Aerosol characterization of Morocco with AERONET and intercomparison with satellite data: TOMS, MODIS, and MISR
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).
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
TL;DR: In this paper, a comparison between the extinction coefficient measured at ground level and the aerosol optical thickness measured from space at La Palma observatory was made in order to study the reliability of the satellite instruments.
Abstract: In site selection processes, one key parameter is the extinction coefficient. This parameter depends on aerosol load, water
vapor content and atmospheric gazes. Actually a lot of satellite instruments give the aerosol optical thickness over the
earth with good spatial and temporal resolutions.
The determination of the extinction coefficient at elevated altitudes from photometric surface measurements at lower
altitudes is very important in the field of site testing.
In the first part of this paper we make a comparison between the extinction coefficient measured at ground level and the
aerosol optical thickness measured from space at La Palma observatory in order to study the reliability of the aerosol
satellite instruments. We used the most popular ones: MODIS Terra and Aqua, MISR and Envisat Meris.
In the second part of the paper, we use three AERONET (Aerosol Robotic Network) stations close to one another at the
Canary Islands; Izana (longitude=16.5°W, latitude=28.3° N, altitude= 2367m), La laguna (longitude=16.32°W,
latitude=28.50°N, altitude=568 m) and Santa-Cruz Tenerife (longitude=16.25°W, latitude=28.5°N, altitude=52 m). The
aerosol optical thicknesses relative to these stations were studied in order to develop some empirical relationships that
help determine photometric quality of an astronomical observatory from satellite measurements (even with very low
resolution) or from in-situ measurements of very low elevated nearby places. LIDAR (Ligth Detection and Ranging) data
of Santa-Cruz Tenerife provided by the MPLNET (Micro-Pulse Lidar Network) network were also used.
Cites background or result from "Aerosol characterization of Morocco..."
...SantaCruz Izana (1) laguna Izana (2) SantaCruz Laguna (3)...
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...[Bounhir et al, 2007-a, 2007-b, 2008] have compared the AERONET data of Morocco and the Canary islands with the aerosol satellite instruments....
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...Then the aerosol optical thickness at Santa-Cruz is equal to the aerosol optical at Izana plus the contribution of the atmospheric portion between Izana and Sanat-Cruz altitudes (τh1); relation (1)....
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...(1) How reliable the satellite instruments are? (2) What is the compromise between reliability and resolution? Is the satellite instrument reliable for stellar photometry? We will try to answer these questions in section 2....
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References
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TL;DR: The operation and philosophy of the monitoring system, the precision and accuracy of the measuring radiometers, a brief description of the processing system, and access to the database are discussed.
6,535 citations
"Aerosol characterization of Morocco..." refers methods in this paper
...…data acquisition, retrieval algorithms and calibration procedure conform to the standards of the AERONET global network, and are described in detail, along with the uncertainty of the final released products, by Holben et al (1998), Dubovik and King (2000), Dubovik et al (1998, 2000, 2002b, 2004)....
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TL;DR: In this paper, the AERONET network of ground-based radiometers were used to remotely sense the aerosol absorption and other optical properties in several key locations, and the results showed robust differentiation in both the magnitude and spectral dependence of the absorption, a property driving aerosol climate forcing.
Abstract: Aerosol radiative forcing is a critical, though variable and uncertain, component of the global climate. Yet climate models rely on sparse information of the aerosol optical properties. In situ measurements, though important in many respects, seldom provide measurements of the undisturbed aerosol in the entire atmospheric column. Here, 8 yr of worldwide distributed data from the AERONET network of ground-based radiometers were used to remotely sense the aerosol absorption and other optical properties in several key locations. Established procedures for maintaining and calibrating the global network of radiometers, cloud screening, and inversion techniques allow for a consistent retrieval of the optical properties of aerosol in locations with varying emission sources and conditions. The multiyear, multi-instrument observations show robust differentiation in both the magnitude and spectral dependence of the absorption—a property driving aerosol climate forcing, for desert dust, biomass burning, urban‐industrial, and marine aerosols. Moreover, significant variability of the absorption for the same aerosol type appearing due to different meteorological and source characteristics as well as different emission characteristics are observed. It is expected that this aerosol characterization will help refine aerosol optical models and reduce uncertainties in satellite observations of the global aerosol and in modeling aerosol impacts on climate.
2,653 citations
TL;DR: The developed algorithm is adapted for the retrieval of aerosol properties from measurements made by ground-based Sun-sky scanning radiometers used in the Aerosol Robotic Network (AERONET) and allows a choice of normal or lognormal noise assumptions.
