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Journal ArticleDOI

Aerosol columnar characterization in Morocco: ELT prospect.

01 Jan 2008-New Astronomy (North-Holland)-Vol. 13, Iss: 1, pp 41-52
TL;DR: In this paper, a linear regression analysis was performed between satellite data and AERONET observations, and good correlations were observed with the Pearson correlation coefficient, R, varying from 0.68 to 0.92 for MODIS, MISR and TOMS OMI.
About: This article is published in New Astronomy.The article was published on 2008-01-01 and is currently open access. It has received 6 citations till now. The article focuses on the topics: AERONET & Ozone Monitoring Instrument.

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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01 May 2002
TL;DR: In this paper, a shape mixture of randomly oriented polydisperse spheroids is used for the retrieval of the optical properties of non-spherical aerosol in remote sensing.
Abstract: Received 4 December 2001; revised 5 February 2002; accepted 8 February 2002; published 24 May 2002. [1] Numerous studies indicate the need to account for particle non-sphericity in modeling the optical properties of dustlike aerosols. The methods for simulating the scattering of light by various non-spherical shapes have rapidly evolved over the last two decades. However, the majority of aerosol remote-sensing retrievals still rely on the Mie theory because retrievals accounting for particle non-sphericity are not as well defined methodologically and are demanding computationally. We propose a method for the retrieval of the optical properties of non-spherical aerosol based on the model of a shape mixture of randomly oriented polydisperse spheroids. This method is applied to angular and spectral radiation measurements from the Aerosol Robotic Network (AERONET) in locations influenced by desert dust. Comparisons with Mie-based retrievals show a significant improvement in dust-particle phase functions, size distributions, and refractive indices. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0933 Exploration Geophysics: Remote sensing; 0994 Exploration Geophysics: Instruments and techniques

355 citations

Journal Article
TL;DR: In this paper, the spatial and temporal variation of the AOD 440nm, aerosol particle size characterization (Angstrom exponent (I± 440-675nm), and aerosol optical depth (AOD440nm) at four locations (Agoufou, Banizoumbou, Cape Verde and Ilorin) over a period of January 2005 to December 2009 were evaluated.
Abstract: The Sahelian West Africa (Long 20W:20E, Lat 0:30N) by its climatological and geographical conditions is a key region for the characterization of global atmospheric aerosol optical properties This study evaluates the spatial and temporal variation of the Aerosol Optical Depth (AOD440nm), aerosol particle size characterization (Angstrom exponent (I±440-675nm) at four locations (Agoufou, Banizoumbou, Cape Verde and Ilorin) over a period of January 2005 to December 2009 Results of the day-to-day AOD440nm variations as well as the seasonal and annual variations are presented in order to establish the aerosol climatology in the region We compared satellite derived data of Total Ozone Mapping Spectrometer - Aerosol Index (TOMSAI), MODIS (Terra and Aqua) with those of ground-based Sunphotometer AERONET measurements In general, there exits good relationship between MODIS (Terra and Aqua) and the ground-based AERONET measurements with correlation coefficients, R2 â€o 08 reported in all stations However low coefficients (as low as 040) were obtained in all the stations for regressions between TOMS AI and ground-based Sunphotometer AERONET data

10 citations


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....

    [...]

  • ...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....

    [...]

  • ...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)....

    [...]

Journal ArticleDOI
TL;DR: In this paper , the optical properties of aerosols on daily and seasonal scales with the use of the aerosol optical depth (AOD) and Angström exponent (AE) data retrieved from AErosol RObotic NETwork (AERONET) and collected at four stations in Southern Morocco.
Abstract: The present study investigates the optical properties of aerosols on daily and seasonal scales with the use of the aerosol optical depth (AOD) and Angström exponent (AE) data retrieved from AErosol RObotic NETwork (AERONET) and collected at four stations in Southern Morocco—Saada (31.63° N; 8.16° W), Ouarzazate (30.93° N; 6.91° W), Oukaïmeden (31.21° N; 7.86° W) and Ras-El-Aïn (31.67° N; 7.60° W). An evaluation of the aerosol volumetric size distribution (AVSD) is also obtained for Saada and Ouarzazate. An AOD inter-comparison is performed between AERONET data and satellite sensors (MODerate resolution Imaging Spectroradiometer—MODIS), as well as assimilation products (Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) and Copernicus Atmosphere Monitoring Service (CAMS)), by the means of a linear regression. Regardless of site location and elevation, the results show the prevalence of the annual cycle of AOD, with a maximum in summer and a minimum in winter. In association with this seasonal variation, the variations in AE and AVSD showed an increase in coarse mode over Ouarzazate and Saada during summer (July to August), underlining that Southern Morocco is prone to the regular transport of desert dust on a seasonal basis. The inter-comparison reveals that the MERRA-2 dataset is slightly more appropriate for the study region, since it shows correlation coefficients (r) ranging from 0.758 to 0.844 and intercepts ranging from 0.021 to 0.070, depending on the study site. The statistical analysis of the back-trajectories simulated by the HYbrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model were consistent with the observations and confirmed the dominance of desert dust aerosols during the summer over the study region. On the other hand, the winter season reveals a predominance of anthropogenic and oceanic aerosols originating from the north and the west of the study site.

