JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D2, PAGES 2261-2278, JANUARY 27, 1999
Retrieval of aerosol optical thickness and size distribution over ocean from the MODIS airborne simulator during TARFOX
D. Tanr6,1L. A. Remer, 2 Y. J. Kaufman, 2 S. Mattoo, 3 P.V. Hobbs, 4 J.M. Livingston, 5 P. B. Russell, 5 A. Smirnov 6 Abstract. Radiationand in-situmeasurements collectedduringthe TroposphericAerosol RadiativeForcingObservational Experiment(TARFOX) areusedto testthe methodfor remote sensingof aerosolpropertiesandloadingfrom theMODIS instrument. MODIS, a Moderate ResolutionImagingSpectroradiometer, will be launchedin 1999 aboardthe first EOS (Earth ObservingSystem).Followingthe MODIS procedure[Tanrd et al., 1997],the spectralradianceat the top of the atmosphere (TOA) measuredoverthe oceanin a wide spectralrange(0.55-2.13 !,tm) is usedto derivethe aerosolopticalthickness(proportionalto the aerosoltotalloading)andthe aerosolsizedistribution(integratedoverthe verticalcolumn)of the ambient(undisturbed)aerosol
bycomparing measured radiances withvalues in look-up table(LUT).TheLUT includes thegasphaseoxidationaccumulation mode,cloud-phase accumulation mode,anda coarsemodethat represents maritimeparticles(salt)anddust.In eachinversion,oneaccumulation andonecoarse modecanbe retrieved.The inversionretrievestheratioof thecontribution to the optical thicknesses of the two particlemodesandthe meanparticlesizethatbestfits the measurements. This algorithmis successfully appliedto the datasetsacquiredduringTARFOX. The MODIS _airborne simulator(MAS) aboardtheNASA ER-2 aircraftflew severaltimesduringthe experimentabovethe Universityof WashingtonC-131A researchaircrafton whichthe six-channel AmesAirborneTrackingSunPhotometer(AATS-6) wasmounted.It flew alsoabovesurfacebasedSunphotometers. Opticalthicknesses (at)• = 550 nm) aswell asthespectraldependence from the variousdatasetscomparevery well. 1. Introduction
assuming an aerosol model representative of the local
Interestin global aerosolmonitoringhas increasedrecently following the recognition that their contributions to atmosphericand Earth processesare very important. Since frequent and global-scale measurements of the Earth's atmosphereare achievableonly by observationsfrom space, efforts have been made to develop new satellite sensors and new strategiesfor retrieving key aerosolparameters[Kaufman et al., 1997]. Early studies [Griggs, 1975; Fraser, 1976; Mekler et al., 1977; Carlson, 1979; Koepke and Quenzel, 1979; Norton et
al., 1980] showedthe potentialof visible satellite imagery to
conditions [Jankowiak and Tanrd, 1992; Husar et al., 1997'
Moulin et al., 1997]. New techniquesusingthe long recordsof previous satellite data, like the UV channels of TOMS/Nimbus-7 instrument[Herman et al., 1997] and the two visible channelsof AVHRR/NOAA [Nakajima and Higurashi, 1997], are also being developed;they are expectedto better characterizethe aerosol componentthrough an estimate of their size (AngstrOmexponent)and absorptionproperties. With the new generation of satellite sensors, like MODIS/EOS [Salomonson, 1989], MISR/EOS [Diner et al., 1989], EOSP [Travis, 1993], and POLDER/ADEOS
[Deschampset al., 1994], more parameterscan be derived. Correspondinginversionschemesuse the spectraldependence channels on the initial instruments, one channel for GOES and of the multiple channelsatellite radiances[Tanrdet al., 1996; Meteosat and two channels for AVHRR/NOAA, these Stowe et al., 1997], the multiangle capability [Martonchick algorithms can derive only the total aerosol loading, and Diner, 1992], the polarized ratio of the reflected light [Mishchenko and Travis, 1997] or both [Deschampset al., •Laboratoire d'OptiqueAtmosph6rique,Villeneuved'Ascq, 1994]. Although more sophisticated,all of these new methods FRANCE. still require assumptionsand have their own limitations. 2Laboratory forAtmospheres, NASAGoddard Space FlightCenter, This paper focuseson the multi-wavelength approach to Greenbelt,Maryland. aerosol retrieval from remote sensing. The algorithm 3Science Application Corporation, Vienna, Virginia. developed for the Moderate Resolution Imaging 4University ofWashington, Seattle. Spectroradiometer(MODIS)is applied to the data acquiredby 5NASAAmesResearch Center,MoffettField,California. the M__ODISairborne simulator (MAS) [King et al., 1996]
detect aerosols over ocean. Because of the limited
number of
6Science Systems andApplications Inc.,Laboratory for Terrestrial aboard the NASA ER-2 aircraft during the Tropospheric
Physics,NASA GoddardSpaceFlight Center,Greenbelt,Maryland.
Copyright1999by the AmericanGeophysicalUnion.
Aerosol
Radiative
(TARFOX)
Forcing
Observational Experiment
[Russell et al., this issue (a)]. The TARFOX
Papernumber 1998JD200077.
campaign was conductedJuly 10-31, 1996, off the United States mid-Atlantic coast and was designed to reduce
0148-0227/99/1998JD200077509.00.
uncertainties 2261
in the aerosol
radiative
effects.
The retrieved
2262
TANR• ET AL.'RETRIEVALOFAEROSOL PROPERTIES OVERTHEOCEAN
aerosolpropertiesare testedagainst groundand airborne measurements providedby the Aerosol Robotic Network (AEReNE2)field networkof Sun-skyradiometers [Holbenet al., 1998; Rerneret al., this issue],andby the in situ and
the coarsemode(radius> 1.0 gm), the optical thickness, and
et al., this issue (b)].
measurements.