Abstract: The problem of deriving a complete set of aerosol optical properties from Sun and sky radiance measurements is discussed. Algorithm development is focused on improving aerosol retrievals by means of including a detailed statistical optimization of the influence of noise in the inversion procedure. The methodological aspects of such an optimization are discussed in detail and revised according to both modern findings in inversion theory and practical experience in remote sensing. Accordingly, the proposed inversion algorithm is built on the principles of statistical estimation: the spectral radiances and various a priori constraints on aerosol characteristics are considered as multisource data that are known with predetermined accuracy. The inversion is designed as a search for the best fit of all input data by a theoretical model that takes into account the different levels of accuracy of the fitted data. The algorithm allows a choice of normal or lognormal noise assumptions. The multivariable fitting is implemented by a stable numerical procedure combining matrix inversion and univariant relaxation. The theoretical inversion scheme has been realized in the advanced algorithm retrieving aerosol size distribution together with complex refractive index from the spectral measurements of direct and diffuse radiation. The aerosol particles are modeled as homogeneous spheres. The atmospheric radiative transfer modeling is implemented with well-established publicly available radiative transfer codes. The retrieved refractive indices can be wavelength dependent; however, the extended smoothness constraints are applied to its spectral dependence (and indirectly through smoothness constraints on retrieved size distributions). The positive effects of statistical optimization on the retrieval results as well as the importance of applying a priori constraints are discussed in detail for the retrieval of both aerosol size distribution and complex refractive index. The developed algorithm is adapted for the retrieval of aerosol properties from measurements made by ground-based Sun-sky scanning radiometers used in the Aerosol Robotic Network (AERONET). The results of numerical tests together with examples of experimental data inversions are presented.
2,122 citations
TL;DR: In this article, a new inversion concept for simultaneously retrieving aerosol size distribution, complex refractive index, and single scattering albedo from spectral measurements of direct and diffuse radiation was proposed.
Abstract: Sensitivity studies are conducted regarding aerosol optical property retrieval from radiances measured by ground-based Sun-sky scanning radiometers of the Aerosol Robotic Network (AERONET). These studies focus on testing a new inversion concept for simultaneously retrieving aerosol size distribution, complex refractive index, and single- scattering albedo from spectral measurements of direct and diffuse radiation. The perturbations of the inversion resulting from random errors, instrumental offsets, and known uncertainties in the atmospheric radiation model are analyzed. Sun or sky channel miscalibration, inaccurate azimuth angle pointing during sky radiance measurements, and inaccuracy in accounting for surface reflectance are considered as error sources. The effects of these errors on the characterization of three typical and optically distinct aerosols with bimodal size distributions (weakly absorbing water-soluble aerosol, absorbing biomass-burning aerosol, and desert dust) are considered. The aerosol particles are assumed in the retrieval to be polydispersed homogeneous spheres with the same complex refractive index. Therefore we also examined how inversions with such an assumption bias the retrievals in the case of nonspherical dust aerosols and in the case of externally or internally mixed spherical particles with different refractive indices. The analysis shows successful retrieval of all aerosol characteristics (size distribution, complex refractive index, and single-scattering albedo), provided the inversion includes the data combination of spectral optical depth together with sky radiances in the full solar almucantar (with angular coverage of scattering angles up to 100" or more). The retrieval accuracy is acceptable for most remote sensing applications even in the presence of rather strong systematic or random uncertainties in the measurements. The major limitations relate to the characterization of low optical depth situations for all aerosol types, where high relative errors may occur in the direct radiation measurements of aerosol optical depth. Also, the results of tests indicate that a decrease of angular coverage of scattering (scattering angles of 75" or less) in the sky radiance results in the loss of practical information about refractive index. Accurate azimuth angle pointing is critical for the characterization of dust. Scattering by nonspherical dust particles requires special analysis, whereby approximation of the aerosol by spheres allows us to derive single-scattering albedo by inverting spectral optical depth together with sky radiances in the full solar almucantar. Inverting sky radiances measured in the first 40" scattering angle only, where nonspherical effects are minor, results in accurate retrievals of aerosol size distributions of nonspherical particles.
1,562 citations
TL;DR: In this paper, the authors present a cloud-screening algorithm for ground-based sun-photometer measurements of aerosol optical depth in the Aerosol Robotic Network (AERONET).
1,311 citations
"Aerosol characterization of Morocco..." refers methods in this paper
...An automatized and computerized cloud-screening procedure algorithm [Smirnov et al, 2000] was applied to the direct sun measurements....
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