Characterisation of dust aerosols in the infrared from IASI and comparison with PARASOL, MODIS

19 Pages·2017·2.56 MB·English

Characterisation of dust aerosols in the infrared from
IASI and comparison with PARASOL, MODIS, MISR,
CALIOP, and AERONET observations
S. Peyridieu, A. Chédin, V. Capelle, C. Tsamalis, C. Pierangelo, Raymond
Armante, C. Crevoisier, L. Crépeau, M. Siméon, F. Ducos, et al.
To cite this version:
S. Peyridieu, A. Chédin, V. Capelle, C. Tsamalis, C. Pierangelo, et al.. Characterisation of dust
aerosols in the infrared from IASI and comparison with PARASOL, MODIS, MISR, CALIOP,
and AERONET observations. Atmospheric Chemistry and Physics, 2013, 13 (12), pp.6065-6082.
￿10.5194/acp-13-6065-2013￿. ￿hal-01091227￿
HAL Id: hal-01091227
https://hal.archives-ouvertes.fr/hal-01091227
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doi:10.5194/acp-13-6065-2013 Chemistry Ac Chemistry Ac
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Discussions
Atmospheric O Atmospheric O
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Measurement A Measurement A
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Techniquese Techniquese
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Characterisation of dust aerosols in the infrared from IASI and Discussions
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comparison with PARASOL, MODIS, MISR, CALIOP, and p p
e Biogeosciencese
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Biogeosciences A A
AERONET observations cc Discussionscc
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S.Peyridieu1,*,A.Che´din1,V.Capelle1,C.Tsamalis1,C.Pierangelo2,R.Armante1,C.Crevoisier1,L.Cre´peau1,
M.Sime´on1,F.Ducos3,andN.A.Scott1 Op Climate Op
Climate e e
1LaboratoiredeMe´te´orologieDynamique,UMR8539,CNRS/IPSL,EcolePolytechnique,Palaiseau,France n n
A of the Past A
2CentreNationald’EtudesSpatiales,Toulouse,France of the Pastcc cc
e e
3Laboratoired’OptiqueAtmosphe´rique,Universite´ desSciencesetTechnologiesdeLille,Villeneuved’Ascq,Frsance Discussionss
s s
*nowat:M3Systemsc/oCLS,SpaceOceanographyDivision,Ramonville-St-Agne,France
O O
Correspondenceto:V.Capelle([email protected]) p Earth System p
Earth System e e
n n
A Dynamics A
Received:10August2012–PublishedinAtmos.Chem.Phys.Discuss.:6September2012 Dynamicsc c
c c
Revised:8May2013–Accepted:23May2013–Published:25June2013 es Discussionses
s s
Geoscientific Geoscientific
Abstract. Infrared Atmospheric Sounder Interferometer 865nm,bothexpectedtoberepresentativeofthOedustcoarse O
p p
(IASI) observations covering the period from July 2007 to mode, lead to conclusiIonnsstdriuffmeriengntaactcioorndi neg to the re- Instrumentation e
n n
December 2011 are interpreted in terms of monthly mean, gionconsidered.ThesediffeMreenctehsoadresd iascnudsse Acdinthelight Methods and Ac
1◦×1◦,10µmdustAerosolOpticalDepth(AOD),meanal- of the MODIS Angstro¨m exponent (865–550cenm). At lo- ce
Data Systemsss Data Systemsss
titude and coarse mode effective radius. The geographical calscale,aroundsixAerosolRoboticNetwork(AERONET)
study area includes the northern tropical Atlantic and the sites,closeorfarfromthedustsources,asimilarsatisfactory Discussions
northwestArabianSea,bothcharacterisedbystrong,regular agreementisfoundbetweenIASIandthevisibOleAODsand GeoscientificO
p p
dust events. The method developed relies on the construc- the differences between thGeseeporosdcuicetsnatriefischoewn and anal- e
n n
tionofLook-Up-Tablescomputedforalargeselectionofat- ysed.IASI-retrieveddustlayermeanaltitudes Aalsocompare Model Development A
Model Developmentcc cc
mosphericsituationsandobservingconditions.Ataregional wellwiththeCloud-AerosolLidarwithOrthogoesnalPolariza- Discussionses
s s
scale, a good agreement is found between IASI-retrieved tion (CALIOP/CALIPSO) aerosol mean layer altitude, both
10µm AOD and total visible optical depth at 550nm from in terms of climatology and of zonal evolution throughout
either the Moderate resolution Imaging Spectroradiometer the Atlantic. ComparisoHnsybdertwoeleongthye aIAnSdI- rOetrieved dust Hydrology and O
p p
(MODIS/Aqua or Terra), or the Multi-angle Imaging Spec- coarsemodeeffectiveradEiuasarntdhr eStryiesvatlesmfromen AAERONET Earth Systemen A
troRadiometer (MISR), or the Polarization and Anisotropy atthesixsitesbringsintoevidenceanalmostsycstematicbias c
c c
of Reflectances for Atmospheric Science coupled with Ob- of about +0.35µm (IASI-AEROSNcEiTe)n. Rceemsovesing this bias Scienceses
s s
servations from a Lidar (PARASOL). Taking into account leads to a satisfactory agreement between the climatologies
Discussions
the ratio existing between infrared and visible AODs, the ofthesetwoproducts.Overall,theseresultsillustratethedust
O O
diversity between the different 550nm AODs is similar to westward transport characterised by a fast depcrease of the p
e Ocean Sciencee
the difference between these and the IASI AODs. The in- dust optical depth, a sOomcewehaant s lSowceiredneccreeasn Ae of the alti- n A
fraredAODtovisibleAODratio,partlyreflectingthevary- tude,andaneffectiveradiusremainingalmostccconstantdur- Discussionscc
e e
ing distribution of the dust layer between the dust coarse ing summer throughout the northern tropical Astlantic. They s
s s
mode particles seen by IASI, and the fine mode seen by also demonstrate the capability of high resolution infrared
the other instruments, is found to vary with the region ob- sounders to contribute improving our understanding of pro-
O O
served with values close to already published values. Com- cesses related to the aerosols (transport, sourceps, cycles, ef- p
e Solid Earthe
n n
parisonsbetweentheclimatologiesofthe10µmIASIAOD fectofaerosolsontheterrestrSiaolrlaiddia Etioan,retthc.) A. A
and of the PARASOL non-spherical coarse mode AOD at cc Discussionscc
e e
s s
s s
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion. O O
p p
e The Cryospheree
n n
The Cryosphere A A
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c Discussionsc
e e
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s s
6066 S.Peyridieuetal.: Characterisationofdustaerosolsintheinfrared
1 Introduction bothproperties.Retrievalofthedustcoarsemodeaerosolef-
fective radius, a variable of prime importance for the study
Aerosols originate either from natural sources (dust, vol- of dust radiative forcing, transport and deposition, uses a
canic,seasaltaerosols,etc.)orfromanthropogenicsources channel at about 9.3µm (Pierangelo et al., 2005) selected
(smoke,sulfates,soot,etc.).Initsfourthreport,theIntergov- for its high sensitivity to dust size and relative insensitivity
ernmentalPanelonClimateChange(IPCC,2007)notesthat, tootheratmosphericcomponents(ozone,sulfates)ortodust
ifaerosolforcingsarenowbetterunderstoodthanatthetime asphericity.