2. Descriptionof the Algorithm
combinationof two lognormal laws, which correspond to the accumulation(small) and coarse (large) modes.The physical
the mean particle size. A complete descriptionof the algorithmis given by Tanrdet al. [1997]. The algorithmis basedon a comparisonbetweenthe measurements and the precomputedradiances;the solution correspondsto the radiometric measurementsobtained aboard the University of Washington C-131Aaircraft[Hobbset al., this issue;Russell aerosol model within a look-up table that best fits the We assume that the aerosol size distribution
follows
a
TheMEDIS algorithm 'tested inthispaper uses thesolar- propertiesof the aerosolmodel(i.e., the particlerefractive reflected spectral radiances, normalized to a central index, the moderadius,and the width of the mode) are given in wavelength to derivethe aerosolsizedistribution; it thenuses Table 1. These characteristics have been derived from in situ the retrieved aerosol size distribution and the corresponding measurementsand are representative of several different phasefunctionfor an accurate derivation of the aerosoloptical aerosolregimes.Six valuesof the aerosolloading,expressed by theopticalthickness 'ca at •k= 550 nm are considered in the thickness.Even with the large spectralcoverageprovidedby MEDIS, only a limitednumberof the primaryparameters of look-uptable (LUT) ('ca= 0.0, 0.2, 0.5, 1.0, 2.0, and3.0). To the aerosol size distribution can be retrieved; the rest of the
describethe surface,we use the rough ocean model proposed
parameters haveto be assumed. A sensitivity study[Tanrdet al., 1997] showedthat the mainaerosolparameters, whichcanbe derived,arethe optical
by Coxand Munk [ 1954];for the Fresnelreflectionon the sea surface,threetypical valuesof wind speedareused:2.0, 6.0, and10.0 m s'•. The percentage of the seacoveredby foam followsKoepke's[1984] model,and the spectralreflectanceof
thicknessratio of the accumulationmode (radius< 0.5 gm) and
Table1. Description of Aerosol Models Usedin theLook-Up Table:Values of Real(mr)andImaginary (mi)Partsof the
Spectral Refractive Index, Median Radius (rg),Standard Deviation ({J),andEffective Particle Radius (reff) oftheLognormal Size Distributions
mr - i mi
)• = 470- > 860 nm
mr - i mi
)• = 1240 nm
mr - i mi
mr - i mi
)•=1640 nm
)•=2130 nm
rg
•J
reff
Comment
0.035
0.40
0.05
watersoluble
Smallparticles
1
1.45-0.0035i
1.45-0.0035i
1.43-0.01i
1.40-0.005i
type
2
1.45-0.0035i
1.45-0.0035i
1.43-0.01i
1.40-0.005i
0.07
0.40
0.10
water soluble type
3
1.45-0.0035i
1.45-0.0035i 1.43-0.01i
1.40-0.005i
0.06
0.60
0.15
water•oluble type
4
1.40-0.0020i
1.40-0.0020i
1.39-0.005i
1.36-0.003i
0.08
0.60
0.20
water soluble with humidity
5
1.40-0.0020i
1.40-0.0020i
1.39-0.005i
1.36-0.003i
0.10
0.60
0.25
water soluble
0.40
0.60
0.98
wet sea-salt
with humidity
Largeparticles
6
1.45-0.0035i
1.45-0.0035i
1.43-0.0035i
1.43-0.0035i
type
7
1.45-0.0035i
1.45-0.0035i
1.43-0.0035i
1.43-0.0035i
0.60
0.60
1.48
wet sea-salt type
8
1.45-0.0035i
1.45-0.0035i
1.43-0.0035i
1.43-0.0035i
0.80
0.60
1.98
wet sea-salt type
9
1.53-0.008i
1.46-0.008i
1.40-0.008i
1.22-0.009i
0.40
0.60
0.98
dust-liketype
10
1.53-0.008i
1.46-0.008i
1.40-0.008i
1.22-0.009i
0.50
0.80
2.50
dust-liketype
11
1.53-0.008i
1.46-0.008i
1.40-0.008i
1.22-0.009i
1.00
0.80
4.95
dust-liketype
Smallparticles correspond totheaccumulation (small) modeandlargeparticles tothecoarse (large) mode.
TANRI• ET AL.: RETRIEVALOF AEROSOLPROPERTIES OVERTHE OCEAN the foam is taken from Whitlock
et al. [1982].
Recent
measurements [Frouin et al., 1996] lead to a different spectral behavior
of the foam reflectance.
Since the use of these new
valueswouldrequireadjustments in the LUT, these adjustments are not done here, but we recognizethat this omission may result in additional uncertainties. Finally, the water-leaving radianceis 0.0 in all channelsexceptat X,= 555 nm where a constant value of 0.005 (expressedin reflectance units) is assumed[Gordon, 1997]. This assumptionis questionablein coastal regions. The inversion
scheme uses six channels between X, = 555
nm and 2130 nm. The blue MODIS channel (470 nm) is not consideredbecauseof the uncertainties of the water-leaving radiance which result from fluctuations in the chlorophyll content. We apply a version of the operational mask [Ackerman et al., 1998] which is based on threshold values in the visible and thermal infrared channels. In this study, we do not correct for ozone contamination
in channels centered at X,
= 555 and 659 nm but will do so in the satellite
version
of the
algorithm. This point is discussed in section 6. The water vapor(channelscenteredat X,= 865 and2130 nm) absorption
2263
of the track were separatedby 10 km. Figure 1 shows the flight patternon July 25, 1996. Looping aroundthe racetrack allowed us to look at the same target at different view angles and
also
to
monitor
the
time
evolution
of
the
derived
parameters.
The ER-2 was directedto fly either in the solar plane or in the plane perpendicularto the solar plane to avoid Sun glint. For low Sun elevations the ER-2 flew in the solar plane, scanningside to side since there was no probability of glint along the scan. For high Sun elevations the ER-2 flew in the perpendicularplane to obtain glint free measurementsin the backscatteredpart of the scan. The ER-2 was also directed to overtly the Meridian ship on its approachto New York on July 20. On July 26 it overflew groundinstrumentationon Bermuda Island as well as on the Meridian ship as it left this island. The ER-2 flew on nine daysin TARFOX (Table 2). Data from six flights are analyzedhere; they correspondto a large range of atmosphericconditions,from very clean on July 20 and 21 to very hazy on July 17 and 24 with intermediate conditions on July 25 and 26.
featuresare weak enoughto be neglected.Sincethe time period Validation Data Set of TARFOX was free of major volcanic effects, no 3.2. stratosphericcorrectionwas applied. To avoid measurements To validate the MAS optical thickness results, the sixcontaminatedby Sun glint, we rejecteddata acquiredwithin a channel Ames Airborne Tracking Sun Photometer (AATS-6) coneof 30ø half angle around the speculardirection. Since the aboardthe University of WashingtonC- 131A aircraft was used wind speedvalue was not available during the experiment, a [Matsumoto et al., 1987; Russell et al., this issue (b)]. AATS-
singlevalue(6.0 m s'l) is usedin thealgorithm. Theimpactof
this assumptionon the retrieval is also discussedin section6. The retrievedaerosolparameterswe examine are the optical
thickness('Ca)at )• = 550 nm, the effectiveradius(reft) of the size distribution, and the ratio rl between the two modes. The rl corresponds to ('cs/('cs+ 'c•))where'csand 'c• are respectively, the optical thicknessof the small (accumulation)and the large (coarse) modes at 550 nm. The general accuracy of the retrieved parameterswas estimatedin a previous paper to be
A'ca = _+0.05+_0.05'c a (at X, = 550 nm), Areff = 0.3reft, and Arl = _-/-0.25[Tanrd et al., 1997].
