scispace - formally typeset

Journal ArticleDOI

Aerosol columnar characterization in Morocco: ELT prospect.

01 Jan 2008-New Astronomy (North-Holland)-Vol. 13, Iss: 1, pp 41-52

Abstract: The work presented in this paper focuses on site testing in terms of aerosol loadings where ground based measurements are essential. In our case they are materialized by the aerosol optical thickness (AOT) and are provided by the Aerosol Robotic Network (AERONET) network from four stations, Dakhla and Marrakech in Morocco and Santa-Cruz and Izana in the Canary Islands. To fully scan all the area of the Moroccan territories, satellite measurements are certainly the most efficient way. We used the most popular and reliable products. Total ozone mapping spectrometer (TOMS) aerosol index (AI) provided by both TOMS Earth Probe and TOMS ozone monitoring instrument (OMI) along with aerosol optical thicknesses provided by moderate resolution imaging spectroradiometer (MODIS) and multiangle imaging spectroradiometer (MISR) instruments, onboard Terra platform. The idea is to compare sensing capabilities of each instrument in the region under study, in order to know which is suitable for a given place and when. For that purpose linear regression analysis were performed between satellite data and AERONET observations. Good correlations were observed with the Pearson correlation coefficient, R, varying from 0.68 to 0.92 for MODIS, MISR and TOMS OMI. However for TOMS EP the correlations are fairly poor (from 0.54 to 0.74). A ten years analysis of the TOMS EP index has been performed with a calibration of the aerosol index into TOMS retrieved aerosol optical thickness in the area of interest (Morocco and Canary Islands) and an inter-comparison with the other products was achieved. In the frame of the extremely large telescope (ELT) project prospect, once the appropriate satellite instrument have been chosen and the area scanned, the next step would be to scan aerosol loadings at higher altitude locations. Since vertical distribution of aerosol optical thickness and microphysical properties are not well understood and modelized, we used the relationships related to Izana (Izana’s altitude is 2367 m), as a first attempt, to extrapolate the aerosol optical thickness at higher locations in the Moroccan mountains. Izana and Santa-Cruz very close to each other (30 km) are located in the same satellite pixel and then have the same satellite (AOT) or (AI) whereas AERONET gives very distinct aerosol optical depths. A good linear correlation (R = 0.92) has been observed between the AERONET aerosol optical depths at Izana and Santa-Cruz. The seasonal correlation coefficients are 0.85 for winter, 0.87 for spring, 0.91 for summer and 0.87 for autumn The ratio AODSanta-Cruz/AODIzana has a seasonal behavior, reaches the average of 4.5 in winter and 2 in summer time and the subtraction of the aerosol optical thicknesses has an average of 1.3. Finally we retrieved the aerosol optical thickness at Oukaimeden: a Moroccan observatory located at 2700 m above sea level, and about 70 km from Marrakech city. We then converted the aerosol optical depth into astronomical light extinction and compare with previous records measured at the observatory.

Content maybe subject to copyright    Report

HAL Id: ird-00390757
https://hal.ird.fr/ird-00390757
Submitted on 4 Jun 2009
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entic research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diusion de documents
scientiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
Aerosol columnar characterization in Morocco: ELT
prospect.
A. Bounhir, Z. Benkhaldoun, Bernard Mougenot, B. Sarazin, E. Siher, L.
Masmoudi
To cite this version:
A. Bounhir, Z. Benkhaldoun, Bernard Mougenot, B. Sarazin, E. Siher, et al.. Aerosol columnar
characterization in Morocco: ELT prospect.. New Astronomy, Elsevier, 2008, 13 (1), pp.41-52. �ird-
00390757�

Accepted Manuscript
Aerosol columnar characterization in Morocco: ELT prospect
A. Bounhir, Z. Benkha ldoun, B. Mo ugenot, M. Sarazin, E. Siher, L. Masmoudi
PII: S1384-1076(07)00061-9
DOI: 10.1016/j.newast.2007.06. 011
Reference: NEASPA 181
To appear in:
New Astronomy
Received Date: 1 April 2007
Revised Date: 10 June 2007
Accepted Date: 20 June 2007
Please cite this article as: Bounhir, A., Benkhaldoun, Z., Mougenot, B., Sarazin, M., Siher, E., Masmoudi, L., Aerosol
columnar characterization in Morocco: ELT prospect,
New Astronomy
(2007), doi: 10.1016/j.newast.2007.0 6.011
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
1
Aerosol columnar characterization in
Morocco: ELT prospect
A.Bounhir
1,2,6
, Z. Benkhaldoun
2
, B.Mougenot
3
, M.Sarazin
4
, E. Siher
5
and L.Masmoudi
6
1 Département de Physique Appliquée, Faculté des Sciences et Technique, BP 549. Marrakech, Morocco.
2 LPHEA, Département de Physique, Faculté des Sciences Semlalia. BP. 2390, Marrakech, Morocco.
3 CESBIO, 18 Avenue Edouard Belin 31401 Toulouse cedex 09, France
4 ESO, Karl-Schwarzschild-Str. 2 D-85748 Garching (Germany)
5 Département de Physique Appliquée, Faculté des Sciences et Techniques, BP : 523, Béni Mellal, Maroc.
6 LETS, Faculté des sciences, avenue Ibn Battouta Bp 1014, Rabat, Morocco.
Abstract
The work presented in this paper focuses on site testing in terms of aerosol loadings where ground based
measurements are essential. In our case they are materialized by the aerosol optical thickness (AOT) and are
provided by the AERONET (Aerosol Robotic Network) network from four stations, Dakhla and Marrakech in
Morocco and Santa-Cruz and Izana in the Canary Islands. To fully scan all the area of the Moroccan territories,
satellite measurements are certainly the most efficient way. We used the most popular and reliable products. TOMS
(Total Ozone Mapping Spectrometer) aerosol index (AI) provided by both TOMS Earth Probe and TOMS OMI
(Ozone Monitoring Instrument) along with aerosol optical thicknesses provided by MODIS (Moderate Resolution
Imaging Spectroradiometer) and MISR (Multiangle Imaging Spectroradiometer) instruments, onboard Terra
platform. The idea is to compare sensing capabilities of each instrument in the region under study, in order to know
which is suitable for a given place and when. For that purpose linear regression analysis were performed between
satellite data and AERONET observations. Good correlations were observed with the Pearson correlation
coefficient, R, varying from 0.68 to 0.92 for MODIS, MISR and TOMS OMI. However for TOMS EP the
correlations are fairly poor (from 0.54 to 0.74). A ten years analysis of the TOMS EP index has been performed with
a calibration of the aerosol index into TOMS retrieved aerosol optical thickness in the area of interest (Morocco and
Canary Islands) and an inter-comparison with the other products was achieved. In the frame of the ELT (Extremely
Large Telescope) project prospect, once the appropriate satellite instrument have been chosen and the area scanned,
the next step would be to scan aerosol loadings at higher altitude locations. Since vertical distribution of aerosol
optical thickness and microphysical properties are not well understood and modelized, we used the relationships
related to Izana (Izana’s altitude is 2367 m), as a first attempt, to extrapolate the aerosol optical thickness at higher
locations in the Moroccan mountains. Izana and Santa-Cruz very close to each other (30 Km) are located in the same
satellite pixel and then have the same satellite (AOT) or (AI) whereas AERONET gives very distinct aerosol optical
depths. A good linear correlation (R=0.92) has been observed between the AERONET aerosol optical depths at
Izana and Santa-Cruz. The seasonal correlation coefficients are 0.85 for winter, 0.87 for spring, 0.91 for summer and
0.87 for autumn The ratio AOD
Santa-Cruz
/AOD
Izana
has a seasonal behavior, reaches the average of 4.5 in winter and 2
in summer time and the subtraction of the aerosol optical thicknesses has an average of 1.3. Finally we retrieved the
aerosol optical thickness at Oukaimeden: a Moroccan observatory located at 2700 m above sea level, and about 70
km from Marrakech city. We then converted the aerosol optical depth into astronomical light extinction and
compare with previous records measured at the observatory.
Key words: site testing, atmospheric effects, data analysis, photometer.
1- 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
ELT (Extremely Large Telescope) 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 [Munoz Tunon et al, 2004; Sarazin et al

ACCEPTED MANUSCRIPT
2
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 et al 1977-a, 1977-b].
[P. 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: I
λ
= I
λ0
*exp(τ
tot
*m); I
λ
is the direct solar
irradiance, I
λ0
is the extraterrestrial irradiance, λ is the wavelength, τ
tot
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 τ
tot
the aerosol optical depth of Rayleigh scattering τ
R
and the Ozone absorption optical
depth τ
O3
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*τ
tot
. 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.
2- Study area, data and method
2-1 Study area
Two AERONET stations located in Morocco; Marrakech (lat=31.6°; lon=-8.15°; alt=420m) and Dakhla (lat=23.7°;
lon=-15.95°, alt=12m), 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=2367m) and Santa-Cruz(lat=28.5°, lon=-
16.25°, alt=52m) 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 in figure1. These sites span desert dust aerosol
predominant environment.
2-2 Data source
a- AERONET aerosol optical thickness
We used data acquired by The AERONET (AErosol RObotic NETwork) program established by NASA and LOA-
PHOTONS (CNRS). The instruments deployed are automatic sun and sky scanning radiometers (CIMEL). The
photometer makes measurements of, direct sun and diffuse sky radiances, within the spectral range 340-1020 nm.
The direct sun measurements are acquired in eight spectral channels. The 940 nm band is used to estimate total
precipitable water content. The bandwidths of the interference filters vary from 2 to 10 nm. The total spectral optical
depth is computed through the Beer-Lambert-Bouguer law after correction taking into account gaseous absorption.
Sky measurements are performed at 440, 670, 870 and 1020 nm with two basic sky observations “almucantar” and
“principal plane”. The philosophy is to acquire aureole and sky radiances observations through a large range of
scattering angles from the sun through a constant aerosol profile to retrieve size distribution, phase function and
single scattering albedo.

