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Monitoring GOME-2 instrument degradation using the Absorbing Aerosol Index (AAI)

Monitoring GOME-2 instrument degradation using the Absorbing Aerosol Index (AAI). L.G. Tilstra, O.N.E. Tuinder, P. Stammes Royal Netherlands Meteorological Institute (KNMI). GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14–15 June 2011. Outline Short introduction of the AAI

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Monitoring GOME-2 instrument degradation using the Absorbing Aerosol Index (AAI)

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  1. Monitoring GOME-2 instrument degradation using the Absorbing Aerosol Index (AAI) L.G. Tilstra, O.N.E. Tuinder, P. Stammes Royal Netherlands Meteorological Institute (KNMI) GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14–15 June 2011

  2. Outline • Short introduction of the AAI • GOME-1 instrument degradation • GOME-2 instrument degradation • Summary and conclusions GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  3. Introduction: Absorbing Aerosol Index (AAI) and the residue – The AAI represents the scene colour in the UV – • Definition of the residue: where the surface albedo A for the simulations is such that: (A is assumed to be wavelength independent: A340 = A380) no clouds, no aerosols : r = 0 clouds, no absorbing aerosols : r < 0 absorbing aerosols : r > 0 B. Definition of the AAI: AAI = residue > 0 (and the AAI is not defined where residue < 0) GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  4. Typical global aerosol distribution according to the AAI: The “Global Dust Belt”: Desert Dust Aerosols (DDA) (dust storms, all year) Biomass Burning Aerosols (BBA) (dry season, anthropogenic) AAI from other UV satellite instruments: TOMS, GOME-1, SCIAMACHY, OMI. Combined with GOME-2 there are more than three decades (1978–2011) of AAI data available for studies of trends in desert dust and biomass burning aerosol. GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  5. Instrument degradation: GOME-1 The global mean residue (GMR), the mean of all residues on a day between 60°N and 60°S, is about constant, showing only a very mild seasonal variation. dashed green line: 12-MAR-1996, end of the GOME-1 commissioning phase 22 June 2003: loss of global coverage The seasonal cycle is caused primarily by seasonal variations in aerosol presence and scattering geometry First signs of the effects of instrument degradation visible after ~4 years after launch. GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  6. GMR for each individual GOME-1 scan-mirror position (East, Nadir, West): dashed green line: 12-MAR-1996, end of the GOME-1 commissioning phase GOME-1 commissioning phase: reduced footprint size, symmetry around exact nadir broken Krijger et al., ESA SP-636, 2007: Clearly, GOME-1 is showing a scan-angle dependent instrument degradation. Scan-angle dependency is modest in the first 4 years, but starts up almost immediately after launch. GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  7. Instrument degradation: GOME-2 GOME-1 experience: the global mean of all residues on a day between 60°N and 60°S is about constant, showing only a very mild seasonal variation. black : GOME-2 green : GOME-2 (GOME-1 swath) red : GOME-1 (parameterisation) pink 1 : PMD band def. 1.0  3.1 pink 2 : PPF 4.2  4.3 pink 3 : throughput test Sept. 2009 Similar seasonal cycle as GOME-1. Downward trend caused by instrument degradation. GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  8. Global mean residue per scanner mirror position versus time (1): [ indicated scan pixels only ] • Global mean residue at the swath edges is higher than in the middle of the swath • Global mean residue of eastern side of the swath shows a very deviating & varying behaviour • Growing east/west bias: ~2.0 index points after ~4 years For GOME-1 the viewing angles at the swath edges are much smaller. As a result, the global mean residue at the swath edges roughly equals that of the middle of the swath.   GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  9. Global mean residue per scanner mirror position versus time (2): [ GOME-1 swath ] • Growing east/west bias in this case: ~1.0 index points over a period of ~4 years • GOME-1: east/west bias grew with ~1.0 index points over the first 4 years after launch So, for the GOME-1 part of the GOME-2 swath the (effect of the) scan-angle dependent component of the degradation is similar to that of GOME-1 in the first 4 years. GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  10. The effects of (scan-angle dependent) degradation on the GOME-2 AAI product: 06-JAN-2007 31-DEC-2010

  11. Overview of results: GOME-1: scan-angle dependent degradation Slow degradation started directly after launch. Extreme degradation after ~4 years; East/west bias ~5.5 index points after ~6 years. Average degradation after 4 years : ~0.0 pts East/west bias after 4 years : ~1.0 pts GOME-2: scan-angle dependent degradation Degradation started directly after launch. If comparable to GOME-1, then there should be extreme reflectance degradation in 1-2 years. Average degradation after 4 years : ~–1.0 pts East/west bias after 4 years : ~2.0 pts (1.0 pts G1 swath) GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  12. Summary and conclusions: • AAI is strongly affected by instrument degradation  sensitive monitoring tool • Instrument degradation for GOME-1 and GOME-2 seems to be similar, at least for the residue, which depends on the Earth reflectance • Scan-angle dependency for AAI: 2.0 index points (east/west) after 4 years • Need for instrument degradation correction is growing rapidly for the AAI GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  13. Extra slides (R) GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  14. R1: The “Global Dust Belt” GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  15. R2: General impression of the global aerosol distribution reported by GOME-2: east/west bias is gone since PPF 4.0 Features at the eastern side of the swath at high latitudes are under investigation. GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  16. R3: Global mean versus scanner mirror position: [also including backscan (BS) pixels] FW BS • global mean residue is roughly symmetrical w.r.t. the exact nadir position right after launch • growing east/west bias destroys this symmetry In conclusion, the GOME-2 AAI has shown to be reasonably stable over the last four years, but the need for a degradation correction is growing. GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  17. R4: Kasatochi volcanic eruption on 7 August 2008: (Kasatochi, Aleutian Islands, 52.17°N, 175.51°W; summit 314 m) The ash plume resulting from the Kasatochi eruption was well captured by the GOME-2 AAI GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  18. R5: Global mean residue per scanner mirror position versus time: [ forward scan pixels only ] • Global coverage is now a factor 24 lower, yet there are enough data points for a reliable daily “global mean residue” Downward trend is for sure caused by instrument degradation; easternmost scan mirror positions are apparently affected in a different manner than the rest. GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  19. R6: GMR per scan-mirror position relative to mean over scan-mirror positions: “Delta[east/nadir/west]” = GMR[east/nadir/west] – GMR[all] • Scan-angle dependent degradation starts immediately after launch • Maximum east/west bias: ~5.5 index points reached after ~6 years • East/west bias changed with ~0.8 index points over the first 3 years • Periodic behaviour: geometry effect due to changing solar position Scan-angle dependent degradation + small seasonal cycle due to geometry. GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  20. R7: GMR per scan-mirror position relative to mean over scan-mirror positions: “Delta_Mean[IndexInScan]” = GMR[IndexInScan] – GMR[all] [ indicated scan pixels only ] • Western and nadir scan-mirror positions degrade in more or less the same manner and rate • Eastern mirror positions degrade very differently GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

  21. R8: Global mean per scan-mirror position, shifted to zero for t=0: [ indicated scan pixels only ] • growing east/west bias: ~1.4 index points in ~3 years Downward trend is for sure caused by instrument degradation; easternmost scan mirror positions are apparently affected in a different manner than the rest. GOME-2 level-1 meeting, DLR, Oberpfaffenhofen, Germany, 14-15 June 2011

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