3. 3.1.
Data MAS/ER-2
Data
The MODIS airborne simulator (MAS) flew on the NASA ER-2 aircraft in TARFOX. The instrumentis describedby King et al. [1996]. It is a scanningspectrometerwith 50 channels
coveringthe rangeX,= 550 nm to 14,300 nm, but only the five channelscentered at X,= 549, 655, 867, 1643, and 2105 nm, equivalent to the MODIS/EOS channels [Salomonson et al., 1989], are used. The instrument scans in the across-track direction.The half scanangle is 42.96ø, which correspondsto
6 measured the solar direct-beam
transmission
in four aerosol
bands centered at X,= 380, 451, 525, and 1021 nm. Aerosol optical depthsand uncertaintieswere derivedfrom the AATS-6 measurements,as describedby Russell et al. [1993]. These data allow validation of the optical thickness derived at 550 nm from MAS and also the spectral dependencerelated to the aerosolsize distribution.To compare the MAS inversion with the C-131A measurements,the flight of the ER-2 was timed to be above the C-131A when it was at its lowest altitude, just above sea level. In this way the optical thicknessmeasuredby the AATS-6 looking upwardrepresentsthe sametotal column value retrievedfrom the MAS instrumentlooking downward. The retrievedoptical thicknesseswere also comparedwith surface-basedSun Photometer measurements[Remer et al., this issue]. Instruments were put on two ships cruising between New York City and the BermudaIslands. We also used a Sun Photometer of the AERONET program [Holben et al., 1998] locatedat the biological stationin the BermudaIslands. The MAS inversions represent the whole atmospheric column. The optical thicknessesmeasuredby Sun Photometers
on the ground or from a low-flying aircraft are also representative of the whole column. The in situ aerosol size distribution measurementsmade with the p_.assivecavity a_erosolspectrometer p_.robe(PCASP) [Hobbs et al., 1998] a swath width of 36 km at the nominal altitude of 20 km. The spatial resolutionis 50 m at nadir, and pixels are aggregated aboardthe C-131A aircraft are representativeof one or more into boxes of 40 x 40 pixels in order to derive the aerosol atmosphericlayers.The PCASP has 15 bins covering the size parameters on a scaleof 2 x 2 km2. Thecalibration of the range 0.10 to 3.00gm diameter.The aerosol size distribution shortwavechannels(from X,= 550 to 2500 nm) is performed derivedfrom the PCASP has not been adjustedto the ambient relative humidity, which may produce differences with the on the ground using an integrating sphere. There is no onboard calibration calibration
is tested
for the shortwave channels, but the
actual aerosol
in
distributionis performedusingthe effective radiusreft defined
a thermal
vacuum
chamber
and
a
temperaturecorrection applied during data processing; the accuracyof the calibration is approximately5%. The ER-2 usually flew along a "racetrack"patternover open water, approximately160 km in length in which the two legs
by
size distribution.
reft =
The
validation
I•0 r3n(r)dr 0•r2n(r)dr
of
the
size
(1)
2264
TANRI• ET AL.: RETRIEVAL OF AEROSOLPROPERTIESOVER THE OCEAN
July 17, 1996, flight 10 Tau
Radius
Ratio
July 17, 1996, flight 11 0.8
0.6
0.6
0.5
1.0
:.
-....... 0.9
....
0.4
0.8
0.3
0.7
0.2
0.6
0.1
0.5
0.4
0.2
0.0
July 17, 1996, flight 12
II
:-am
Plate 1. Maps of aerosol parametersderivedfrom the MAS measurementson July 17. Optical thickness (left), effective radius (middle) and ratio rl (right). Informationon tracks 10, 11, and 12 is given in Table 3.
TANR• ET AL.: RETRIEVAL OF AEROSOLPROPERTIESOVER THE OCEAN
I• I,.I,•,HT g!!. ] $3
25 ,,IIJLY ] EJq5
n/ [' '7.ca
PIFIS-5 O / ¾J:3 f LR5E
Figure 1. Exampleof an ER-2 flight patternduringthe TARFOX experiment.This pattern was performedon July 25, 1996.The distancebetweenthe two legsis around10 km, andthe legsare 160 km long. Table 2. Summaryof MAS Data CollectedDuring TARFOX Flight
Time,
Location
Comment
Atmospheric
Number
1996
UTC
Conditions
96-146
July14
1707-1825
mid-Atlantic states
fightaborted
/
96-147
July16
1709-1826
Delawarepeninsula/
testflight
/
Atlantic Ocean
96-148
July17
1712-2130
Delawarepeninsula/ Atlantic Ocean
96-149
July20
1723-2230
Delawarepeninsula/ Atlantic Ocean
96-150
July21
1720-2047
Delawarepeninsula/ Atlantic Ocean
96-151
July22
1759-1936
mid-Atlantic coast/ Atlantic
96-152
July24
1710-1949
July25
1508-1815
mid-Atlantic coast/ mid-Atlantic coast/ Atlantic Ocean
96-154
July26
1809-2305
AtlanticOcean/ Bermuda
coordination
C-131A/ship coordination
C-130/ship coordination
testflight
cloudy,somecloudless holes,hazy very clean conditions, low humidity very clean conditions, low humidity
/
Ocean
Atlantic Ocean
96-153
C-131A
C-131A
very hazy and cloudy
coordination
C-131A,C-130
cloud free and hazy
coordination
ship coordination
cloudy near the coast and clear later
2265
2266
TANRl•ET AL.' RETRIEVALOFAEROSOLPROPERTIES OVERTHEOCEAN
4. Results
4.1.
July
17, 1996
For each selectedflight, maps of the following aerosol The meteorologicalconditions on July 17 were not ideal productsare provided:(1) the optical thicknessXa at )• = 550 sincelow-level "fog-likeclouds"weremoving from the shore nm; (2) the effective radius reft of the complete size eastwardandhigh-level cirruswerealso moving eastwardand expanding in extent. Nevertheless a hole within the cloud cover developedandprovidedabout 1 hourof clear-skydata, coincident measurements were obtained from the two aircraft starting at around1830 UTC, which correspondedto ER-2 or from the ER-2 and the surface. Summaries of the ER-2 flight tracks 10, 11, and 12 (Plate 1). At that time, the C-131A flightsare givenin Table 3. We will comparethe MAS optical wasat its lowest altitude(100 feet) at 37.44øN/74.09øW,just thicknessderivedat )• = 550 nm and its spectraldependence below the ER-2 on track 10 when there was Sun glint in the computed from the retrieved aerosol model with the Sun middleof thescan.A few minuteslaterat 1900 UTC the image Photometermeasurements. The effectiveparticleradiusderived was almostglint free (track 11), but the C-131A was then at an from the MAS is comparedwith the values obtained from the altitudeof 3750 m. The ER-2 passed againoverthe first leg of in situ measurements aboard the C-131A aircraft over a vertical the track pattern(track 12). profile usingequation(1). In only two flights did the C-131A In Plate 1, we definethe glint mask to be +30ø aroundthe complete a vertical profile coincident with a MAS retrieval. Fresnelreflection.This thresholdvalue shouldbe increased up Thus comparisons of reft are only madetwice. to 35ø sincethe image was obviouslystill contaminated(i.e., distribution; (3) the ratio q. We analyzed those parts of the flight tracks where
Table 3. Characteristicsof ER-2 Flight Tracks That Were Selectedand Analyzed.
