1. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Validation of SCIAMACHY’s reflectance and polarisation measurements Juan Acarreta, Martin de Graaf, Gijs Tilstra, Piet Stammes (KNMI, De Bilt, NL), Matthijs Krijger (SRON, Utrecht, NL)

2. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Overview of talk Introduction and approach Selected SCIAMACHY nadir state Comparison SCIA vs. model reflectance in 300-400 and 400-800 nm ranges Effect of UV calibration on AAI Conclusions

3. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Introduction R. Siddans et al. (RAL) have found a radiometric calibration offset of the SCIAMACHY nadir reflectances in the UV (320-390 nm cluster) by comparison with GOME, amounting to about –20 %. Is this due to the polarisation sensitivity of SCIAMACHY and/or to another calibration problem? NOTE: reflectance (or: reflectivity) R =   radiance / (  0  solar irradiance)

4. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Approach For cloudfree scenes with small aerosol loadings the reflectance and polarisation at TOA can be predicted quite accurately using a radiative transfer model, given the spectral albedo of the surface. Especially in the UV, where most surfaces are dark. We use the polarised Doubling-Adding KNMI (DAK) model. The spectral surface albedo is taken from the database of Koelemeijer et al. (JGR, 2002, in press), based on GOME data for 1995-2000. This database contains the Lambertian equivalent reflectivity of the surface at 11 wavelengths between 335 and 772 nm, at a spatial resolution of 1°x1°.

5. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Sketch of the situation sun SCIAMACHY Sahara surface albedo O 3 absorption Rayleigh scattering no aerosols no clouds

6. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Selected orbit: 2509 (from verif. database)

7. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002

8. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 MODIS image

9. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 SeaWifs image + selected state (no. 11)

10. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Four selected areas of 60 x 280 km² (per area 7 pixels along track) 4 (West) 3 (CenterW) 2 (CenterE) 1 (East)

11. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 SCIA’s solar irradiance spectrum (ch.1-6)

12. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 SCIA’s earth reflectance spectrum (ch.1-6)

13. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 SCIAMACHY – DAK comparison Input parameters for DAK: Surface pressure from meteorological observations Ozone column from the GOME fast delivery product Surface albedo from GOME LER database (1995-2000) Sun-viewing geometry from level 1b data product Output: Reflectance (  I/  0 E), Q/I, and U/I

14. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 DAK spectrum 300-800 nm of R (‘’I”), Q/I and U/I East pixel West pixel

15. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 SCIAMACHY – DAK comparison Comparison for 320-390 nm (cluster 9), with polarisation correction off. Comparison with calculated polarisation effect, using instrumental sensitivity and calculated Q and U. Comparison for 300-800 nm range (clusters 10, 9,15, 17, 24, 26), with polarisation correction on/off.

16. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Pixel 1 (East) – SCIA vs. DAK

17. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Relative difference

18. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Pixel 2 (CenterEast) – SCIA vs. DAK

19. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Relative difference

20. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Pixel 3 (CenterWest) – SCIA vs. DAK

21. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Relative difference

22. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Pixel 4 (West) – SCIA vs. DAK

23. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Relative difference

24. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 SCIA’s polarisation sensitivity for scan mirror position of East pixel

25. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Calculated effect of polarisation – East pixel

26. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Actual relative difference

27. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 SCIA’s polarisation sensitivity for scan mirror position of West pixel

28. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Calculated effect of polarisation – West pixel

29. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Actual relative difference

30. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Effect of polarisation correction in SCIAL1C for 320-390 nm

31. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Pixel 1 (E) for 300-800 nm

32. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Pixel 2 (CE) for 300-800 nm

33. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Pixel 3 (CW) for 300 – 800 nm

34. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Pixel 4 (W) for 300-800 nm

35. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 SCIA’s polarisation sensitivity for scan mirror position of East pixel

36. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Calculated effect of polarisation – East pixel

37. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 SCIA’s polarisation sensitivity for scan mirror position of West pixel

38. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Calculated effect of polarisation – West pixel

39. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Conclusions of polarisation correction: Polarisation correction at 320-390 nm: Polarisation correction goes in the right way for the east pixel. Polarisation correction does not remove the negative offset of the reflectance spectrum. Polarisation correction should not have an effect for the west pixel, because light is almost unpolarised there. However, in the product there is a polarisation effect for west pixels. Polarisation correction at longer wavelengths: Small effect; it cannot explain a large offset.

40. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Effect of level 1 calibration offset on AAI The Absorbing Aerosol index (TOMS residue method) uses the reflectance pair at 340/380 nm The AAI is normally in the range 0 (no absorbing aerosol) to 4 (high absorbing aerosol load) A negative offset in the reflectance will affect the AAI: less reflectance means a higher AAI

41. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002

42. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002

43. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002

44. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Correlation between Level 2 and own AAI

45. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Correlation between Level 2 and own AAI

46. Validation of SCIA’s reflectance and polarisation (Acarreta, de Graaf, Tilstra, Stammes, Krijger ) Envisat Validation Workshop, Frascati, 9-13 December 2002 Conclusions Comparison of the clear sky TOA reflectance measured by SCIA with a polarised model, shows that SCIA has a UV polarisation correction problem for west pixels and an offset of around –20 % for all pixels. At the longer wavelengths the surface albedo of the Sahara is dominating the TOA reflectance. This albedo may be variable. It is not clear what the offset is there. There are some strange spikes in the reflectance.