of the Third Assessment Report due to improved measure- This method is applied here to Infrared Atmospheric
ments and more comprehensive modelling, they remain the Sounder Interferometer (IASI) observations for the period
dominant uncertainty in radiative forcing, on the one hand fromJuly2007toDecember2011.ComparedtoAIRS,IASI
because they show a very high spatio-temporal variability, hastheadvantageofahigherspectralresolution(0.50cm−1
ontheotherbecausetheyinfluencemanycomplexprocesses apodized), being constant throughout the whole spectrum,
oftheplanet.Amongstallaerosolspecies,mineraldustisa andofafullcoverageofthespectraldomain,allowingthese-
major contributor to total aerosol loading and has been the lectionofchannelsmorespecifictothevariableconsidered.
subjectofanincreasingnumberofstudies(e.g.Maheretal., Itsdrawbackisalargernoiseatshortwavelengthswhichmay
2010; Mahowald et al., 2010; Formenti et al., 2011; Shao beovercomebyaveragingafewadjacentandradiativelysim-
et al., 2011; Knippertz et al., 2012). Remote sensing in the ilarchannels.
visible domain has been widely used to obtain better char- Data and method are described in Sect. 2. Section 3
acterisation of these particles and their effect on solar radi- presents the results from IASI and comparisons with other
ation. After a long period of relative disinterest in aerosol aerosol products. For the AOD, these products are derived
remote sensing in the infrared (one of the oldest reference from the Moderate Resolution Imaging Spectro-radiometer
is by Legrand et al. (1989) using the low spectral resolu- (MODIS/Aqua and Terra), from the Multi-angle Imag-
tion, one window-channel, radiometer on board Meteosat), ing SpectroRadiometer (MISR), from the Polarization and
a marked growing interest in the infrared is now observed AnisotropyofReflectancesforAtmosphericSciencecoupled
with the emergence of hyperspectral instruments as AIRS with Observations from a Lidar (PARASOL), and from the
andIASI(Pierangeloetal.,2004,2005;DeSouza-Machado, AErosol RObotic NETwork (AERONET). For the altitude,
2006; Klu¨ser et al., 2011, 2012). Indeed, from infrared re- comparisonsaredonewiththeCloud-AerosolLidarwithOr-
motesensingacoherentensembleofaerosolcharacteristics thogonal Polarization (CALIOP/CALIPSO). Finally, the ra-
can be retrieved simultaneously: AOD, mean altitude and diusiscomparedwithAERONETcoarsemoderadius.These
size; this allows the impact of aerosols on terrestrial radia- resultsarediscussedinSect.4.