Flight Track
Time, Lat(ø)/Long(ø) Hour Minute Second
SolarZenithand AzimuthAngles(ø)
Heading (ø)
July 17, 1996 10
1833:27 1849:24
37.46øN/74.19øW 38.53øN/72.47øW
{25.6o;236.7 ø} {30.00;242.8 ø}
48.6 ø
11
1852:38 1906:59
38.64øN/72.53øW 37.55øN/74.17øW
{30.50;242.9 ø} {31.4ø;246.9 ø}
230.5 ø
12
1910:45 1926:33
37.43øN/74.24øW 38.50øN/72.52øW
{32.1ø;248.6 ø} {36.70;252.7 ø}
49.7 ø
July 20, 1996 04
1818:42 1834:06
36.88øN/73.32øW 38.44øN/74.33øW
{23.90;232.0 ø} {26.60;233.9 ø}
333.6 ø
15
2109:14 2122:36
37.18øN/72.54øW 37.93øN/70.91øW
{56.8ø;271.7 ø} {60.70;274.2 ø}
59.6 ø
July 24, 1996 08
1857:20 1908:08
38.39øN/73.65øW 38.98øN/75.02øW
{31.4ø;241.5 ø} { 32.60;242.3 ø}
296.7 ø
09
1911:30 1920:40
39.21 øN/74.89øW 38.70øN/73.73øW
{33.2ø;243.2 ø} {35.40;247.7 ø}
119.4 ø
03
1544:55
38.04øN/74.01
1555:03
37.07øN/74.76øW
{23.5ø;133.8ø}
38.13øN/73.95øW 37.08øN/74.75øW
{21.3ø;211.8 ø} { 21.4ø;217.8 ø}
210.3 ø
36.91 øN/74.68øW 37.88øN/73.93øW
{21.9ø;221.8 ø} {24.40;226.5ø}
49.7 ø
July 25, 1996
13
1806:00 1815:56
øW
July 26, 1996
04
2102:40 2116:16
33.13øN/65.93øW 32.26øN/64.52øW
{61.6ø;275.4ø} {65.70;278.2ø}
124.8ø
The universaltime, the latitudeandkongitude,andthe solarzenithandazimuthanglesare givenfor the centerof the scan.Valuesat the beginning(top) andat the end (bottom)of each trackare provided.The headingis the ER-2 azimuth.
TANRl• ET AL.: RETRIEVAL OF AEROSOLPROPERTIESOVER THE OCEAN 1.00
,
0.30 17 July 96
0.01
MAS/Track MAS/Track MAS/Track
ß
AATS-6
10 11 12
........
0.2
•
0.20
•
0.15
•
0.10
'-,-,
0.05
0.00 0.4
0.6
0.8
1.0
•
I
I
3.0
0.0
Figure 2. Aerosol optical thickness measuredby the Sun photometer(AATS-6) aboardthe C- 131A aircraft and derived from MAS data as a functionof wavelength.The comparisonis madeover a point located at 37.49øN/74.16øW around 1830 UTC. Information on tracks 10, 11, and 12 is given in Table 3.
the retrievedoptical thicknessdisplays an unexpectedpattern in the middle of the scan). A strong, shallow fog layer was identified by the AATS-6 aboard the C-131A [see Russell et al., this issue (a), Plate 3d] which can explain the larger optical thicknessobservedin the southernpart of the images. The _LidarAtmospheric Sensing _Experiment(LASE) airborne lidar Ilsmail et al., 1997] aboard the ER-2 [see Russell et al., this issue (a), Plate 3c] shows that the layer was not uniform,
but there is no informationon the spatialextent of the layer to our results.
Figure 2 showsthe optical thicknessesmeasuredby the Sun photometeron the C-131A and those derivedfrom the MAS data as a function of wavelength.The comparisonis madeover
a pointlocatedat 37.49øN/74.16øWwhenthe ER-2 wasflying
I
17 July 96 18:30-19:00 N37.49-W74.16
• 0.5
• 1.0
1.5
Wavelength(•tm)
confirm
I
• 0.25
0.10
o [] /x
•
2267
2.0
2.5
Altitude
(km)
3.0
3.5
4.0
Figure 3. Effective particle radiusmeasured by the Passive Cavity Aerosol SpectrometerProbe (PCASP) aboard the C131A aircraft as a function of altitude.
In Figure4, we showthe MAS opticalthicknessaveragedin the cloud free region correspondingto a geographical area defined by points located at 38.17øN/74.0øW and 38.27øN/74.10øW; the time of the acquisition was around 1830 UTC. The time correspondsto a portion of the ER-2 track 4. The comparisonis madewith the Sun photometer measurements aboard the C- 131A acquired over 38.20øN/73.98øWat 1842 UTC. The agreementis still good (A'C= 0.015); the retrievedvalueof 'ca at )• = 550 nm is 0.036 when the Sun photometer measured around 0.05. The agreementbetweenspectraldependence is not so good since the MAS gave an almost constant value, while the Sun photometer data show a slight decrease as a function of wavelength. Becauseof the extremely clean conditions, the relative accuracyof the inversion is decreasedsignificantly.
Nevertheless,the algorithm gives the optical thickness'ca within a +0.015 accuracy.
above the C-131A. The agreement is very good (i.e., the
optical thicknessat )• = 550 nm is very close to the Sun photometer value). The derived spectral dependencealso matches the measurementvery well, which means that the aerosol model derived from the ER-2 remote sensing measurementshas the right optical properties. The aerosoleffective radiusmeasuredby the PCASP during its profile is plotted as a function of altitude in Figure 3. Values are in the range 0.10 to 0.15 •tm. The median values computedfrom the ER-2 measurements over the uniform part of the aerosollayer for tracks10, 11, and 12 are 0.219, 0.184, and 0.183 gm, respectively. Mdszaros [1970] showedthat an ammoniumsulfate particle with a dry radiusof 0.14gm can increaseits size by a factor of 2 at a relative humidity of 80%. It is excludedto state that similar effects occurredduring the TARFOX experiment, but correcting the aerosol size distribution for relative humidity, which is not a trivial matter, would definitely producelarger sizes. The decreasein effective radius observed for tracks 11 and 12 may actually occur, but there are no in situ measurements to confirm it.
