Validation of Aerosol Optical Thickness Retrieved by BAER...
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Shaddowtext iup Validation of Aerosol Optical Thickness Retrieved by BAER in the Mediterranean Area Wolfgang von Hoyningen-Huene (1) , Alexander Kokhanovsky (1) , John P. Burrows (1) , Maria Sfakianaki (2) , Maria Kanakidou (2) 1) Institut f ¨ ur Umweltphysik (iup), Universit ¨ at Bremen FB1, Otto-Hahn-Allee 1, D-28359 Bremen Phone: +49/421/218-2915, Fax: +49/421/218-4555, e-mail: [email protected] 2) University of Crete, ECPL, GR-71409 Heraklion, Greece Summary The pattern of aerosol loading over Europe can be affected by various source processes, like desert dust transports, fo- rest fires, urban and industrial pollution as well as by the natural variability of the aerosol. The most sources are over land. Satellite remote sensing of aerosol properties over land and ocean surfaces yields the regional distribution of aerosol load in terms of aerosol optical thickness (AOT). For this purpose successfully the BAER (Bremen AErosol Retrieval) approach is used, Fig. 1, using multi-spectral separation tech- niques between spectral surface and atmospheric properties, c.f. von Hoyningen-Huene et al. 2003 . ESA ENVISAT’s MERIS provide sufficient spectral coverage to investigate the main atmospheric aerosol loading. MERIS-L1b data, giving top-of-atmosphere (TOA) reflectan- ce of cloud less scenes are used for the determination of spec- tral AOT for the MERIS channels 1 - 7. The pattern of AOT in August 2003 shows the distribution of forest fire plumes form Iberian peninsula over almost whole Europe. The results of AOT for Europe are in close agreement with ground-based AOT measurements. The BAER Method Ocean Cloud Land (13) < 0.1 (13) > 0.2 (13) > 0.1 Clear Ocean Coastal Water z = 0 m T = 288 - SST z = z(x,y) T = 288 Vegetation Cover / Bare Soil Ocean Land AOT LUT for different Aerosols Aerosol-Reflectance Surface Reflectance Rayleigh Reflectance Main p- + T-profiles Tropical Mid-Latitude S/W Sub-Arctic S/W ( , z) NDVI DEM Digital Elevation Model Discrimination L1A Data Preparation - TOA-Radiance (Radiance MDS) - Irradiation-/View-Geometry (Scaling Factor GADS/ Tie point ADS) - TOA-Reflectance ( ) TOA λ R λ ( ) Surf λ ( ) = ( ) - ( , z) - ( ) λ λ λ λ TOA R Surf A λ ρ λ Spectral Smoothness of AOT ρ ρ ρ ρ ρ ρ ρ ρ ρ TOA TOA ρ TOA Veget. Cover Model AOT( ) = f( , ) SeaWiFS MERIS L1 Data MERIS L2 Data Fig. 1: Main steps used within the retrieval method BAER (Bremen AErosol Retrieval) for the derivation of spectral aerosol op- tical thickness. The method determines an aerosol reflectance by • correcting the path radiance produced by the Rayleigh scat- tering, using a digital elevation model for the surface ele- vation, • correcting the surface reflectance. Over land: using a model of linear mixture of reflectance spectra of vegatation and bare soil, tuned by the NDVI. Over ocean: linear mixing of coastal water and clean ocean using the NDPI. • consideration of constraints for the spectral aerosol optical thickness. • look-up-tables (LUT) transfer aerosol reflectance to AOT. LUT use aerosol parameters (phase function and single scattering albedo), obtained during LACE-98 experiment. MERIS Retrievals -10 -5 0 5 10 60 55 50 45 40 35 Longitude Latitude 0.0 0.2 0.4 0.6 0.8 1.0 MER_RR_20030805 AOT(0.443 μm) -10 -5 0 5 10 55 50 45 40 35 Longitude Latitude 0.00 0.25 0.50 0.75 1.00 MER_RR_20030808_L1L AOT(0.443 μm) 0 5 10 15 20 60 55 50 45 40 35 Longitude Latitude 0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000 MER_RR_20030809 AOT(0.443 μm) -10 -5 0 5 10 55 50 45 40 35 Longitude Latitude 0.0 0.2 0.4 0.6 0.8 1.0 MER_RR_20030811_L1 AOT(0.443 μm) -30 -20 -10 0 10 20 30 40 50 ★ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ★ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ★ ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ★ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ★ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ★ ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 1000 1500 2000 2500 3000 3500 4000 4500 1750 06 00 08/09 18 12 06 00 08/08 18 12 06 00 08/07 18 12 06 00 08/06 18 12 06 00 08/05 18 12 06 00 08/04 18 12 NOAA HYSPLIT MODEL Backward trajectories ending at 10 UTC 09 Aug 03 FNL Meteorological Data Source ★ at 53.03 N 8.48 E Meters AGL Job ID: 313775 Job Start: Tue Sep 20 14:21:44 GMT 2005 lat.: 53.03 lon.: 8.48 hgts: 1750, 1850, 1950 m AGL Trajectory Direction: Backward Duration: 144 hrs Meteo Data: FNL Vertical Motion Calculation Method: Model Vertical Velocity Produced with HYSPLIT from the NOAA ARL Website (http://www.arl.noaa.gov/ready/) Fig. 2 a) and b): Aerosol optical thickness for 443 nm (channel 2) retrie- ved from the MERIS (Medium Resolution Imaging Spectrometer) for the scenes of 05. (upper left), 08.(upper right), 09. (lower left) and 11. (lower right) August 2003 with fire plumes of the Portuguese forest fire season in 2003. Hysplit back-trajectories for the 9. August for Bremen show the transport from Iberian peninsula. Black areas are clouds or pixels without results. In August 2003, forest fire plumes from Iberian peninsula ha- ve been spread over south and central Europe. The effect of fire plumes increased the AOT for 0.443 μm in wide parts of Europe of about 0.2 and more. Validation by Ground-Based Data The validation of AOT retrievals using MERIS L1 data is mainly based on comparisons with ground-based sun- photometer measurements from AERONET or other insti- tutions. ENVISAT overflights over Europe are used. Ave- rages and standard deviation of retrieval results of 5x5 pi- xel groups around the sites of the ground-based instruments are compared with the best time match. Results for scenes from August 2003 are presented. The obtained correlation is: AOT GROUND (0.443μm)=0.795 · AOT BAER (0.443μm)+ 0.092 with a correlation coefficient of r =0.94 and a stan- dard deviation σ =0.045. For wavelength of channels 1 - 4 similar correlations are obtained. Further inter-comparisons with retrieval results of the ME- RIS L2 aerosol product - AOT from ’dark dense vegetation’ (DDV) pixels (Santer et al., 2000) are made. The advantage of BAER is its better coverage for a wider range of different land surface types compared with the widely restricted DDV approach. Both comparisons demonstrate the good agreement of the retrieved AOT and the capability of BAER for investigations of aerosol properties in climate research and environmental control. 0.00 0.20 0.40 0.60 0.80 1.00 AERONET AOT(0.440) 0.00 0.20 0.40 0.60 0.80 1.00 MERIS AOT(0.443) 0.00 0.40 0.80 1.20 1.60 AOT1-L2-DDV 0.00 0.40 0.80 1.20 1.60 AOT1-BAER MER_RR_20030811 Fig. 3: Comparison of retrieval results of the aerosol opti- cal thickness of BAER for the channels 2 (0.443 μm) deri- ved from MERIS L1 data with ground-based sun-photometer measurements (left). Inter-comparison of different retrie- val results for aerosol optical thickness for the channels 1 (0.412 μm) derived with the BAER approach and the ’dark- dense-vegetation’ approach in the MERIS L2 aerosol product (right). Reference: von Hoyningen-Huene, W., Freitag, M., Burrows, J.P.: Retrieval of aerosol optical thickness over land surfaces from top-of-atmosphere radiance. JGR 2003 108 D9 4260. doi:10.1029/2001JD002018, 2003. Santer, R., Carrere, V., Dessaily, D., Dubuisson, P., Roger, J.-C., 2000. MERIS ATBD 2.15 : Algorithm theoretical ba- sis document, atmospheric correction over land. Technical report PO-TN-MEL-GS-0005, LISE.
