Recent results of in-flight calibration of SCIAMACHY reflectances

1. Recent results of in-flight calibration of SCIAMACHY reflectances Gijsbert Tilstra and Piet Stammes KNMI

2. Reflectance • Reflectance = π × Earth radiance / solar irradiance at TOA = π I / µ0 E0 The reflectance is the basic L1 quantity needed for retrieval of L2 products.

3. Absolute radiometric calibration of the reflectance is needed for e.g.: • Nadir ozone profile algorithm • Cloud algorithm (e.g. FRESCO) • Absorbing aerosol index algorithm • Aerosol optical thickness algorithm • Surface albedo algorithm

4. Approach of in-flight reflectance calibration 1.Use a radiative transfer model in the UV, where Rayleigh scattering and ozone absorption are determining the signal, which can be accurately computed 2. Compare reflectances with other satellite instruments: • MERIS • GOME • POLDER

5. SCIAMACHY vs RTM (DAK) in the UV (240–400 nm): (old key data) orbit 2509, 23-08-2002, SV 5.00 orbit 11251, 25-04-2004, SV 5.04 orbit 13028, 27-08-2004, SV 5.04 Tilstra et al., JGR 110, in press, doi:10.1029/2005JD005853, 2005.

6. Alternative radiative transfer comparison Large-scale comparison between reflectances from SCIAMACHY and an RTM (LidortA, polarization corrected) by Van Soest et al. (Atmos. Chem. Phys., 5, 2171-2180, 2005) gives: RSCIA – RRTM = -20 ± 5 % for 250 – 400 nm.

7. SCIAMACHY vs MERIS: Acarreta & Stammes, GRSL 2, 31–35, doi:10.1109/LGRS.2004.838348, 2005. Acarreta et al., KNMI Internal Report, 3 November 2004. (old and new key data) MERIS spectral bands: 442, 510, 665, 708, 885 nm

8. SCIAMACHY vs GOME: (old key data) • strong (scan-angle dependent) degradation of GOME in the UV up to 500 nm. • above 500 nm no scan-angle dependency + result accurate. • result above 500 nm agrees with MERIS comparison. Tilstra et al., Appl. Opt, submitted, 2005.

9. SCIAMACHY vs POLDER: (old key data) Summarized: all comparisons point to the same 15–20 % calibration error in the reflectance

10. Reflectance ratio R(new key data) / R(old key data) This ratio is fully determined by the new key data irradiance. This ratio is the same for all scenes, because the radiance appears to be unchanged (!) with the new key data.

11. All information put together: At higher wavelengths no agreement between the effect of new key data and the reflectance comparisons with other satellite instruments.

12. Spectral features: The new key data introduces spectral features in the reflectance. Effect on level-2 (DOAS) retrievals unknown.

13. Why the new key data is not the solution • It does not correct the reflectance error for wavelengths > 600 nm. • It introduces new spectral features, especially between 340 and 400 nm, whereas we know from GOME, OMI and UV RTMs that the continuum spectrum there should be straight.

14. Conclusions • The new key data does not solve the calibration error of the reflectance, esp. at longer . • The new key data adds nonphysical spectral features to the reflectance, esp. at shorter . • For the time being it seems better to apply a spectrally smooth empirical correction of the reflectance spectra produced with the old key data, based on in-flight calibration.

15. Reflectance corrections applied • AAI: 340 nm: 1.21 380 nm: 1.13 • FRESCO: 760 nm: 1.20