4.2.
July
20,
1996
July 20 was a very clean day with low humidity, so we expectedvery low aerosol optical thickness. The ER-2 flew perpendicularto the solar plane.
1.00
i
i
i
i
o
MAS/Track
ß
AATS-6
i
i
i
]
04
20 July 96
O.lO
:
:
I
O.Ol 0.2
0.4
i
i
0.6
i
i
0.8
i
I
1.0
i
3.0
Wavelength (•tm) Figure 4. Aerosol optical thickness measuredby the Sun photometer (AATS-6) aboardthe C- 131A aircraft and derived from MAS data as a function of the wavelength. The Sun
photometer data were acquired over a point at 38.20øN/73.98øW at 1842 UTC. The optical thicknesses derivedfrom the MAS are averagedover a geographical area defined by points located at 38.17øN/74.0øW and 38.27øN/74.1øW;the time of the acquisitionwas around1830 UTC. Informationon track 4 is given in Table 3.
2268
TANRI•ET AL.:RETRIEVALOFAEROSOLPROPERTIES OVERTHEOCEAN 1.00
i
]
i
!
i
[
[
cloudsto theobservation area,andbandsof cirruswerequickly movingover the oceanfrom the continent.We analyzedER-2 tracks8 and9, whichcorrespond to thebestpartsof the flight consideringthe occurrence of cloudsandthe presenceof Sun glint duringthe first loops. Track 8 corresponds to the time whenthe C-131Aaircraftmadeits verticalprofile, although the exactlocationwaswithin the part of the MAS image
[
20 July 96
0.10
affectedby the Sun glint.
Plate 3 showsmapsof the derivedaerosolpropertiesfor both tracks8 and9. The imageis affectedby Sunglint, but thereis sufficientunaffected oceansurfacein the scanaway
SHIP
MAS/Track 0.01
I
i
0.2
I
15
I
0.4
I
0.6
I
0.8
I
fromtheSun. Notethatourcloudmaskdoesnotrejectthetiny
[
i
cloudsin the top left part of the images,but this will not be
1.0
3.0
Wavelength(gm)
the casewith the operational MODIS cloudmask. An aerosol
plumecanbe seenin the centerof both ER-2 flight tracks,
whichmeansthat the plume persistedin the areafor at least a Figure 5. Aerosol optical thicknessmeasuredby the CIMEL half hour.The largeropticalthicknessis connectedto smaller instrumentaboardthe Meridian ship and derivedfrom the MAS data as a function of wavelength. The ship data are averaged particles(as seenin the ratio B). The aerosol optical thickness measuredby the Sun over a geographical area defined by points located at 37.44øN/70.28øW
and 38.02øN/70.50øW;
the time of the
acquisition was between 2011 and 2124 UTC. The optical thicknesses derived from the MAS are averaged over a geographical area defined by points located at 37.40øN/71.86øW and 37.50øN/71.56øW; the time of the acquisitionwas around 2120 UTC. Information on track 15 i s given in Table 3.
The ER-2 then flew over the cruiseshipMeridian (track 15). In Figure 5, the aerosol optical thicknesses measuredby the CIMEL instrumentaboardthe Meridian ship and derivedfrom MAS dataare plottedas a functionof the wavelength.The ship data are averagedover a geographical area defined by points located at 37.44øN/70.28øW
and 38.02øN/70.50øW;
the time
of the acquisition was between 2011 and 2124 UTC. The optical thicknessesderivedfrom MAS are averagedover a cloud free geographical area defined by points located at 37.40øN/71.86øW
and 37.50øN/71.56øW;
compared in Figure6. The Sunphotometerdatawereacquired over point 38.80øN/74.33øWat 1900 UTC. The optical thicknessesderived from MAS are averagedover two geographical areas defined by points located at 38.90øN/74.47øW for zone a and at 38.82øN/74.52øW for zone
b; zone a is within the aerosolplume, and zone b is on the
edgeof theplumecloseto the endof the scan.Theagreement is perfectin zonea but the spectraldependence is differentfor zoneb. AlthoughtheSunphotometer datawereacquiredwithin the glint, we can reasonablyassumethat the AATS-6 was samplingthe aerosolplume observedin zone a. 4.4.
July
to be nonuniform.
These
inversion
variations
scheme
and/or
could
result
from
in the calibration
artifacts
in
1.00
•
did not confirm
was around0.15gm.
this
of the instrument.
since the effective
radius
24 July 96
0.10
the
It can also be attributedto the higher humidity in the presence of cloudsat the beginningand at the end of the track.The large valuesof reft are consistent with the weak spectraldependence observed in the Sun photometer data but the PCASP measurements
1996
The conditions on this day were not perfect because althoughthe sky was clearat the beginning of the flight, the
Maps of the aerosol parametersderived from track 15 are shown in Plate 2. The optical thickness shows an angular dependenceand varies between 0.045 and 0.095 along the scan.
25,
the time of the
acquisition was around 2118 UTC. The agreement is better thanfor the case shown in Figure 4. In this case, the spectral dependenceis better retrieved, although the optical thickness derivedat 550 nm is again smaller than the Sun photometer measurements. Note that the optical thicknessis twice as great as in Figure 4, which can explain the better agreement.The aerosollayer was also observedby the Sun photometeron the C-131A
photometer on the C-131A and derived from MAS data are
ß
AATS-6
[] ¸
MAS/Track MAS/Track
I
0.01
0.2
I
0.4
I
I
0.6
08/b 08/a I
I
I
I
0.8 1.0
I
3.0
Wavelength(gm) Figure 6. Aerosol optical thicknessmeasuredby the Sun photometer (AATS-6) aboard the C- 131A aircraft and derived from MODIS airborne simulator (MAS) data as a function of
wavelength. The Sun photometer data were acquiredat 38.80øN/74.33øW at 1900 UTC. The optical thicknesses derivedfrom the MAS are averagedover two geographical On this day the meteorologicalsituationwas againnot ideal areasdefinedby pointslocatedat 38.90øN/74.47øWfor zone a for deriving aerosolparametersfrom the MAS. A stationary andat 38.82øN/74.52øW for zoneb; the timeof the acquisition front, orientatedeast-westand crossing the shoreline over was around1904 UTC. Information on track 8 is given in North Carolina, was slowly moving northward,bringing Table 3. 4.3.