tion to be quantified. Moreover, infrared and visible spectra
arenotsensitivetothesamerangesofparticlesize:thecoarse
mode(sizerange>1µm)ispreferentiallyobservedinthein- 2 Methodanddata
frared,whereasthefinemode(<1µm)ismostlyobservedin
thevisible. 2.1 Method
The new generation of high spectral resolution infrared
sounders,suchastheAtmosphericInfraredSounder(AIRS) The method for the retrieval of dust characteristics from
on Aqua, or the Infrared Atmospheric Sounder Interferom- IASI observations was originally developed for application
eter (IASI) on Metop, have already shown promising capa- to AIRS (Pierangelo et al., 2004). The present study uses a
bilities to retrieve dust aerosol properties such as the 10µm slightly modified version of this method already described
optical depth, the mean altitude, and the mean coarse mode in detail in Pey2010. It is a three-step algorithm based on
particle size (Pierangelo et al., 2004, 2005; Peyridieu et al., a“Look-Up-Table”(LUT)approach.Thetwofirststeps,de-
2010,hereafterreferredtoas“Pey2010”). terminationoftheatmosphericstateobserved(usingsixIASI
In this study, dust aerosol characteristics are retrieved channels)andsimultaneousdeterminationofthe10µmAOD
fromamethodbasedonaLook-Up-Table(LUT)approach, andaerosollayermeanaltitude(usingeightIASIchannels),
already applied to more than 9yr of AIRS observations havebeendescribedinPey2010.Thethirdstepofthealgo-
(Pey2010), with results being obtained at a space-time res- rithmistheretrievalofthedustcoarsemodeeffectiveradius
olutionof1degree-1month.Twospectralbandsareofpar- (R ):theapproachfollowstheonedescribedinPierangelo
eff
ticular interest for the retrieval of aerosol properties in the et al. (2005). Here, it must be recalled that in the thermal
infrared:between8and12microns,wherechannelsaresen- infrared, at the reference 10µm wavelength, the number of
sitive to both the dust optical depth and the altitude of the parameters describing the Particle Size Distribution (PSD)
dustlayer,andaround4microns,wherechannelsareessen- can be reduced to one: the effective radius R (Pierangelo
eff
tially sensitive to the dust optical depth (Pierangelo et al., et al., 2005). In fact, the PSD may be considered as being
2004; Pey2010: see their Fig. 1). Combining channels from monomodal and the impact of its width is negligible once
these two spectral regions allows simultaneous retrieval of the AOD is given. “Radius-LUT” are built for a channel
Atmos. Chem. Phys.,13,6065–6082,2013 www.atmos-chem-phys.net/13/6065/2013/
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S.Peyr2i dieu etal.: Characterisationofdustaerosolsintheinfrared 6067
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Fig.1a.4I AS1IF- riegtru iervee 1da1. 0IAµmSIA-OreDtricelvimeadt o1l0og µy.mA vAeOragDe cislipmeraftoorlmoegdyo. vAerve4ryargfeo risJ apneurafroyrtmoeJudn oevaenrd 4o vyeera5rsy rfofrro mJulytoDecember.
5 2J anu ary to June and over 5 years from July to December.
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Fig.1b.2007–2011averagedzonalevolutionoftheIASIAOD(top-left),theMODIS(Terra)AOD(topright),theMISRAOD(bottomleft),
28 Figure 1b. 2007-2011 averaged zonal evolution of the IASI AOD (top-left), the MODIS (Terra)
andthePARASOLAOD(bottom-right),fromtheAfricancoasttotheCaribbean,inJune(blue),July(red),andAugust(green).Dataare
29 AOD (top-right), the MISR AOD (bottom-left), and the PARASOL AOD (bottom-right), from the
averagedoverthelatitudeband10◦N–30◦Nandover±3◦longitude,aroundthelongitudeconsidered.Blacksolidline:numberofitemsfor
30 African coast to the Caribbean, in June (blue), July (red), and August (green). Data are averaged
July(rightordinate).
31 over the latitude band 10N-30N and over ±3° longitude, around the longitude considered. Black
32 solid line: number of items for July (right ordinate).
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40 Figure 2. Boundaries of the four regions of study: EA: East Atlantic, MA: Mid-Atlantic, WA:
41 West Atlantic, AS: Arabian Sea. Blue stars indicate the locations of the AERONET sites
42 considered.
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6068 S.Peyridieuetal.: Characterisationofdustaerosolsintheinfrared
Table1.EntriesusedtobuildtheIASILook-Up-Tables(LUT)ofsimulatedbrightnesstemperatures(BTs)usedinthethree-stepalgorithm.
BTsforbold-facedvaluesarecalculatedusingtheforwardcoupledradiativetransfermodel4AOP-DISORT,BTsfortheothervaluesare
interpolated(linearlyorquadratically).
step1:LUT– step2:LUT– step3:LUT–
Atmosphere AOD,altitude effectiveradius
NumberofIASIchannels 6 8 1
◦
Angles( ) 0,5,10,15,20,25, 0,5,10,15,20,25,30,35, 0,5,10,15,20,25,30
30,35,40,45,50 40,45,50
AODvalues N/A 0.0,0.1,0.2,0.3,0.4, 0.1,0.2,0.3,0.4,
0.5,0.6,0.7,0.8 0.5,0.6,0.7,0.8
Altitudevalues(m) N/A 757,1258,1756,2411, 757,1258,1756,2411,
3254,4116,4965,5795 3254,4116,4965,5795
Radiivalues(µm) N/A 2.3(Reff)or3.0(rmodV) 0.5,0.8,1.0,1.5,2.0,
OPAC/MITR 2.5,3.0,4.0,5.0
selected at 9.315µm and for the values of the dust infrared 2.2 Data
optical depth, mean altitude and R of Table 1, using a
eff
monomodallognormalsizedistribution.Inordertosimulate In addition to IASI, the core instrument of this study, sev-
IASI brightness temperatures (BT) for the different effec- eralotherspace-basedinstrumentsandtheiraerosolproducts
tiveradii,theopticalpropertieswerecalculatedusingaMie have been used for the purpose of comparison and valida-
code (Mishchenko et al., 2002 and ftp://ftp.giss.nasa.gov/ tion: MODIS, MISR and PARASOL, all measuring visible
pub/crmim/spher.f)andtherefractiveindicesofthe“Mineral AOD and, therefore essentially sensitive to the fine mode.
Transported”(MITR)aerosolmodelfromthe“OpticalProp- CALIOP, which is also part of the A-train, provides a mea-
ertiesofCloudsandAerosols”(OPAC)database(Hessetal., sureoftheaerosolverticaldistribution.
