1. 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

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

3. 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 REFLECTANCES @ selected aerosol wavelengths AOD @ 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, 8 - 12 May 2006

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

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

6.  Reflectance Correction effects : linear correction for TOA Refl 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 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, 8 - 12 May 2006

7.  Reflectance Correction effects : Spectra for clouds free pixels FRESCO ccf = 0 ESA ESRIN Frascati, 8 - 12 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

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

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

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

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

12. 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 + 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 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, 8 - 12 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

14.  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 (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. 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, 8 - 12 May 2006

15. Atmospheric Science Conference