July
24,
1996
TANRI•ET AL.:RETRIEVALOFAEROSOLPROPERTIES OVERTHE OCEAN
2269
July 20, 1996, flight 15 Tau
Radius
!
Ratio
g
0.20
1.0
1.0
0.9
0.8
0.8
0.7
0.6 0.6 0.10
0.5 0.4 0.4
0.05 0.3
0.2
0.2
o.oo .
:,
4•I' 11
r lB
sa
.
II
,
0.1
.""
gl
ß
!
I .d
II
Plate 2. Sameas Plate 1 but for July 20. Informationon track 15 is given in Table 3.
0.0
2270
TANRI• ET AL.:RETRIEVALOF AEROSOLPROPERTIES OVERTHE OCEAN
July 24, 1996, flight 8 Tau
Radius
0.8
Ratio
0.6
1.0
0.5
0.9
0.4
0.8
0.3
0.7
0.2
0.6
0.1
O.5
0.6
0.4
0.2
0.0
July 24, 1996, flight 9
0.8
0.6
1.0
0.5
0.9
0.4
0.8
0.3
0.7
0.2
0.6
0.1
0.5
0.6
0.4
0.2
0.0
Plate 3. Same as Plate 1 but for July 24. Informationon tracks8 and 9 is given in Table 3.
TANR!• ET AL.: RETRIEVAL OF AEROSOL PROPERTIESOVER THE OCEAN
2271
July 25, 1996, flight 3 Tau
Radius
0.8
Ratio
0.6
1.0
0.5
0.9
0.4
0.8
0.3
0.7
0.2
0.6
o. 1
0.5
0.6
0.4
0.2
o.o
July 25, 1996, flight 13
0.8
0.6
1.0
0.5
0.9
0.6
0.4
'
0.8
0.4
0.2
o.o
0.3
0.7
0.2
0.6
o. 1
0.5
Plate 4. Same as Plate 1 but for July 25. Informationon tracks3 and 13 is given in Table 3.
2272
TANRI• ET AL.: RETRIEVALOF AEROSOLPROPERTIES OVERTHE OCEAN
July 26, 1996, flight 4 Tau
Radius
Ratio
i
0.8
.
"
0.6
1.0
0.5
0.9
0.4
0.8
0.3
0.7
I m
i
i
i
0.6
0.4
0.2
. 0.2
I .
ß
ß
0.0
I I
I
I
.
I
0.6
.
0.]
0.5
I I ii
I
i
i
m
m
m
t
m"
I'
Plate 5. Same as Plate 1 but for July 26. Information on track 4 is given in Table 3.
TANRI•ET AL.:RETRIEVALOFAEROSOLPROPERTIES OVERTHEOCEAN
MASimages wereaffected bySunglint(track3). Laterin the afternoon,whenthe geometrical conditionswere more
2273
0.50
favorable for avoiding the Sun glint, there was more
cloudiness. Nevertheless, holesin the cloudcovercan be
25 July 96
0.40
foundon tracks 12 and 13 that correspondto coincident Sun
photometer measurements performed aboard theC-131Awhen
it was flying atitslowest altitude.
Oøo o
0.30
c• o oo o
oo
Plate 4 showsthe aerosolpropertiesfor both tracks3 and
13. Track3 wasstronglyaffected by Sunglint, andlarge
0.20
instabilities in the aerosolretrievalcanbe seenon the edgeof the glint, mainly in the valuesof the ratio fl. An aerosol
0.10
plume canbeseenin thebottom portion of theplate.Track
o
AATS-6
ß ß
MAS/Track MAS/Track
13 12
13 is moreinterestingbecausebothedgesof the scanare glint
free.Again,theaerosol layerisnotuniform asseen alsoin the
0.00
I 37.6
Sun photometermeasurements.
37.7
•
•
37.8
37.9
• 38.0
• 38.1
I 38.2
38.3
In Figure7, the aerosoloptical thicknessmeasured by the Latitude (ø) Sunphotometer aboardtheC- 131Aandderivedfrom MAS data are plotted as a functionof wavelength.The Sun photometer Figure 8. Aerosol optical thickness measuredby the Sun data were acquiredat latitude 37.84øN along the transect photometer(AATS-6) aboardthe C- 13IA aircraft and derived from the MAS data as a function of latitude. The Sun performedat 74.15øW around 1849 UTC. The optical photometer data were acquired along the transect performed at thicknessesderivedfrom the MAS were for a point locatedat 74.15øW between 1845 and 1857 UTC. Values were
37.85øN/74.15øW; the time of the acquisition was around 1815 UTC. The agreement is again very good, the small discrepanciescan be attributedto different acquisitiontimes. Aerosol optical thicknessesmeasured by the Sun photometer
interpolatedat )•= 550 nm. The optical thicknessesderived from the MAS were obtained over the same meridian 74.15øW at 1750 UTC for track 12 and at 1815 UTC for track 13.
along the transectare plotted as a functionof the latitude in Figure8. Values have been interpolatedat 3•=550 nm for comparisonwith the MAS retrievals. Two points resulting from the MAS inversionhave been plotted' one for a pixel of
140
•
track 12 located on the transect and the other for track 13. The 120
-
100
-
imageshowsvaluesbetween0.25 and 0.28gm (Figure9a),
80
-
which is twice larger than the PCASP measurementsalong its profile (Figure 9b). Again, the PCASP measurements are not
60
-
40
-
agreementis perfect. Finally, the histogram of the effective particle radius derivedfrom the MAS for track 13 over the left part of the
adjusted totheambient relative humidity.
20 0
1.00
0.00
25 July 96
0.05
0.10
0.15
0.20
750
0.25 I
0.30 I
0.35
0.40
I
800
25 July 96 0.10
850
0.01
'
0.2
o
MAS/Track
ß
AATS-6
i
i
0.4
i
i
0.6
900
13
i
i
i
[
0.8 1.0
'
-
1000
-
ß.
o
o
3.0 1050
Wavelength(gm) Figure
950
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Effective Radius (gm)
7. Aerosol optical thickness measuredby the Sun
photometer(AATS-6) aboardthe C- 13IA aircraft and derived from the MAS data as a function of wavelength. The Sun
Figure 9. (a) Histogram of the effective particle radius derivedfrom the MAS for track 13 over the left part of the image (see Plate 4). (b) Effective particle radius measuredby
photometerdata were acquiredat 1849 UTC over a point at latitude 37.84øN on the transect performed at 74.15øW. The optical thicknessesderivedfrom the MAS were obtained at 37.85øN/74.15øW; the time of the acquisition was around
during a vertical profile performedbetween 1845 and 1930
1815 UTC.
UTC.
the PCASP aboard the C-131A
aircraft at 38,64øN/74.15øW
2274
TANR• ET AL.: RETRIEVAL OF AEROSOLPROPERTIESOVER THE OCEAN 1.00
,
,
'
'
'
'
'
I
i
:
0.50
. 0.40
-
26 July
ß _
Bermuda Island 0.30
ß 0.10
-
..........................