3 citations

Proceedings ArticleDOI
TL;DR: In this article, the authors used the AERONET network and satellite data to characterize the aerosol loadings, their optical and microphysical properties over Morocco, by using three AERNET stations in Morocco; Saada, Ras-El-Ain and Dakhla.
Abstract: The aim of this paper is to characterize the aerosol loadings, their optical and microphysical properties over Morocco, by use of the AERONET network and satellite data. Three AERONET stations in Morocco; Saada, Ras_El_Ain and Dakhla are considered in this work. The aerosol parameters studied are the aerosol optical thickness, the Angstrom parameter, the size distribution, the single scattering albedo and the refraction index. An inter-comparison with satellite data has been achieved. The most popular satellite products, TOMS (Earth Probe and OMI), MODIS and MISR have been considered. It comes out from this study that the mean aerosol optical thickness (550 nm) vary from 0.22 to 0.3, with a peak in summer time of 0.56 for Dakhla, 0.42 for Ras_El_Ain and 0.35 for Saada. The Angstrom parameter mean is 0.6 for Dakhla and 0.75 for Saada and Ras_El_Ain with a summer minimum of 0.32 for Dakhla and 0.55 for Saada and Ras_El_Ain. The size distribution is bimodal with a predominance of the coarse mode except for Saada in winter and autumn. This region depicts desert dust predominant environment with a single scattering albedo varying from 0.72 to 0.96. Saada and Ras_El_Ain being at 52 km apart, their coincident daily AOT correlate with a correlation coefficient; R=0.93. Concerning the correlation between satellite data and AERONET AOT, TOMS EP has a correlation coefficient of 0.53 for Saada (all data), 0.68 for Dakhla (all data). TOMS OMI correlation coefficient is 0.68 for Saada (all data) and 0.71 for Ras_El_Ain (year 2006). MISR (level 3 data) correlation coefficient is 0.77 for Saada (all data) and 0.85 for Dakhla (all data). MODIS (level 3) correlation coefficient is 0.86 for Saada (all data) and 0.92 for Dakhla (all data). Level 2 MODIS correlation coefficient is 0.69 for Saada (year 2006), 0.86 for Ras_El_Ain (year 2006) and 0.97 for Dakhla (year 2003).

2 citations

Journal Article
TL;DR: In this paper, the spatial and secular variant of AOD-441nm, aerosol particulate size categorization Angstrom exponent β 441-676nm at different four sites IIorin, Cape Verde, Banizoumbou and Agoufou.
Abstract: Sahelian in West Africa within Long 20 o W: 20 o E, Lat 0:30 o N through its climatic process and geographical environmental state is a major region for the distinctive of universal atmospheric aerosol optical properties. This research evaluates the spatial and secular variant of Aerosol Optical Depth AOD-441nm, aerosol particulate size categorization Angstrom exponent β 441-676nm at different four sites IIorin, Cape Verde, Banizoumbou and Agoufou. Outcomes for everyday AOD-441nm differences as well as the regular and yearly changes are presented to ascertain the aerosol climate changes in the region. We evaluated satellite resultant data of Total Ozone Mapping Spectrometer for Aerosol Index MODIS Terra and Aqua, TOMS AI with individuals of ground based Sunphotometer AERONET dimensions. Generally, presently exit good connection between ground-based AERONET dimensions and MODIS Aqua and Terra with correlation coefficients, R 2, 0.9 details in all stations. Though low coefficients low as 0.41 were ascertained in all the locations for regressions amid TOMS AI and ground based Sunphotometer AERONET data.