1998).Forthismodel,thecomplexrefractiveindexat550nm Developed by CNES in collaboration with EUMETSAT,
is1.53+0.0055iandthemoderadiusis0.5µm.Usingtheat- the IASI instrument (Chalon et al., 2001; http://smsc.cnes.
mospheric and aerosol properties (AOD, altitude) retrieved fr/IASI), onboard the MetOp-A polar platform, is a Fourier
fromthetwofirststeps,theobservedBToftheIASIradius- Transform Spectrometer that measures Earth-emitted in-
sensitivechannelselectediscomparedtotheLUT-simulated frared radiation. Launched in October 2006 (operational
BTs for all sampled R values and the two values framing sinceJuly2007),itprovides8461spectralchannels,between
eff
the observed BT are kept. The retrieved R value results 15.5µmand3.63µm,withaspectralresolutionof0.50cm−1
eff
fromalinearinterpolationbetweentheirassociatedR val- afterapodisationandaregularspectralsamplingintervalof
eff
ues. 0.25cm−1. Metop-A crosses the Equator at 9:30p.m. on its
Entries used to build the IASI simulated brightness tem- ascendingnode.IASIprovidesanearglobalcoveragetwice
perature LUTs used in the three-step algorithm are given adayataspatialresolutionof12kmatnadir.
in Table 1. In this Table, the entries in bold face corre- MODIS (http://modis.gsfc.nasa.gov/) is an imaging ra-
spondtoLUTsBTscalculatedusingtheforwardcoupledra- diometer flying on board the Terra platform since Decem-
diative transfer model 4A/OP-DISORT (Scott and Che´din, ber 1999 and on board the Aqua platform since May 2002.
1981; Stamnes et al., 1988, http://4aop.noveltis.com). The Its data products are widely used in the aerosol community
other entries correspond to LUTs BTs interpolated linearly (Tanre´ etal.,1997;Remeretal.,2005,2008).Aqua(Terra)
orquadratically. crossestheEquatorat01:30a.m.(10:30a.m.)onitsdescend-
Several aspects of the retrieval algorithm such as the ro- ing node. Aqua (Terra) crosses the Equator at 01:30a.m.
bustness to aerosol model (size distribution, shape, and re- (10:30a.m.)onitsdescendingnode.Thistemporalshiftwith
fractive index), the possible contamination by other aerosol IASI is not significant at the monthly scale. Here, we use
species, the radiative transfer model bias removal, or the the Level 3 monthly products (total and fine mode) from
cloud mask including discrimination between clouds and Terra, which provide the optical depth reported at 550nm
aerosols, etc., were investigated in Pierangelo et al. (2004) ataspatialresolutionof1◦×1◦(see:http://disc.sci.gsfc.nasa.
and Pey2010. Most of the methodological work developed gov/giovanni/overview/index.html).MODISfinemodeAOD
here has consisted in the adaptation to IASI of the AIRS is obtained by multiplying the total AOD by the fine mode
channelsselectionprocedure. fraction.Wehaveverifiedthatnosignificantdifferencesare
observed between Terra and Aqua, at least at the monthly
scaleconsideredhere.
Atmos. Chem. Phys.,13,6065–6082,2013 www.atmos-chem-phys.net/13/6065/2013/
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28 Figure 1b. 2007-2011 averaged zonal evolution of the IASI AOD (top-left), the MODIS (Terra)
29 AOD (top-right), the MISR AOD (bottom-left), and the PARASOL AOD (bottom-right), from the
30 African coast to the Caribbean, in June (blue), July (red), and August (green). Data are averaged
31 over the latitude band 10N-30N and over ±3° longitude, around the longitude considered. Black
32 solid line: number of items for July (right ordinate).
33
S.Peyridieuetal.: Characterisationofdustaerosolsinthe3i4n fr ared 6069
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MISR (http://www-misr.jpl.nasa.gov; Diner et al., 2002),
flying on board the Terra platform since December 1999,
viewsthesunlitEarthsimultaneouslyatninewidelyspaced
angles.TheMISRLevel3globaldataproductsarereported
onarectangulargrid0.5◦ by0.5◦ degree.Here,useismade
oftheAODreportedat555nm.
Launched in December 2004, PARASOL carries a 38
39
POLDER-like(PolarizationandDirectionalityoftheEarth’s 40 FFiiggu.re2 2.. BBoounudnardieas roife thse ofofurt hreegiofnos uofr srtuedgy:i oEAn:s Eoasft Asttluandtiyc,: MEAA: M:iEd-AastltanAtict, lWanAt: ic,
41 West Atlantic, AS: Arabian Sea. Blue stars indicate the locations of the AERONET sites
Reflectances) wide field radiometer. The instrument (De- 42 McoAnsi:deMredi.d -Atlantic,WA:WestAtlantic,AS:ArabianSea.Bluestars
schamps et al., 1994) measures the polarized light in dif- 4434 i ndicatethelocationsoftheAERONETsitesconsidered.
ferentdirectionsforretrievingaerosolandcloudopticaland 25
physical properties (Tanre´ et al., 2011). The mission takes
3 Results
also advantage of the other instruments in the A-Train for-
mation.OriginalAODvaluesaregivenat865nmintheof-
The period analysed extends from the beginning of IASI
ficialPARASOLarchiveprovidedbytheICAREdatacenter
operational activity (July 2007) to December 2011. Results
(http://www.icare.univ-lille1.fr/).Theyareherereferencedto
are presented for the northern tropics of the Atlantic Ocean
550nm using the Angstro¨m exponent (865–670) from the
andnorthwestArabianSea.Onlynight-timeobservationsare
Level 2 products (orbits) of the PARASOL archive, then
analysed because of the use of shortwave channels signifi-
combined into global, monthly products, gridded at a reso-
cantlysensitivetosunradiation.