ß
.le 0.20
::
ß e e.e ee
ß
eel
-
MAS/Track 04 SHIP
o
0.10
26 July96
ß
(a)
0.01
i
i
0.2
i
0.4
i
i
i
0.6
i
J
0.8 1.0
3.0
0.00 8:00:00
12:00:00
16:00:00
20:00:00
1.00
Time (GMT)
• ••
Figure 10. Aerosol optical thickness measuredat the biologicalstationin BermudaIsland as a functionof the time.
26 July 96:
0.10
4.5.
July
26,
1996
This flight can be dividedinto two parts.The ER-2 first flew a "racetrack"pattern over the target area in order to obtain coincident
measurements with
the C-131A.
However,
cloudiness
was variable
with
broken
clouds.
The
aerosol
optical thickness varied between 0.15 and 0.50 along the transect;a similar increase was observedby the ground-based CIMEL Sun photometer during the day at the biological stationin Bermuda (Figure 10). In Figure 11 we comparethe MAS-retrieved values to the Sun photometer data from the Meridian ship as it left the BermudaIslands(Figure 1la) and to the CIMEL measurements from the biological stationnear the time of the ER-2 overpass time (Figure 1lb). Again, the agreementis very good.
MAS/Track
o /•
Biological Station 20:56 Biological Station 21:06
¸
the
flight did not go as plannedbecauseof cloudiness.The ER-2 then flew southeastto overtly the Bermuda Islands. Plate 5 illustrates the results. During the flight the
ß
04
Biological Station 21:17 i
0.01
i
0.2
i
0.4
i
i
i
0.6
0o)
i J
0.8
i
1.0
3.0
Wavelength(pm) Figure 11. (a) Aerosol optical thickness measuredby the CIMEL instrument aboardthe ship and derivedfrom the MAS instrument. The ship data correspondto 32.23øN/64.34øW; the acquisition time was around2145 [ITC. The MAS optical thicknesses are averagedover a zone from 32.34øN/64.83øW to 32.11 øN/64.78øW; the time was around 2115 UTC. (b) Aerosol optical thicknessesmeasuredat the biological station in Bermuda
Island at three different
times.
5. Summary of Results To illustrate the performancesof the algorithm, we will summarize the results for those days when coincident verification measurements were available. The optical
thicknesses Xa derivedat t = 550 nm from theMAS (Figure 12) comparewith the "ground-truth"measurementswith a better accuracythan expected.The error can be describedby Axa = +0.01+0.05Xa (instead of the theoretical value of Axa = +0.05+0.05xa [Tanr6 et al., 1997] for the aerosol type encounteredduring the experiment. Concerning the effective particle radius,comparisonson July 17 (reft = 0.18 to 0.22 pm from the MAS, and reff = 0.10 to 0.15 pm from the PCASP) andon July 25 (reft = 0.25 to 0.28 !.tmfrom the MAS, and reft = 0.10 to 0.15 pm from the PCASP)give an accuracy
lowerthanourpreviousestimate(Areff= 0.3reft),buta part of the discrepancycan clearly be attributedto humidityeffects. Becauseof the difficulty in makingan accuratevalidationof reft, we also comparedthe Angstr6m exponent which is representative of the aerosol size distribution (Table 4). Except for July 20 the agreementis good. Recall that the Angstr6m exponent is not computedfrom exactly the same wavelengthsfor the different instruments, which may explain
0.60 0.50
0.40
/ / / / /
0.30
/
/
/
0.20
/ / /
/
0.10
/
0.00 0.00
,
0.10
I
0.20
,
I
0.30
,
I
0.40
,
I
,
0.50
0.60
Measured Optical Thickness()•=550nm) Figure 12. Comparisonof the aerosoloptical thicknessat t = 550 nm measuredby the CIMEL instrument,aboardthe C131A aircraft and values derived from the MAS all coincident measurements.
instrument
for
TANRl• ET AL.: RETRIEVAL OF AEROSOLPROPERTIESOVER THE OCEAN
2275
Table 4. Angstr6mExponentDerived from the MAS Instrument(C•MAS) and from the SurfaceMeasurements:Sun photometer(c•sp) and AirborneSunphotometer(C•AATS_6)
Flight
XMAS
{XMAS
OtAATS_ 6
asp
Comment
Track
July 17 10 11 12
x = 0.57 x = 0.49 x = 0.55
1.60 1.65 1.68
1.58 1.58 1.58
-
goodmatches(time and position) between ER-2 (track 10) and C-131A; high spatial variability
0.55 (ship)
aerosolopticalthicknessis too small to derive the size
July 20 04 15
x < 0.05 x < 0.07
-0.15 -0.11
1.13 -
distribution
July 24 08
x = 0.49
1.62 a
x = 0.37
1.47 •
1.73
goodmatches(time and position) between ER-2 and C-131A;
presenceof an aerosol plume
July 25 13
x = 0.29
1.23
1.60
goodmatch(position)between ER-2 andC-131A, gap in time (45min); high spatialvariability
July 26 04
x = 0.28
0.97
1.24 (Bermuda Islands)
good match (time) between ER-2 and surface instrumentation;not the same location
t•MAS is computed at 3,=550and865 nm,t•Sp at 3,= 499 and865 nm,O•AATS_ 6 at 3,= 525 and1025nm.
a Corresponds to thetwozonesdefinedin section 4.3.
b Corresponds tothetwozones defined insection 4.3. part of the discrepancy. In addition, the larger discrepancy (Aot= 0.35) occurred when there was a delay of 45 min betweenmeasurements.When there is both a temporal and a geographicalmatch, the differenceis aroundA•z = 0.10.
wind speed. Larger values of x are obtained the smaller the wind speed because the corresponding foam contribution decreases and this decreasehas to be compensatedby a larger optical thickness. Also, the effective particle radius (Figure
13b) is hardly affected.Sincethe aerosolcontributionis larger in the visible when the wind speedis smaller, the resulting 6. Discussion stronger spectral dependence is then interpreted as a In this section we discussthe impactsof potential sources contribution of smaller particles. Similar computationswere performedfor the clearestday, July 20 (Figure 14). The above of errors, such as surfaceconditions, the glint mask, and the conclusions were still valid althoughthe impact was twice as ozone correction. large (Axa = +0.02). Since the relative contribution of the atmosphereis smallerin this case, resultsare slightly more 6.1. Impact of Wind Speed affectedby the changesin the surfacereflectance. To check the impact of wind speed on the retrieval, we performedthe inversion for the three values included in the
LUT (i.e., 2.0, 6.0, and 10.0 ms'i). In Figure13, results obtainedfor the two extremevalues(2.0 and 10.0 ms-1) are plotted as a function of the resultsobtained for the mean value
(6.0 ms'i). July 26 wasselected because therewasno glint within the image, and the opticalthicknesses displayeda large variability. The impact on 'ca (Figure 13a) is very limited, around0.01, which meansthat the coupling term betweenthe atmosphereand the oceansurfaceis almost independentof the
6.2.