2 citations


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
More filters
Proceedings ArticleDOI
TL;DR: In this paper, the authors compare the aerosol index data of TOMS/Earth Probe observations and atmospheric optical extinction coefficient from CAMC in the period 1996-2004, showing that there is no linear correlation between the index and the extinction coefficient.
Abstract: Satellite data measuring aerosols over the Canary Islands need to be correctly interpreted in accordance with the spatial resolution and spectroscopic channels used. In situ data are still a necessary reference for calibrating and interpreting the aerosol index provided by different spectrometers onboard satellites. Most of the airmass flux component arriving at the Canarian Archipelago comes from the North Atlantic Ocean and consists of sea aerosols, i.e. absorbent chloride in the UV which does not affect the extinction in the visible range. African dust intrusions affect the western and eastern Canary Islands differently. Moreover, the presence of a stable inversion layer and the sharp orography of the western islands (La Palma at Tenerife) produce different mass flux patterns in the low (mixing) layers closer to the sea and in the median-upper (or free) troposphere layer. The aerosol index provided by the TOMS (Total Ozone Mapping Spectrometer) is one of the most widely accepted products to detect the daily aerosol content. On the other hand, several techniques have been developed in situ to characterize the presence of dust locally at the Canarian Observatories. In particular, a parameter related to sky transparency, the atmospheric extinction coefficient, has been measured at the Roque de los Muchachos Observatory (ORM) on La Palma since 1984 by the Carslberg Automatic Meridian Circle Telescope (CAMC). In situ aerosols are also available for the Teide Observatory (OT) on Tenerife, from 1986 onwards. In this work we compare aerosol index data of TOMS/Earth Probe observations and atmospheric optical extinction coefficient from CAMC in the period 1996-2004. A preliminary comparison of both techniques (in situ and remote) is discussed in this paper, showing there is no linear correlation between the aerosol index and the extinction coefficient. Here, we present an interpretation of different situations arising and discuss critically the correct interpretation of the aerosol index and images provided by the TOMS in accordance with the dust presence over the Observatories.

7 citations

Journal ArticleDOI
TL;DR: In this article, the authors used data of nine years (1989 to 1997) at all IRIS sites (Culgoora, Kumbel, Oukaimeden, Izana, La Silla and Stanford, devoted to the study of the solar velocity oscillation) to study of diurnal atmospheric extinction coefficient behaviour at OUKaimeden observatory and to compare it to the others.
Abstract: In this work, we use data of nine years (1989 to 1997) at all IRIS sites (Culgoora, Kumbel, Oukaimeden, Izana, La Silla and Stanford, devoted to the study of the solar velocity oscillation) to study of diurnal atmospheric extinction coefficient behaviour at Oukaimeden observatory and to compare it to the others. By this study, we can conclude that the mean extinction coefficient at Oukaimeden is around 0.120 mag airmass-1. Besides, we show that the daily extinction coefficient varies under the influence of the seasonal effect. By the comparison of Oukaimeden to the other sites, we showed that Oukaimeden has a good photometric sky quality and constitutes a potential site for astronomical observations in the north hemisphere.

6 citations

Proceedings ArticleDOI
TL;DR: In this paper, a comparison of both techniques (in situ and remote) is discussed in this paper, showing there is no linear correlation between the aerosol index and the extinction coefficient, and the authors present an interpretation of different situations arising and discuss critically the correct interpretation of the aircraft's images provided by the TOMS in accordance with the dust presence over the Observatories.
Abstract: Satellite data measuring aerosols over the Canary Islands need to be correctly interpreted in accordance with the spatial resolution and spectroscopic channels used. In situ data are still a necessary reference for calibrating and interpreting the aerosol index provided by different spectrometers onboard satellites. A comparison of both techniques (in situ and remote) is discussed in this paper, showing there is no linear correlation between the aerosol index and the extinction coefficient. Here, we present an interpretation of different situations arising and discuss critically the correct interpretation of the aerosol index and images provided by the TOMS in accordance with the dust presence over the Observatories. The aerosol index measured at sectors centered at both Observatories (Observatorio del Roque de los Muchachos -ORM-, on La Palma and Observatorio del Teide -OT-, on Tenerife) is also compared.

5 citations

Journal ArticleDOI
01 Nov 2005
TL;DR: The ESO strategy for short listing ELT candidate sites is reviewed: it uses a specially designed tool to allow the simultaneous use of existing databases with various relevant parameters.
Abstract: The ESO strategy for short listing ELT candidate sites is reviewed: it uses a specially designed tool to allow the simultaneous use of existing databases with various relevant parameters.

4 citations

Proceedings ArticleDOI
07 Jul 2004
TL;DR: In this article, the site selection for the future large European telescope is addressed within the Euorpean proposal to be submitted to be funded by the EECC within the ELT design studies proposal.
Abstract: The site selection for the future large European telescope is a key issue, which will be addressed within the Euorpean proposal to be submitted to be funded by the EECC within the ELT design studies proposal. The organization, working scheme and baseline frameworks have been discussed and planned and are summarized here. For the definition of the working package (the WP 12000 of the proposal) important use have been done of all efforts been carried out during the last decade in the definition of techniques and tools reliable for the study of the atmosphere over observing sites. Also important is the relevance that the studies of the atmosphere have acquired, becoming key projects for most important astronomical sites. Therefore we have also taken advantage of the number of data already available which have naturally defined a ranking among the known places, which have also been taken as a base line for pre-selecting the candidate sites.

2 citations