lutionof0.25degree.Useisalsomadeofthenon-spherical
coarsemode(NSCM)monthlymeanAODat865nm. 3.1 IASI10µmopticaldepth1◦×1◦climatology
Launched in April 2006, the satellite CALIPSO with
the on board two-wavelength depolarization lidar CALIOP Figure 1a shows IASI-retrieved monthly climatology (1◦×
permits an accurate determination of the aerosol altitude 1◦) of the dust aerosol mean 10µm optical depth for the
(Winker et al., 2007, 2009; Kim et al., 2008). The depo- period 07/2007–12/2011. Blank areas correspond to cloudy
larization at 532nm allows discriminating between the dust conditions or no retrievals. This figure clearly shows dust
aerosolsandothertypesofaerosolswhichingeneraldonot transport, through the so-called Saharan Air Layer (SAL),
depolarize light. It should be noted that CALIPSO, a near- fromtheSaharansourcestotheCaribbeanandNorthAmer-
nadir viewing instrument, does not offer a full coverage of icainsummerandtotheAmazonbasininwinterasalready
the Earth (beam diameter of 70m at the Earth’s surface, 16 described in several publications (see, for example, Chia-
daysrepetitioncycle)whichmakesitdifficulttostatistically pello et al., 1995, 2005; Karyampudi et al., 1999; Kaufman
interprettheresult.Here,theL25kmaerosollayerproduct etal.,2005;Huangetal.,2010).Notethe“clean”periodin
(version3.01)isusedtocalculatemonthlymeanaltitudesat November–December as shown in Ben-Ami et al. (2012).
regionalscale.Twoclassesofaerosolsareusedforthecom- Figure 1a is similar to the AIRS climatology of Pey2010
parisonoftheIASIaltitudewiththeL2product:“dust”and as well as to the MODIS 550nm AOD climatology (not
“polluted dust”. The polluted dust class, which corresponds shown). Figure 1a shows a fast westward decrease of the
todustmixedwithothertypesofaerosols,asbiomassburn- AODmoreclearlyseenonFig.1bwhichdisplaysthe2007–
ing or urban pollution, is also taken into account. For one 2011averaged zonalevolution ofthe AODfrom IASI(top-
CALIOPmeasurement,uptoeightaerosollayerscanbere- left), and of the total AOD from MODIS(Terra) (top-right),
ported. With the purpose of a simple and robust evaluation MISR(bottom-left),andPARASOL(bottom-right),fromthe
ofIASIretrievals,onlythecaseswhereoneaerosollayeris AfricancoasttotheCaribbean,inJune,July,andAugust,for
detected and measured by the lidar are selected. The sub- the latitude band 10◦N–30◦N (±3◦ longitude, 1◦ moving
set of CALIOP data resulting from this selection represents average).Allproductsshowasimilarfastwestwarddecrease
about65%ofallCALIOPdatawhereaerosollayersarede- up to 50◦W and then remaining approximately constant,
tected(seePey2010formoredetails).ContrarytoCALIOP, with differences of the same order of magnitude between
theIASIdustretrievalalgorithmoutputsthemeanaltitudeof 0.55µmAODsasbetweenIASIandtheotherthreeproducts
thewholedustlayer,i.e.thealtitudeatwhichhalfoftheAOD (apartfromtheratioobservedbetweenIASI10µmAODand
is belowand half ofthe AOD isabove, with lowsensitivity the0.55µmAODsanddiscussedinthenextsection).
toacomplexlayeringofthedust.
Finally, IASI-retrieved 10µm AOD and coarse mode ef- 3.2 Comparison between IASI 10µm AOD and visible
fectiveradiusarecomparedtoAERONET(see,forexample, MODIS, MISR, and PARASOL AODs at regional
Duboviketal.,2002)visibleAOD(550nm)andcoarsemode scale
radius for several sites of the northern tropical Atlantic and
onesiteoftheArabianSea. ForeachofthefourregionsillustratedinFig.2,East-Atlantic
(EA), Mid-Atlantic(MA), West Atlantic (WA)and Arabian
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6070 S.Peyridieuetal.: Characterisationofdustaerosolsintheinfrared
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Fig.3.ClimatologyoftheIASIAOD(10µm,red,thiswork),andclimatologiesoftheAODofMODIS-Terra(total:dashedblue;finemode:
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dottedblue2)7,M ISFRig(utoreta 3l:. gCreliemn)a,taonlodgPyA oRf AthSeO ILAS(tIo tAalO:dDa s(h1e0d µgmre,y r;efidn, ethmiso wdeo:rpku),r palned) fcolirmeaactohloreggiieosn oof fthsteu dAyO.ADl lvisibleAODsarescaled
bytheemp2i8ri calloyfd MetOerDmIiSne-TdeIrRra/V (tIoStarla:t idoasshhoewd nbloune;e faicnhe gmraopdhe:( dtoopttleedf tbllaubee)l,) M.AIvSeRra (gtoetoavl:e grr4ee(nJa),n uanardy PtAoRJuAnSe)OtLo 5(JulytoDecember)
years. 29 (total: dashed grey; fine mode: purple) for each region of study. All visible AODs are scaled by
30 the empirically determined IR/VIS ratio shown on each graph (top-left label). Average over 4
31 (January to June) to 5 (July to December) years.
Sea (AS),32F igu re 3 shows climatologies of the IASI 10µm oftheconcentrationoffinetocoarsemode,followedbythe
AOD, the33M OD IS-Terra, total and fine modes AODs, the effective radius of the coarse mode have the largest influ-
MISR tot3a4l AO D , and the PARASOL total and fine modes ence. Given the range of values obtained from AERONET
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AODs,fortheperiodJuly2007toDecember2011(January– retrievals (http://aeronet.gsfc.nasa.gov) during the IASI pe-
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June:4-yraverage;July–December:5-yraverage).Itisworth riodonseveralsites,wefoundanexpectedIR/VISratioly-
pointing out that, for the sake of an easier comparison, all ingbetween0.3and0.6,withhighervaluescorrespondingto
visible AODs are scaled by the factor shown on each graph smaller fine mode concentrations. These values are consis-
(top-leftlabel)ofthisfigure.Thisscalingfactoractuallycor- tentwiththeratiosempiricallyfoundhere.Ingeneral,IASI
responds to the infrared (10µm) to visible (0.55µm) AOD compareswellwiththesescaledtotalvisibleAODclimatolo-
ratio, determined here empirically, so as to ensure the best giesbutsignificantdifferencesarealsoobserved.