Impact
of the Glint
Mask
We have noticed previously that the glint mask is very important since the reflection of the directSunbeam is very dependenton the wind speed.To checkthis, we selecteda day affected by the presence of the glint (July 17) and we performed the inversion for the three wind speeds. The differencesin the optical thickness between two inversions are shownin Figure 15 as a functionof the glint angle. Angles
2276
TANRI•ET AL.:RETRIEVALOF AEROSOLPROPERTIES OVERTHE OCEAN These results show that the Sun glint requiresa very large
0.50
' 0.40
I
'
I
'
I
'•
maskaroundthe specularreflection. it has to be testedagain on actual MODIS data since there may be contamination by stray light in the airborne MAS version, which increasesthe sensitivity.
ß v=2ms -1
6.3.
0.30
0.20
••,c• 0.10
July 26,Track 04
0.20
0.30
0.40
0.50
Retrieved Optical Thickness (atv=6ms 4) 0.70
I
'
I
of Ozone Correction
The impact of the ozone correction is estimated by assuminga variation of 80 Dobson units in the apparent ozone content; this correspondsto a variation of 27 Dobson
(a) '•• , I , I )•=550nm , I.... (a)
0.10
Impact
'
unit for an air mass of 3. Recall that the difference
between the
two extreme models (tropical and sub arctic winter) is around 25 Dobson
units.
The quality of the inversion is clearly not affected by variationsin the ozone content.For the clearestday (July 20) the differencebetween both optical thicknesses is less than
0.01 at )• = 550 nm (Figure 16). 0.60
o
v=10ms4
ß
v=2ms -1
7.
Conclusion
0.50
The potential of the MODIS data for retrieving aerosol parameters (optical thickness, the effective radius of the particle size distribution,and the ratio of the accumulationand coarsemodes),is demonstrated by the presentstudy.The data acquiredduringTARFOX confirm the expectedaccuracyof the retrievedsize distribution. The accuracyof the derivedvalues of the optical thicknessis even better than anticipated.
o o
0.40
o
0.30
July26, Track 04 (b)
0.20
•
0.10
0.10
0.20
0.30
0.40
0.50
I
0.60
0.70
0.10
RetrievedEffectiveRadius (at v=6ms-1) Figure
0.00
13. (a) Aerosol optical thickness derived from the
MAS instrumentassuminga wind speedof 2.0 ms-• and
10.0ms '•asa function oftheoptical thickness assuming a '•' =
-0.10
windspeedof 6.0 ms-•.Thecomparison is doneat 550 nm for July 26. (b) As in Figure 13a but for the effective particle radius.
m ii
July 17, Track12
-0.20
v=6.0ms4 and v=2.0ms4
! smaller
than
30 ø are not
shown
because this
was the
value
previouslyselectedfor the mask. It is clear that directionsare increasingly affected as specular reflection is approaching. For an angle of 35ø from the specularreflection the impact is aroundAxa = + 0.10, and the impact is completelynegligible for anglesgreater than 45 ø. 0.10
-0.30
-
ß
ß
(a) -0.40
I
I
20.0 25.0 30.0 0.10
I
0.00
Iß
35.0
•1
40.0
45.0
50.0
55.0
I
I
I
I
~ß ..........
...............
&
60.0
•
0.O9
-010
0.08 0.07
-0.20
0.06 -0.30
0.05 0.04 0.03
0.02
%
-
-
-
July17,Track12
v=6.0rns qand v=10.0ms 4
-
•
July 20,Track 15
-/ 0.02
-]
k=õ50nm (a)1 0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
Retrieved Optical Thickness (atv=6ms q) Figure 14. As in Figure 13a but for July 20.
-0.40
I
I
I
I
I
(b) i
I
20.00 25.00 30.00 35.00 40.00 45.00 50.00 55.00 60.00
Glint Angle (ø) Figure 15. Differences in the retrieved optical thickness as a function of the glint angle assumingdifferent wind speed
valuesin the inversion.(a) Wind speedsof 2.0 ms-• and 6.0 ms-•. (b) Windspeeds of 6.0 ms4and10.0ms4.
TANRl• ET AL.: RETRIEVAL OF AEROSOLPROPERTIESOVER THE OCEAN
2277
Earth ObservingSystemera, J. Geophys.Res., 102, 17,081-17,107, 1997.
Griggs,M., Measurements of atmospheric aerosolopticalthicknessover waterusingERTS-1 data,J. Air Pollut. ControlAssoc.,25, 622-626,
0.11
1975.
0.10
Herman, J., P. K. Bhartia, O. Torres, C. Hsu, C. Seftor, and E. Celarier, Global distributionof UV-absorbing aerosolsfrom Nimbus 7/TOMS data,J. Geophys.Res., 102, 16,911-16,922, 1997. Hobbs, P. V., An overview of the University of Washingtonairborne
0.09
0.08
measurements and resultsfrom the TroposphericAerosol Radiative Forcing ObservationalExperiment (TARFOX), J. Geophys. Res.,
0.07
this issue.
0.06
-
0.05
_//if-
0.04
0.04
_•-
0.05
0.06
July20,Track15 -
Holben, B. N., et al., AERONET--A federated instrument network and data archive for aerosol characterization, Remote Sens. Environ., 66, 1-16, 1998.
L=550nm _ 0.07
0.08
0.09
0.10
0.11
AerosolOpticalThickness Figure 16. Impact of the ozone correctionon the aerosol opticalthickness.The comparison is doneat 550 nm for July 20.
Althoughthe presentresultsneed to be confirmedfor other aerosol types (e.g., dust and smoke), the MODIS aerosol productsover ocean can be usedwith some confidence.The problem of the glint mask is still unresolved,but actual MODIS data are requiredto solveit.
Husar, R. B., J. M. Prospero,and L. L. Stowe, Characterizationof troposphericaerosolsover the ocean usingthe NOAA advanced very high resolution radiometer optical thickness operational product,J. Geophys.Res., 102, 16,889-16,909, 1997.
Ismail, S., E. V. Browell, A. S. Moore, W. C. Edwards, K. Browns, S. A.
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(ReceivedAugust28, 1998;revisedNovember2, 1998; acceptedNovember 3, 1998.)
