ∗ τtot. The scattering by molecules has a predictable profile depending on the wavelength, altitude and the refractive index of molecules. The optical thickness of ozone is calculated using daily averages from TOMS satellite.
The rest of the paper is organized as follows. In Section 2, we describe the data used in the current study, the area of interest, and the methodology adopted. In Section 3, we show the details and the results of the regression analysis performed between ground measurements and the different satellite data. Section 4 deals with comparison of ground measurements of Izana and Santa-Cruz. In Section 5, first the TOMS EP index is converted to aerosol optical thickness mapping the area of study, and then we made a comparison of long term satellite AOT retrievals. Finally, we retrieve the AOT at Oukaimeden observatory, by different approaches. In order to compare to previous measurements of astronomical light extinction coefficient performed at the observatory during the year 1997, we converted the AOT to astronomical light extinction coefficient over 1997 using TOMS EP records.
Section snippets
Study area
Two AERONET stations located in Morocco; Marrakech (lat = 31.6°; lon = −8.15°; alt = 420 m) and Dakhla (lat = 23.7°; lon = −15.95°, alt = 12 m), have delivered data during at least two years. Close to Morocco, two other AERONET stations located in the Canary Islands; Izana (lat = 28.3°, lon = −16.5°; alt = 2367 m) and Santa-Cruz (lat = 28.5°, lon = −16.25°, alt = 52 m) have delivered data during relatively a long period of time. The mape showing the locations of the AERONET sun-sky radiometers used in this study is shown
TOMS aerosol index and AERONET aerosol optical thickness correlation
The only available long term record of atmospheric aerosols over both oceanic and continental areas is provided by TOMS. Indeed TOMS data are delivered since 1978 until now; Nimbus 7 from November 1978 until May 1993, Meteor 3, from August 1991 until December 1994, Earth Probe from September 1996 until December 2005 and OMI from 2004 until now. The primary importance of this instrument resides in the fact that it is possible to study the variability of the aerosols over the last three decades
Altitude effect
Vertical distribution of aerosol properties is not well understood; there is no systematic way of deducing aerosol microphysical and optical properties at a given height from aerosol properties measured on the ground just below. Active sensors like LIDAR can give the average position of the aerosol layer. In this section we will focus on AERONET AOT at Izana and Santa-Cruz and make regression analysis. As the AERONET data at Santa-Cruz and Izana are level 1.5 and then are not manually
Retrieval of Aerosol optical thickness and light extinction coefficient at Oukaïmeden observatory
First, we want to confront long term satellite aerosol loading previsions from different instruments. Fig. 5 illustrates the average aerosol optical thicknesses of MODIS and MISR instruments from 2000 until 2006. These images (MODIS and MISR) were acquired using the GES-DISC Interactive Online Visualization and Analysis Infrastructure (Giovanni) as part of the NASAs Goddard Earth Sciences (GES) Data and Information Services Center (DISC). MISR AOT forecast for the Canary Islands and the
Conclusion and perspectives
In this work we have characterized sky transparency by means of aerosol optical properties trough the aerosol optical thickness parameter in the area of Morocco and the Canary Islands. Ground based measurements are provided by the AERONET Network in four locations: Dakhla, Marrakech, Santa-Cruz and Izana. TOMS Earth Probe aerosol index have been used to have long term aerosol cover in the four AERONET stations. Seasonal effects of aerosol cover have been clearly demonstrated through ten years
Acknowledgements
We would like to thank TOMS and AERONET groups from NASA Goddard Space Flight Center for providing data. Thanks to GIOVANNI group. We thank the investigators: Emilio Cuevas, Hamad Benchekroun, Brent Holben, Philippe Goloub, and Benoit Duchemin and their staff for establishing and maintaining the sites of Marrakech, Santa-Cruz, Izana and Dakhla used in this investigation.
References (28)
- et al.
Atmos. Res.
(2002) - et al.
J. Remote Sens. Environ.
(1998) Atmos. Environ.
(1977)- et al.
Atmos. Environ.
(1977) - et al.
Remote Sens. Environ.
(2000) - et al.
Adv. Space Res.
(2002) - Benkhaldoun, Z., Siher, E.A., 1998. Photometry análisis using IRIS database. SOHO6/Gong98 Workshop. Boston, USA, ESA...
- et al.
J. Geophys. Res.
(2000) - et al.
J. Geophys. Res.
(1998) - et al.
J. Geophys. Res.
(2000)
Geophys. Res. Lett.
J. Geophy. Res.
J. Geophys. Res.
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2010, Proceedings of SPIE - The International Society for Optical EngineeringAerosol characterization of Morocco with AERONET and intercomparison with satellite data: TOMS, MODIS and MISR
2008, Proceedings of SPIE - The International Society for Optical Engineering