Transcript of Validation of Aerosol Optical Thickness Retrieved by BAER...
Shaddowtext
iup
Validation of Aerosol Optical Thickness Retrieved by BAER inthe Mediterranean Area
Wolfgang von Hoyningen-Huene(1), Alexander Kokhanovsky(1), John P. Burrows(1),
Maria Sfakianaki(2) , Maria Kanakidou(2)
1)Institut fur Umweltphysik (iup), Universitat Bremen FB1, Otto-Hahn-Allee 1, D-28359 BremenPhone: +49/421/218-2915, Fax: +49/421/218-4555, e-mail: [email protected]
2)University of Crete, ECPL, GR-71409 Heraklion, Greece
Summary
The pattern of aerosol loading over Europe can be affectedby various source processes, like desert dust transports, fo-rest fires, urban and industrial pollution as well as by thenatural variability of the aerosol. The most sources are overland. Satellite remote sensing of aerosol properties over landand ocean surfaces yields the regional distribution of aerosolload in terms of aerosol optical thickness (AOT). For thispurpose successfully the BAER (Bremen AErosol Retrieval)approach is used, Fig. 1, using multi-spectral separation tech-niques between spectral surface and atmospheric properties,c.f. von Hoyningen-Huene et al. 2003 . ESA ENVISAT’sMERIS provide sufficient spectral coverage to investigate themain atmospheric aerosol loading.
MERIS-L1b data, giving top-of-atmosphere (TOA) reflectan-ce of cloud less scenes are used for the determination of spec-tral AOT for the MERIS channels 1 - 7. The pattern of AOTin August 2003 shows the distribution of forest fire plumesform Iberian peninsula over almost whole Europe. The resultsof AOT for Europe are in close agreement with ground-basedAOT measurements.
The BAER Method
Ocean Cloud Land (13) < 0.1 (13) > 0.2 (13) > 0.1
Clear Ocean
Coastal Water
z = 0 mT = 288 - SST
z = z(x,y)T = 288
VegetationCover /Bare Soil
Ocean Land
AOT
LUT for different Aerosols
Aerosol-Reflectance
Surface Reflectance
Rayleigh ReflectanceMain p- + T-profiles Tropical Mid-Latitude S/W Sub-Arctic S/W
( , z)
NDVI
DEM DigitalElevation Model
Discrimination
L1A Data Preparation- TOA-Radiance (Radiance MDS)- Irradiation-/View-Geometry (Scaling Factor GADS/ Tie point ADS)- TOA-Reflectance
( )TOA
λ
R λ
( )Surf
λ
( ) = ( ) - ( , z) - ( )λ λ λ λTOA R SurfA
λ ρ λSpectral Smoothness of AOT
ρ
ρ
ρ
ρ ρ ρ ρ
ρ ρTOA TOAρTOA
Veget.CoverModel
AOT( ) = f( , )
SeaWiFS MERISL1 Data
MERISL2 Data
Fig. 1:Main steps used within the retrieval method BAER (BremenAErosol Retrieval) for the derivation of spectral aerosol op-tical thickness.
The method determines an aerosol reflectance by
• correcting the path radiance produced by the Rayleigh scat-tering, using a digital elevation model for the surface ele-vation,
• correcting the surface reflectance.Over land: using a model of linear mixture of reflectancespectra of vegatation and bare soil, tuned by the NDVI.Over ocean: linear mixing of coastal water and clean oceanusing the NDPI.
• consideration of constraints for the spectral aerosol opticalthickness.