match between the IR and the various visible AODs, tak- ForregionEA,closetothedustsources,IASIAODwell
ing into account both their climatology and their time se- matches the shape of the MODIS, MISR, and PARASOL
ries. A physical evaluation of this ratio is not straightfor- AODs.TheIR/VISratiois0.45.
ward because of its strong dependence on the microphysi- For region MA, farther from the sources, the agreement
cal prope3rt7ie s of the aerosol considered. It can be obtained is still good with even smaller di fferences than for the EA
by a Lore3n8z -MFieigucrael c4u. lCaltiimona,togloivgeiens oasf tihnep MutOtDheIS rAefOraDc,t itvoetal (solriedg biolune.) Tanhde fIiRne/V mIoSdrea (tdiootitsed0 .b4l2u.e), left
39 ordinate, and of the MODIS Angström exponent (865-550 nm) (green, right ordinate), for the two
index and a size distribution (see Sect. 2.1). For a bimodal Region WA shows an IASI AOD season lagging behind
40 regions EA and AS.
lognormal volumic size distribution, that is supposed to be theMODIS(orMISRandPARASOL)dustseason.Thisbe-
41
the more appropriate, it requires knowing its modal volu- haviour had already been pointed out for AIRS in Pey2010
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mic radii,43w idt hs, and volume concentrations for both fine who explained it by a late arrival of the dust coarse mode
and coarse modes for the situation considered (Mishchenko compared to the fine mode. This late arrival was confirmed
26
etal.,2002;Duboviketal.,2002).Adirectcomparisonbe- bybothAERONET0.55µmAODandAngstro¨mexponentat
tween infrared and visible AODs is thus difficult. However, LaParguera(18◦N,67◦W)station,showingafasttransition
thecomparisonofbothproductsbringsinformationondust between the two aerosol modes in May/June (see Pey2010
size distribution, in particular, on the relative abundance of for more details). Note that for regions MA and WA, the
fineversuscoarsemodeparticles.Sensitivitytestshavebeen MODISfinemodeAODappearssignificantlylargerthanthat
performedinordertodeterminewhichvariablesarethemain fromPARASOL.TheIR/VISratiois0.40.
contributors to the IR/VIS ratio. Results show that the ratio
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27 Figure 3. Climatology of the IASI AOD (10 µm, red, this work), and climatologies of the AOD
28 of MODIS-Terra (total: dashed blue; fine mode: dotted blue), MISR (total: green), and PARASOL
29 (total: dashed grey; fine mode: purple) for each region of study. All visible AODs are scaled by
30 the empirically determined IR/VIS ratio shown on each graph (top-left label). Average over 4
31 (January to June) to 5 (July to December) years.
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S.Peyrid3ie3u et al.: Characterisationofdustaerosolsintheinfrared 6071
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Fig.4.ClimatologiesoftheMODISAOD,total(solidblue)andfinemode(dottedblue),leftordinate,andoftheMODISAngstro¨mexponent
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(865–550nm)(green,rightordinate),forthetworegionsEAandAS.
38 Figure 4. Climatologies of the MODIS AOD, total (solid blue) and fine mode (dotted blue), left
39 ordinate, and of the MODIS Angström exponent (865-550 nm) (green, right ordinate), for the two
40 regions EA and AS.
For reg4i1o n A S, covering the Arabian Sea, south of the modes (and, obviously, the difference between the observa-
Arabian P42en ins ula, the situation is more complex. Signifi- tionwavelengths).
cantdiffe4re3n ces areobservedbetweenthevariousproductsin We also compared the 10µm IASI AOD with the NSCM
June–Julyand, inAugust–September,IASIdisplaysslightly26 AODat865nmderivedfromPARASOL,bothbeingspecific
larger AODs. Differences are also seen for the four months ofthedustparticlecoarsemode.ThesetwoAODclimatolo-
October–January, with significantly lower IASI AODs. The gies are shown in Fig. 5 together with that of the MODIS
IR/VIS ratio is 0.31. These differences can partly be ex- Angstro¨mexponent(865–550nm).Toapproximatelymatch
plainedbythedifferentbehaviouroftheMODISAngstro¨m the IASI AOD, the NSCM AOD has been scaled by a ra-
exponent (865–550nm) shown on Fig. 4 together with the tio,actuallytheIR/VIScoarsemoderatio,choseninsucha
climatologies of the MODIS total and fine modes AODs way that the best agreement is obtained around the peak of
for the two regions EA and AS, closest to source regions. thedustseason.Indeed,morecoarsemodeisexpectedtobe
WhileEAregionshowsarelativelyflatAngstro¨mexponent, presentatthistimeoftheyear.Thisratiocorrespondstothe
withmaximainSeptember–NovemberandminimainJuly– only coarse mode and can be estimated theoretically using
