SCIAMACHY TOA Reflectance Correction Effects on Aerosol Optical Depth Retrieval

1. Atmospheric Science Conference SCIAMACHY TOA Reflectance Correction Effects on Aerosol Optical Depth Retrieval W. Di Nicolantonio, A. Cacciari, S. Scarpanti, G. Ballista, E. Morisi, R. Guzzi ESA ESRIN Frascati, 8-12 May 2006

2. ESA ESRIN Frascati, 8 - 12 May 2006 Contents SCIAMACHY data Processing for Aerosol retrieval  Measured Reflectance Correction effects on AOP retrieval  Comparison with AERONET measurements  Conclusion

3.  SCIAMACHY data Processing for Aerosol retrieval : main Phases and Tools PRE-Processing PROCESSING POST-Processing AEROSOL DB >Spectral Optical Properties >Aerosol Profile DEM DB GTOPO30 ( 0.25º res.) SURFACE ALBEDO DB Spectral monthly MLER from GOME data ( 1º res.) RTM DOWNSTREAM for simulated reflectances INVERSE METHOD 2min.by LMFMfor simulated and measured spectral reflectances on AOD and aerosol class • PIXELSELECTION on • > Cloud Coverage Fraction • (FRESCO SC-v3) <0.05 • > SZA < 70 deg • > | latitude| < 70 deg • > sea OR land pixel ( no coast) • DATA READING • ( based on BEAT library ) • > Earthshine radiance • > Solar Irradiance • > Geolocation • > Observation Geometry • > Acquisition Time • WAVELENGTHS SELECTION • > CRITERIA: Tg(i) > 99.7 % and • i+1 - i > 20 nm • sea set: 364, 387,429, 683,754,775 nm • land set: up to 683 nm QUALITY CHECK Data Filtering on: >2values and >Discrimination Index on aerosol class VALIDATION Spatial Coincidence check on AERONET sites VISUALIZATION Data Format for: >daily/monthly map visualization Aerosol Level-2 Product and selected pixels for AOD Comparison / Validation MEASURED SPECTRAL REFLECTANCES @ selected aerosol wavelengths AOD @ 500 nm AEROSOL CLASS ESA ESRIN Frascati, 8 - 12 May 2006 A S P Data downloading & calibration LEVEL 1B DATA > from KNMI (via SDC link) > from ESA (via ftp or DVD) NADIR Observations Selection &Calibration by SciaL1C from EnviView Level 1-C data

4.  SCIAMACHY data Processing for Aerosol retrieval : Aerosol types in use a & w ESA ESRIN Frascati, 8 - 12 May 2006

5.  SCIAMACHY data Processing for Aerosol retrieval : Aerosol types in use  Derivative of aerosol TOA reflectance - in SS approx - with respect to AOD @ 500 nm ESA ESRIN Frascati, 8 - 12 May 2006

6. Reflectance Correction effects : linear correction for TOA Refl Rscia-corr(la) = Csl(la) Rscia(la)+ Coff(la) ESA ESRIN Frascati, 8 - 12 May 2006 Known SCIAMACHY underestimation for UV and VIS-NIR reflectance by about 10 to 25% and 10 to 20% Tilstra et al. 2005 derived spectral correction factors in the range 240 - 400 nm by comparing SCIAMACHY spectral reflectance and DAK simulated reflectance over different surfaces Under the hypothesis of a linear relation between SCIAMACHY and MERIS reflectances, Acarreta & Stammes 2005 provided a set of correction factors at 442, 510, 665, 708, and 885 nm taking account different underlying surfaces

7.  Reflectance Correction effects : Spectra for clouds free pixels In the NIR, the adopted correction doesn’t introduce a significant variation in the TOA reflectance values, mainly over ocean. This can be due to the hypothesis of linearity assumed also in low reflectance domain (< 0.2) in SCIAMACHY - MERIS refl. comparison ESA ESRIN Frascati, 8 - 12 May 2006 FRESCO ccf = 0 Sep 26, 2004 Sep 26, 2004

8.  Reflectance Correction effects on AOP : Example of daily global retrieval September 15, 2004 ESA ESRIN Frascati, 8 - 12 May 2006 SCIAMACHY AOD @ 500 nm

9.  Reflectance Correction effects on AOP : Example of daily global retrieval September 15, 2004 ESA ESRIN Frascati, 8 - 12 May 2006 SCIAMACHY AOD @ 500 nm

10.  Reflectance Correction effects on AOD : Example of monthly global retrieval September 2004 ESA ESRIN Frascati, 8 - 12 May 2006

11.  Reflectance Correction effects on AOD : Example of seasonal global retrieval Autumn 2004 ESA ESRIN Frascati, 8 - 12 May 2006

12.  Comparison with AERONET measurements: 2003 and 2004 corresponding data 9 AERONET sites AERONET AOD Level 2 data ASP tendency to AOD understimation DAOD =  0.03  0.06 AOD Land : y=0.85 x + 0.07; r= 0.93 Ocean : y=0.24 x + 0.13; r= 0.38  HP: non-linearity effects for small TOA reflectance not considered in the correction factors derivation, introducing a systematic error, resulting in a bias mainly in the retrieval of AOD lower than 0.4 ESA ESRIN Frascati, 8 - 12 May 2006

13.  Reflectance Correction effects on AOP retrieval: retrieval uncertainties Estimates of retrieval uncertainties associated to reflectance correction factors  AOD  > 0.1 in  50% and  20% aerosol type changed  AOD  > 0.2 in  10% and  5% aerosol type changed ESA ESRIN Frascati, 8 - 12 May 2006 Csl-min = Csl - Dsl; Coff-min = Coff - Doff Csl-max = Csl + Dsl ; Coff-max = Coff + Doff

14. ESA ESRIN Frascati, 8 - 12 May 2006 Conclusion • TOA reflectance correction procedure was applied to SCIAMACHY nadir measurements to explore the possibility of obtaining reliable information on tropospheric aerosol retrieval : • Examples of global map of AOD at 500 nm, relative to September, October, and November 2004, display the relatively low aerosol loading over oceanic regions, with AOD < 0.2, and the hot spots in AOD over land, presenting values  0.5, can be clearly identified • SCIAMACHY versus AERONET AOD (2003-2004) result in a good agreement (r = 0.93) in the case of significant aerosol loading over land and a very minor agreement (r = 0.38) for the retrieval over ocean. 1s uncertainty evaluated as DAOD =  0.03  0.06 AOD • On the other hand • Uncertainties in the reflectance correction factors lead to a significant propagated variability in the retrieved AOD of the order of  20% for AOD  1 and up to 90% for AOD  0.1 • Then • Applied correction encourages the adoption of SCIAMACHY sensor for aerosol retrieval and at the same time strengthens the need to use anofficial re-calibrated version of Level 1B data to improve the radiometric accuracy of low reflectance measurements and to increase the accuracy degree of the aerosol retrieval.

15. ESA ESRIN Frascati, 8-12 May 2006 Atmospheric Science Conference