• look-up-tables (LUT) transfer aerosol reflectance to AOT.LUT use aerosol parameters (phase function and singlescattering albedo), obtained during LACE-98 experiment.
MERIS Retrievals
-10 -5 0 5 10
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MER_RR_20030809�AOT(0.443�µm)
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MER_RR_20030811_L1 AOT(0.443 µm)
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NOAA HYSPLIT MODEL Backward trajectories ending at 10 UTC 09 Aug 03
FNL Meteorological Data
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Job ID: 313775 Job Start: Tue Sep 20 14:21:44 GMT 2005 lat.: 53.03 lon.: 8.48 hgts: 1750, 1850, 1950 m AGL Trajectory Direction: Backward Duration: 144 hrs Meteo Data: FNL Vertical Motion Calculation Method: Model Vertical Velocity Produced with HYSPLIT from the NOAA ARL Website (http://www.arl.noaa.gov/ready/)
Fig. 2 a) and b): Aerosol optical thickness for 443 nm (channel 2) retrie-ved from the MERIS (Medium Resolution Imaging Spectrometer) for thescenes of 05. (upper left), 08.(upper right), 09. (lower left) and 11. (lowerright) August 2003 with fire plumes of the Portuguese forest fire seasonin 2003. Hysplit back-trajectories for the 9. August for Bremen show thetransport from Iberian peninsula. Black areas are clouds or pixels withoutresults.
In August 2003, forest fire plumes from Iberian peninsula ha-ve been spread over south and central Europe. The effect offire plumes increased the AOT for 0.443 µm in wide parts ofEurope of about 0.2 and more.
Validation by Ground-Based Data
The validation of AOT retrievals using MERIS L1 datais mainly based on comparisons with ground-based sun-photometer measurements from AERONET or other insti-tutions. ENVISAT overflights over Europe are used. Ave-rages and standard deviation of retrieval results of 5x5 pi-xel groups around the sites of the ground-based instrumentsare compared with the best time match. Results for scenesfrom August 2003 are presented. The obtained correlation is:AOTGROUND(0.443µm) = 0.795 · AOTBAER(0.443µm) +0.092 with a correlation coefficient of r = 0.94 and a stan-dard deviation σ = 0.045. For wavelength of channels 1 - 4similar correlations are obtained.
Further inter-comparisons with retrieval results of the ME-RIS L2 aerosol product - AOT from ’dark dense vegetation’(DDV) pixels (Santer et al., 2000) are made. The advantageof BAER is its better coverage for a wider range of differentland surface types compared with the widely restricted DDVapproach.
Both comparisons demonstrate the good agreement of theretrieved AOT and the capability of BAER for investigationsof aerosol properties in climate research and environmentalcontrol.
0.00 0.20 0.40 0.60 0.80 1.00AERONET AOT(0.440)
0.00
0.20
0.40
0.60
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RIS
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.443
)
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0.00
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0.80
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1.60
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T1-
BA
ER
MER_RR_20030811
Fig. 3: Comparison of retrieval results of the aerosol opti-cal thickness of BAER for the channels 2 (0.443 µm) deri-ved from MERIS L1 data with ground-based sun-photometermeasurements (left). Inter-comparison of different retrie-val results for aerosol optical thickness for the channels 1(0.412 µm) derived with the BAER approach and the ’dark-dense-vegetation’ approach in the MERIS L2 aerosol product(right).
Reference:
von Hoyningen-Huene, W., Freitag, M., Burrows, J.P.:Retrieval of aerosol optical thickness over land surfacesfrom top-of-atmosphere radiance. JGR 2003 108 D9 4260.doi:10.1029/2001JD002018, 2003.
Santer, R., Carrere, V., Dessaily, D., Dubuisson, P., Roger,J.-C., 2000. MERIS ATBD 2.15 : Algorithm theoretical ba-sis document, atmospheric correction over land. Technicalreport PO-TN-MEL-GS-0005, LISE.