August, the AS region shows a pronounced low (∼0.3 in- thesamerefractiveindexthanpreviouslyandamonomodal
stead of ∼0.4) of the Angstro¨m exponent in June–August lognormaldistribution.Thistime,thevariablesthathavethe
and high values (>0.9) in October–January. This indicates strongestimpactonthisratioaretheeffectiveradiusandthe
asummerpeakperiodlargelydominatedbythecoarsemode standarddeviationofthedistribution.Sucharatiodecreases
(Smirnov et al., 2002; Eck et al., 2008), seemingly more in withtheeffectiveradiusandincreaseswiththewidthofthe
phase with the flat PARASOL fine mode AOD than with distribution. For typical values from AERONET, the ratio
the MODIS fine mode peak of June–July (Figs. 3 and 4). 10µm IASI AOD/ 865nm PARASOL NSCM AOD is ex-
Moreover, the high Angstro¨m exponent values of the four pected to lie between 0.7 and 0.9. These values are higher
months October–January coincide with the low IASI AOD than the one found here, which would imply, if IASI actu-
values for this region (Fig. 3) indicating the dominating in- allyretrievesthesamecoarsemodeastheoneobtainedfrom
fluence of the fine mode almost not seen by IASI. For the PARASOL observations, an underestimation of the IASI
sevenfirstmonths,IASIappearsmoreinphasewithMISR. AOD.Ontheotherhand,thePARASOLNSCMAODcould
For IASI and for this region, however, there may be some also be slightly overestimated. An analysis of the monthly
doubtsabouttheuseoftheMITRaerosolmodel,inparticu- ratio 865nm PARASOL NSCM AOD/ 865nm PARASOL
lar, because of the presence observed of carbonate minerals total AOD leads to values sometimes above 0.9 and even
fromdepositsalongtheOman/YemenandUnitedArabEmi- 1.0, higher than the values theoretically expected. Indeed,
ratescoasts(Reidetal.,2005,2008)orthepossiblemixing estimating the coarse mode contribution to the total AOD
ofdustwithanthropogenicpollutionaerosolseitherfromthe at865nm(followingPierangelo,2013)fortypicalvaluesof
petroleumindustryintheArabianpeninsulaorfromIndia. thesizedistributionparameterstakenfromAERONET,leads
TheIR/VISratiosshownonFig.3forregionsEA(∼0.45), to values lower than 0.85. Nevertheless, with a ratio IASI/
MA (∼0.42), and WA (∼0.40) are consistent with the ex- 865nm PARASOL NSCM of 0.65 for the EA and the MA
pectedrangeofratiosobtainedfromAERONETresults,and regions, a relatively good agreement is found between the
are in reasonable agreement with values from Highwood et two products with a slightly faster start of the IASI season.
al. (2003) (see also Pierangelo et al., 2004). For region AS, Forbothregionsandbothproducts,therapiddecreaseofthe
the ratio comes to ∼0.31, much smaller than for the other AODinAugust–Septembercoincideswiththerapidincrease
regions.Theseratiosreflect,atleastinpart,thevaryingdis- of the Angstro¨m exponent. With a ratio IASI/PARASOL
tribution of the dust layer between the coarse and the fine NSCM of 0.55 (resp. 0.40) for the WA (resp. AS) region,
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6072 S.Peyridieuetal.: Characterisationofdustaerosolsintheinfrared
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Fig. 5. IAS2I81 0µm AOD climatology (red, left ordinate) and PARASOL non-spherical coarse mode (CMo) AOD at 865nm climatology
(scaledbyth2e9 empFiirgicuarlely 5d. eItAerSmI i1n0ed µImR /VAIOSDra ctiloimshaotowlongoyn (eraecdh, lgerfatp ohr:dtionpa-tlee)f talnadb ePl)AfRorAtSheOfLo unrorne-gsipohnesroicfaslt ucdoyar(sger ey,leftordinate);cli-
matologyof3t0h eMmOoDdIeS (ACnMgsot)r o¨AmOeDxp oatn e8n6t5( 8n6m5– 5cl5i0mnamto)lo(ggrye e(nsc,arilgehdt boyrd itnhaet ee)m.Apvireircaaglleyo dveetre4rm(Jianneuda rIyRt/oVJIuSn rea)ttioo 5 (JulytoDecember)
years. 31 shown on each graph: top-left label) for the four regions of study (grey, left ordinate); climatology
32 of the MODIS Angström exponent (865-550 nm) (green, right ordinate). Average over 4 (January
33 to June) to 5 (July to December) years.
theagreem3e4n tbe tweenthetwoproductsissignificantlyless theirAODtimeseriesforthefourregionsofstudy(Fig.6).
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good,particularlyoutsidethepeakoftheseason.FortheWA Again, region WA well depicts the late arrival of the coarse
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region, however, both products confirm the late start of the moderelativelytothefinemode.Somediscrepanciesarealso
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dust coarse mode season and show the same sharp Spring found,forexample,inMarch–May2009forregionEA,orin
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transition i3n9 Ma y/June. A similar behaviour is observed for August–September2010forregionMA.
