1. Quality of the official SCIAMACHY Absorbing Aerosol Index (AAI) level-2 product L.G. Tilstra and P. Stammes Royal Netherlands Meteorological Institute (KNMI) SCIAVALIG workshop, KNMI, De Bilt, 30-11-2012

2. (2) Absorbing Aerosol Index: version 3.01 versus version 5.02W –Totally new algorithm setup –New look-up tables (pseudo spherical atmosphere + including polarisation) –Better surface height database –... Changes in L1–2 data processor:  version 5.02W is a huge improvement on version 3.01

3. SCIAVALIG workshop, KNMI, De Bilt, 30-11-2012(3) Scientific product (SC-AAI) Official product (L2-AAI v5.02W) Scientific product SC-AAI versus L2-AAI v5.02W(2003) (grey = sun glint filtering)

4. SCIAVALIG workshop, KNMI, De Bilt, 30-11-2012(4) Scatter plot: Scientific product SC-AAI versus L2-AAI v5.02W(2003) Good agreement (in the year 2003) –Calibration correction SC-AAI –Degradation correction SC-AAI –Ozone dependence taken into account in SC-AAI –L2-AAI: measurements “converted” to the (shorter) integration time of cluster 26 (O 2 –A band) Explanation for the differences:

5. SCIAVALIG workshop, KNMI, De Bilt, 30-11-2012(5) Scientific product (SC-AAI) Official product (L2-AAI) Scientific product SC-AAI versus L2-AAI v5.02W(2011)  The official L2-AAI is suffering from a scan-angle dependence

6. SCIAVALIG workshop, KNMI, De Bilt, 30-11-2012(6) Scatter plot: Scientific product SC-AAI versus L2-AAI v5.02W(2011) Official product suffers from a strong scan-angle dependence in 2011 colours: scan mirror positions

7. SCIAVALIG workshop, KNMI, De Bilt, 30-11-2012(7) Influence of (scan-angle dependent) instrument degradation: Global mean reflectance Global mean residue (AAI) [no m-factors applied] [Tilstra et al., JGR 117, 2012]

8. SCIAVALIG workshop, KNMI, De Bilt, 30-11-2012(8) M-factors applied in calculation of L2-AAI: Global mean residue (AAI) Current m-factor approach is scan-angle independent; the official L2-AAI product therefore shows a scan-angle dependence. Although v5.02W is a large improvement on the previous version, data after 2006 are not to be used without application of necessary corrections by the user.

9. SCIAVALIG workshop, KNMI, De Bilt, 30-11-2012(9) Conclusion: –The official AAI product of level-2 version 5.02W is a large improvement w.r.t. the previous version 3.01. –Instrument degradation has a large impact on the residue. The use of the official AAI product after 2006 is problematic. –This is, however, a level-1 issue.

10. SCIAVALIG workshop, KNMI, De Bilt, 30-11-2012(10) Extra slides

11. SCIAVALIG workshop, KNMI, De Bilt, 30-11-2012(11) (A is assumed to be wavelength independent: A 340 = A 380 ) A.Definition of the residue: where the surface albedo A for the simulations is such that: The AAI represents the scene colour in the UV Introduction: Absorbing Aerosol Index (AAI) B.Definition of the AAI: AAI = residue > 0(and the AAI is not defined where residue < 0) The AAI can be retrieved over land and sea surfaces, even in the presence of clouds. no clouds, no aerosols: r = 0 clouds, scattering aerosols: r < 0 absorbing aerosols: r > 0

12. SCIAVALIG workshop, KNMI, De Bilt, 30-11-2012(12) Example of global aerosol distribution recorded by SCIAMACHY: The “Global Dust Belt”: Desert Dust Aerosols (DDA) (dust storms, all year) AAI from other UV satellite instruments: TOMS, GOME-1, OMI, GOME-2. Combined with SCIAMACHY there are more than three decades (1978–2012) of AAI data available for studies of trends in desert dust and biomass burning aerosol. Biomass Burning Aerosols (BBA) (dry season, anthropogenic)

13. SCIAVALIG workshop, KNMI, De Bilt, 30-11-2012(13) The “Global Dust Belt”

14. SCIAVALIG workshop, KNMI, De Bilt, 30-11-2012(14) Wavelength pair (nm) Equator crossing time Pixel size (km) Days needed for global coverage Platform / Operation period GOME–1 340 / 380 10 : 30 LT 320 × 40 3ERS-2 (1995 – 2003*) SCIAMACHY 340 / 380 10 : 00 LT 60 × 30 6Envisat (2002 – 2012) GOME–2 340 / 380 09 : 30 LT 80 × 40 1.5MetOp-A (2006 – present) OMI 354 / 388 13 : 30 LT 13 × 24 1Aura (2004 – present) AAI products from GOME-1, SCIAMACHY, GOME-2, and OMI: * GOME-1: loss of global coverage on 22 June 2003 ; instrument retired on 4 July 2011