ACCEPTED MANUSCRIPT
3
Data are transmitted through the DCS Data Collection System operating the geosynchronous GOES, METEOSAT,
and GMS satellites and are available at the sun photometer home page http://aeronet.gsfc.nasa.gov
. The instruments
are described in detail in [Holben et al, 1998] and the inversion procedure is well documented in [Dubovic et al;
1998, 2000-a, 2000-b, 2002]. Aerosol optical depth data are computed for three data quality levels: Level 1.0
(unscreened), Level 1.5 (cloud screened), and Level 2.0 (cloud-screened and quality assured). Cloud screening
procedures are detailed in [Smirnov et al; 2000].
b- TOMS aerosol index
The TOMS (Total Ozone Mapping Spectrometer) instruments allow an efficient method of space based aerosol
detection: TOMS Aerosol Index [Torres et al, 1998]. TOMS AI is a measure of the change of spectral contrast in the
near ultraviolet (341 and 380 nm) due to radiative transfer effects of aerosols in a Rayleigh scattering atmosphere.
The AI is defined as:
where I
mes
is the backscattered radiance measured by TOMS and I
calc
the radiance calculated using a radiance
transfer model for a pure Rayleigh atmosphere. TOMS data used in this work are daily gridded level-3 product at a
resolution of latitude and 1.25° longitude, The AI data were acquired from the NASA/GSFC TOMS ozone
processing team at http://toms.gsfc.nasa.gov/ftpdata.html
and are related to TOMS EP and TOMS OMI aboard Earth
Probe and AURA platforms respectively.
c- MISR aerosol optical thickness.
MISR is one of five instruments launched into polar orbit aboard NASA's Terra spacecraft in August 1999. The
MISR instrument provides high sensitivity for a wide range of scene reflectance (0.02% to 100%) without change in
gain. MISR has 9 cameras so the instrument can view successively a same point of the Earth at this number of
angles simultaneously. Most of the information about aerosol and cloud particle properties, and some of the
information about surface structure, come from studying observations taken at different scattering angles. 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. The canter wavelength of each of these bands is 446, 558, 672, and 867
nanometers respectively. MISR has a periodic coverage between two and nine days depending on the latitude
[Martonchick et al, 2002]. Its unique combination of multiple bands and angles enables it to retrieve aerosol optical
thickness and additional particle properties at a resolution of 17.6 Km over both land and ocean, with no assumption
about the absolute land surface reflectance or its spectral characteristics in the aerosol retrieval algorithm
[Martonchick et al, 1998, 2002]. The bands in the red and near-infrared (670 and 865 nm) provide vegetated surface
identification, and are also useful for marine aerosol studies since water is nearly black at these wavelengths. The
green band at 555 nm is near the peak of the solar spectrum, and thus will be given high weight in studies to
estimate broadband reflecting properties (albedos). The MISR wavelengths have been selected to avoid known
ranges of strong atmospheric gas absorption and solar Fraunhofer lines.
d- MODIS aerosol optical thickness:
MODIS (or Moderate Resolution Imaging Spectroradiometer) is a key instrument aboard the Terra (EOS AM)
and
Aqua (EOS PM)
satellites. Terra MODIS and Aqua MODIS are viewing the entire Earth's surface every 1 to 2 days,
acquiring data in 36 spectral bands between 0.405 and 14.385 µm. The MODIS instrument has a viewing swath
width of 2,330 km and it acquires data at three spatial resolutions 250m, 500m, and 1,000m. MODIS retrieval
algorithm is based on the “dark object” method for aerosol estimation [Kaufman et al, 1997-a, 1997-b, 2002]. The
basic assumption is that if the surface is densely vegetated, the middle infrared reflectance around 2.1 µm (MODIS
band 7) ρ
7
is linearly related to surface reflectance at the blue band (band 3 around 0.49 µm) ρ
3
and the red band
(band 1 around 0.66 µm) ρ
1
. Since most aerosol sizes are smaller than the middle-infrared wavelength, aerosol
effects in band 7 are negligible and its surface reflectance can be easily estimated. Based on surface measurements,
Kaufman et al. (1997-b) established the following empirical formulae: ρ
7/
ρ
1
=0.25, and ρ
3/
ρ
7
=0.5. 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. These look-up tables are pre-calculated for each aerosol
model and pre-defined for a given location and time.
Along with all the data from other instruments on board the Terra spacecraft and Aqua Spacecraft, MODIS data are
transferred to ground stations in White Sands, New Mexico, via the Tracking and Data Relay Satellite System
(TDRSS). The data are then sent to the EOS Data and Operations System (EDOS) at the Goddard Space Flight
Center. After Level 0 processing at EDOS, the Goddard Space Flight Center Earth Sciences Distributed Active
Archive Center (GES DAAC) produces the Level 1A, Level 1B, geolocation and cloud mask products. Higher-level
MODIS land and atmosphere products are produced by the MODIS Adaptive Processing System (MODAPS), and
then are parcelled out among three DAACs for distribution.
2-3 Methodology
=
calc
380
331
10
mes
380
331
10
I
I
Log
I
I
Log100- AI

