Aerosol columnar characterization in Morocco: ELT prospect.
Summary (2 min read)
1- Introduction
- Extremely Large Telescopes are considered worldwide as one of the highest priorities in ground based Astronomy.
- At the present time, many countries are involved in the ELT project prospect, from site testing and selection to instrumentation.
- 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.
- 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 τO3 in the Chappuis bands, and the absorptions of water vapor, carbon dioxide, methane and nitroxyde, depending on the wavelength used.
- In section 2, the authors describe the data used in the current study, the area of interest, and the methodology adopted.
2-1 Study area
- The mape showing the locations of the AERONET sun-Sky radiometers used in this study is shown in figure1.
- These sites span desert dust aerosol predominant environment.
2-2 Data source
- The authors used data acquired by The AERONET (AErosol RObotic NETwork) program established by NASA and LOAPHOTONS (CNRS).
- In each of the nine MISR cameras, images will be obtained in four spectral bands, i.e. in four different colours, one each for blue, green, red, and near-infrared.
- After calculating surface reflectance of band 3 and 1 from their linear relationships with band 7 reflectance, aerosol optical depths at band 1 and 3 can be estimated by using look-up tables.
- Marrakech, being located at the bottom of the High Atlas mountains, its AOT will be used for to extrapolate the optical thickness in neighbouring high altitude places, taking advantage of previous results concerning Izana and SantaCruz.
- The satellite data used are the aerosol index (AI) provided by both Earth Probe and OMI, and the aerosol optical thicknesses provided by MODIS and MISR.
3-1 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.
- Their theoretical model simulation has demonstrated that the AI depends on the aerosol optical thickness (AOT), single scattering albedo (SSA), the aerosol layer height and viewing geometry; which makes the quantitative interpretation of the AI very difficult.
- The authors can see from figure 2 that Dakhla’s index is very high, the index at Marrakech is moderate and at Izana and Santa-Cruz, it is relatively low.
- Since at all sites there is not linearity between AOT and AI seasonal curves, the annual relationship is not accurate enough for AOT retrievals.
- TOMS data have been used from astronomers; [Siher et al, 2004] found good correlation between TOMS index and light astronomical extinction over La Palma observatory, however debated [Varela et al; 2004].
3-2 MISR-AERONET and MODIS-AERONET AOT correlation.
- The authors will examine monthly values because MISR provides weekly values and MODIS TERRA provides one value every two days.
- The slope after adjusting wavelengths is 0.98 for MODIS and 0.83 for MISR.
- There are two key requirements associated with this method: (1) existence of large homogeneous dense vegetation in the scene, (2) stable empirical relationships of surface reflectance between band 7 and band 3 and 1.
- The aerosol optical thicknesses measured with sunphotometers at Izana and Santa-Cruz are in very good correlation as well illustrated in the section altitude effect.
4- 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.
- The linear relationships (annual from daily means, annual from monthly means, annual from seasonal means, seasonal) are illustrated in table 3 (AOTIzana=A*AOTSanta-Cruz+B) with corresponding correlation coefficient, number of measurements and AOT means.
- From this table the authors can see that the annual average of Izana is 0.08 which is very low aerosol loading, they must say that the corresponding histogram is very sharp with 75% of the frequency occurring for AOT less than 0.05.
- Santa-Cruz seems to see more AOT events during all the year with a magnitude depending on the season.
- Seasonal behaviour is also clear with low AOT values in winter time, increasing in spring time to a maximum in the early summer and decreasing during summer and autumn to a minimum value in the following winter.
5- Retrieval of Aerosol optical thickness and light extinction coefficient at Oukaïmeden observatory.
- First, the authors want to confront long term satellite aerosol loading previsions from different instruments.
- The authors have already noticed that the MISR signal underestimates the AOT around Marrakech (section 3-2), whereas the MODIS signal gives a better estimation.
- Figure 7 (b) illustrates daily retrievals from TOMS OMI and from AERONET for the year 2005.
- The authors transformed the TOMS EP index to aerosol optical thickness first using the relation related to Izana in order to account for altitude contribution.
- The result is illustrated in figure 9 where the authors added the astronomical light extinction coefficient of La Palma for the same year.
6- Conclusion and perspectives
- In this work the authors 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.
- Comparisions of the TOMS aerosol index with sun-photometer aerosol optical thickness: results and applications.
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Citations
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Cites background from "Aerosol columnar characterization i..."
...For large particles like dust Bounhir et al.( 2008) stated that the wavelength dependence between 340 nm and 440nm is very small and as such the 440nm AOD values is approximately equal to that of 340nm. Ground-based AERONET, TOMS-AI and MODIS AOD show extreme similarity in their daily variations....
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...Bounhir et al. (2008) reported Pearson correlation coefficient varying from 0.68 to 0.92 between AERONET data and satellite derive aerosol optical depth (MODIS, MISR and TOMS OMI) for Morocco....
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...However, the strong dependence of AI on height distribution of aerosol decreases its sensitivity to the aerosol presences at altitude below 1.5km (Bounhir et al., 2008)....
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3 citations
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Cites background from "Aerosol columnar characterization i..."
...Little association coefficient observed in Banizoumbou can be enlightened by a mixture of diverse factors such as feeling of the TOMS algorithm to height of the mineral dust layer, sub-pixel cloud pollution, aerosol composition, size circulation and variety frequency for the Sunphotometer and TOMS algorithm [12,42]....
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References
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