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 Atmospheric Science Conference

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

A S P PRE-Processing -PIXEL SELECTION 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: T g ( i ) > 99.7 % and i+1 - i > 20 nm sea set: 364, 387,429, 683,754,775 nm land set: up to 683 nm MEASURED SPECTRAL selected aerosol wavelengths 500 nm AEROSOL CLASS 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  2 min. by LMFM for simulated and measured spectral reflectances on AOD and aerosol class Aerosol Level-2 Product and selected pixels for AOD Comparison / Validation Comparison / ValidationPOST-Processing QUALITY CHECK Data Filtering on: >  2 values and >Discrimination Index on aerosol class VALIDATION Spatial Coincidence check on AERONET sites VISUALIZATION Data Format for: >daily/monthly map visualization LEVEL 1B DATA > from KNMI (via SDC link) > from ESA (via ftp or DVD) NADIR Observations Selection &Calibration by SciaL1C from EnviView Data downloading & calibration Level 1-C data  SCIAMACHY data Processing for Aerosol retrieval : main Phases and Tools ESA ESRIN Frascati, May 2006

 SCIAMACHY data Processing for Aerosol retrieval : Aerosol types in use ESA ESRIN Frascati, May 2006  & 

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

 Reflectance Correction effects : linear correction for TOA Refl Tilstra et al derived spectral correction factors in the range 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 Known SCIAMACHY underestimation for UV and VIS-NIR reflectance by about 10 to 25% and 10 to 20% R scia-corr  a ) = C sl  a )  R scia ( a ) + C off  a ) ESA ESRIN Frascati, May 2006

 Reflectance Correction effects : Spectra for clouds free pixels FRESCO ccf = 0 ESA ESRIN Frascati, May 2006 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 Sep 26, 2004

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

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

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

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

ASP tendency to AOD understimation  AOD =  0.03  0.06 AOD  Comparison with AERONET measurements: 2003 and 2004 corresponding data AERONET AOD Level 2 data 9 AERONET sites Land : y=0.85 x ; r= 0.93 Ocean : y=0.24 x ; 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, May 2006

 Reflectance Correction effects on AOP retrieval: retrieval uncertainties C sl-max = C sl  sl ; C off-max = C off +  off C sl-min = C sl  sl ; C off-min = C off -  off ESA ESRIN Frascati, May 2006  AOD  > 0.1 in  50% and  20% aerosol type changed  AOD  > 0.2 in  10% and  5% aerosol type changed Estimates of retrieval uncertainties associated to reflectance correction factors

 Conclusion TOA reflectance correction TOA reflectance correction procedure was applied to SCIAMACHY nadir measurements to explore the possibility of obtaining reliable information on tropospheric aerosol retrieval : global map 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 SCIAMACHY versus AERONET AOD ( ) 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. 1  uncertainty evaluated as  AOD =  0.03  0.06 AOD On the other hand significant propagated variability in the retrieved AOD 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  0.1Then the need to use anofficial re-calibrated version of Level 1B Applied correction encourages the adoption of SCIAMACHY sensor for aerosol retrieval and at the same time strengthens the need to use an official 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. ESA ESRIN Frascati, May 2006

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