Citations
More filters

01 May 2002
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
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

9 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)....

    [...]


Proceedings ArticleDOI
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
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]....

    [...]


Proceedings ArticleDOI
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.

References
More filters

Journal ArticleDOI
Brent N. Holben1, Thomas F. Eck2, Ilya Slutsker1, Didier Tanré  +9 moreInstitutions (6)
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.
Abstract: The concept and description of a remote sensing aerosol monitoring network initiated by NASA, developed to support NASA, CNES, and NASDA’s Earth satellite systems under the name AERONET and expanded by national and international collaboration, is described. Recent development of weather-resistant automatic sun and sky scanning spectral radiometers enable frequent measurements of atmospheric aerosol optical properties and precipitable water at remote sites. Transmission of automatic measurements via the geostationary satellites GOES and METEOSATS’ Data Collection Systems allows reception and processing in near real-time from approximately 75% of the Earth’s surface and with the expected addition of GMS, the coverage will increase to 90% in 1998. NASA developed a UNIX-based near real-time processing, display and analysis system providing internet access to the emerging global database. Information on the system is available on the project homepage, http://spamer.gsfc.nasa.gov . The philosophy of an open access database, centralized processing and a user-friendly graphical interface has contributed to the growth of international cooperation for ground-based aerosol monitoring and imposes a standardization for these measurements. The system’s automatic data acquisition, transmission, and processing facilitates aerosol characterization on local, regional, and global scales with applications to transport and radiation budget studies, radiative transfer-modeling and validation of satellite aerosol retrievals. This article discusses 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.

5,936 citations


"Aerosol columnar characterization i..." refers methods in this paper

  • ...The data were collected from the GIOVANNI MOVAS interface that provides monthly global level-3 products (MIL3MAE for MISR and MODO8_03 for MODIS) at 1*1 degree grid average....

    [...]

  • ...The instruments are described in detail in [Holben et al, 1998] and the inversion procedure is well documented in [Dubovic et al; 1998, 2000-a, 2000-b, 2002]....

    [...]


Journal ArticleDOI
Oleg Dubovik1, Brent N. Holben, Thomas F. Eck1, Thomas F. Eck2  +5 moreInstitutions (2)
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.

2,459 citations


Journal ArticleDOI
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.

1,951 citations


Journal ArticleDOI
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.

1,421 citations


Journal ArticleDOI
Alexander Smirnov1, Brent N. Holben1, Thomas F. Eck1, Thomas F. Eck2  +2 moreInstitutions (2)
Abstract: Automatic globally distributed networks for monitoring aerosol optical depth provide measurements of natural and anthropogenic aerosol loading, which is important in many local and regional studies as well as global change research investigations. The strength of such networks relies on imposing a standardization of measurement and processing, allowing multiyear and large-scale comparisons. The development of the Aerosol Robotic Network (AERONET) for systematic ground-based sunphotometer measurements of aerosol optical depth is an essential and evolving step in this process. The growing database requires the development of a consistent, reproducible, and system-wide cloud-screening procedure. This paper discusses the methodology and justification of the cloud-screening algorithm developed for the AERONET database. The procedure has been comprehensively tested on experimental data obtained in different geographical and optical conditions. These conditions include biomass burning events in Brazil and Zambia, hazy summer conditions in the Washington DC area, clean air advected from the Canadian Arctic, and variable cloudy conditions. For various sites our screening algorithm eliminates from ∼20% to 50% of the initial data depending on cloud conditions. Certain shortcomings of the proposed procedure are discussed.

1,237 citations


"Aerosol columnar characterization i..." refers methods in this paper

  • ...The results of linear regression analysis between the MODIS or MISR retrieved AOT and AERONET observations, in the form AOTsatellite=A*AOTAERONET + B, are illustrated in Table 2; R is the correlation coefficient, N, the number of months studied and corresponding means of AOT retrieved by satellite:…...

    [...]