theASregi4o0n .In bothcases,thesharpdecreaseofIASIAOD
in August–4S1 epte mber occurs in phase with the increase of 3.3 ComparisonsbetweenIASI10µmAODandvisible
theAngstr4o¨2m ex ponent,butoccursonemonthsoonerforthe MODIS, MISR, PARASOL and AERONET AODs
PARASOL43N SC M AOD. This makes the IASI season one atsixAERONETsites
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monthlongerthanseenbyPARASOL,againmoreinphase
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ComparisonsbetweenIASI10µmAODandvisibleMODIS,
with the Angstro¨m exponent behaviour. In general, the ra-
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MISR,PARASOLandAERONETAODshavebeendoneat
tio IASI/PA47R AS OL NSCM is smaller to what is expected
(∼0.5,with48th ee xceptionofTenerife).Theimpactofthedust six AERONET sites, namely: Tenerife (La Laguna), Capo
fine mode4o9n the IASI AOD cannot explain this difference: Verde, Dakar, Barbados, La Parguera, all in the Atlantic re-
for dust w5it0h a typical bimodal distribution, the fine mode gions,andKarachi,intheASregion(sitesindicatedbyastar
51 on Fig. 2). Figure 7 shows climatologies of the IASI AOD,
contribution to the total optical depth at 10µm is usually of
52 of the visible AOD from MODIS-Terra (total), MISR (to-
the order of 10% (Pierangelo, 2013). Another explanation
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tal),PARASOL(totalandfinemode),andfromAERONET
couldcomefromthe(hypothetical)presenceofpolluteddust
(500nm AOD) at these sites (N.B.: for Capo Verde, the
(possiblyincludingthehygroscopicgrowthofthefinemode
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AERONET AOD is at 440nm as no 500nm AOD is avail-
particles, Smirnov et al., 2002) that present more spherical
able). Time average is made over 4 (January to June) to
particlesnotfullyaccountedforintheNSCMAOD.Thisre-
5 (July to December) years for the satellite products. For
sultcouldalsoindicatethattheuseoftheMITRmodelisnot
AERONETdata,weconsiderthatatleasttwoyearsmustbe
appropriateforalltheregions,andparticularlyforASdueto
reported for averaging. Space average is made over an area
theheterogeneityoftheaerosoltypesobservedinthisregion
±1.5◦ inlatitudeandlongitude,centredoneachsite;pixels
(Kaskaoutisetal.,2010).
flagged“land”areexcluded.Onthewhole,theagreementbe-
Finally, the interannual variability seen by the visible in-
tweenIASIandtheothersourcesofdataappearssatisfactory
struments is also well reproduced by IASI, as shown by
with,however,differencesfromonesitetoanother.
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S.Peyri1d ieu etal.: Characterisationofdustaerosolsintheinfrared 6073
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Fig.6.Ti3m ese riesofthe10µmIASIAOD(red),andoftheMODIS-Terra(blue),MISR(green),andPARASOL(grey),total550nmAODs,
fortheEA4 regiFoing(utorpe l6e.f tT),itmheeM seArireesg ioofn t(htoep 1ri0g hµt)m,t hIAeWSIA AreOgiDon ((rbeodt)to ,m anledft )o,ft hteheA SMrOegDioInS(-bToettrorma (ribglhute).)A, MllvISisRib leAODsarescaled
bytheem5p iric(agllryedeent)e,r maninde dPAIRR/VAISSOraLtio (sghroewy)n, otonteaalc 5h5g0ra npmh( tAopOlDefst,l afboerl )t.he EA region (top-left), the MA region
6 (top-right), the WA region (bottom-left), the AS region (bottom-right). All visible AODs are
7 scaled by the empirically determined IR/VIS ratio shown on each graph (top-left label).
For Tenerife (La Laguna), IASI and the visible (total) Figure8showstheclimatologiesofthe10µmIASIAOD,
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AOD climatologies display similar shapes with relatively of the PARASOL NSCM AOD at 865nm, scaled by the
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smalldifferences.Amorepronounceddifferenceisobserved IR/VIS ratio (label top-left on each graph) empirically esti-
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between AERONET and the other satellite data, as already mated, as in Fig. 5, to approximately match the IASI AOD
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reported by Kalashnikova and Kahn (2008) for the neigh- around the peak of the dust season, and of the MODIS
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bouringsiteofIzana,wherethisdifferenceisevenlarger.The Angstro¨mexponent.ForTenerife,theagreementisrelatively
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meteorologicalconditionscurrentlyprevailingatthesesites, goodwitharatioof0.95.AtCapoVerde,iftheagreementis
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marked by strong low level intrusions of cold air, may at satisfactoryfromJunetoDecember,IASIstartsmuchmore
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leastpartiallyexplainthisproblem.AtCapoVerde,theIASI rapidly than the 865nm PARASOL NSCM AOD does. It
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dust season starts more rapidly. At Dakar, IASI is more in may however be observed that the Spring sharp gradient of
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phasewithMISRandAERONET(severalmonthsaremiss- theAngstro¨mexponent,indicativeofachangeintheaerosol
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ing)thanwithMODISorPARASOLwhichpeakmonthsap- modes distribution, is more in phase with IASI than with
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peartwotothreemonthslater.Thereasonforthisdifference PARASOL.AlmostthesamesituationisobservedatDakar.
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isstillunclear.AtthesiteofBarbados,farfromthesources, TheIASIAOD,wellinphasewiththeAERONETorMISR
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IASIAODislargerthantheotherAODsinAugustandde- AODs(seeFig.7),peakstwomonthsearlierthanthePARA-
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creaseslessrapidly;atthissite,theAERONETAODissub- SOL NSCM AOD. The situation is different far from the
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stantially smaller than the other visible AODs, a result also sources (at Barbados and La Parguera) and in the Arabian
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observed by Kalashnikova and Kahn (2008) at the nearby sea(at Karachi) withIASIdust seasonssignificantlylonger
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siteofGuadeloup(16◦N,61◦W).AtLaParguera,theover- than the PARASOL NSCM ones, but again appearing more
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all agreement is satisfactory but, as for Barbados, the IASI inphasewiththeMODISAngstro¨mexponentgradients,par-
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seasondecreaseslessrapidlythanshownbytheotherprod- ticularlyatBarbados.AsfortheWAandASregions(Fig.5),
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ucts.Forbothsites,thePARASOLAODpeakissignificantly IASI AOD lies significantly above PARASOL outside the
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largert3h0a nth eothervisibleAODpeaks.IntheArabianSea summermonths.
region, at Karachi site, IASI AOD performs relatively well Alltogether,thesecomparisons,withdifferencesbetween
(again larger in August) in spite of our use of the MITR 28 IASIandtheotherproductsofthesameorderofmagnitude
modelmostprobablylessadaptedtothisregion. as between the other products themselves, demonstrate the
ability of IASI to work properly at small scale. They also
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