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A closer look at aerosol retrieval with the Absorbing Aerosol Index (AAI) from SCIAMACHY

WP 3 : Absorbing Aerosol Index (AAI) WP 10 : Level-1 validation. A closer look at aerosol retrieval with the Absorbing Aerosol Index (AAI) from SCIAMACHY. L.G. Tilstra (1,2) , I. Aben (1) , P. Stammes (2) (1) SRON; (2) KNMI. SRON, 12-12-2008. Outline Introduction

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A closer look at aerosol retrieval with the Absorbing Aerosol Index (AAI) from SCIAMACHY

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  1. WP 3 : Absorbing Aerosol Index (AAI)WP 10 : Level-1 validation A closer look at aerosol retrieval with the Absorbing Aerosol Index (AAI) from SCIAMACHY L.G. Tilstra (1,2), I. Aben (1), P. Stammes (2) (1)SRON; (2)KNMI SRON, 12-12-2008

  2. Outline • Introduction • The AAI as in-flight calibration monitoring tool • Aerosol studies: zonal and regional means of the AAI • Conclusions SCIAvisie meeting, SRON, 12-12-2008

  3. 1 / 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) SCIAvisie meeting, SRON, 12-12-2008

  4. Typical aerosol distribution: AAI according to SCIAMACHY on Envisat The “Global Dust Belt”: Desert Dust Aerosols (DDA) (dust storms, all year) Biomass Burning Aerosols (BBA) (dry season, anthropogenic) SCIAMACHY AAI: 340/380 nm wavelength pair AAI from other UV satellite instruments: TOMS, GOME-1, OMI, GOME-2 SCIAvisie meeting, SRON, 12-12-2008

  5. 2 / The AAI as in-flight calibration monitoring tool GOME-1 experience: global mean residue (60°S–60°N) has a ~constant value SCIAMACHY: instrument degradation strongly affects the AAI/residue Case A: no degradation correction (no degradation correction applied) • Global mean residue increased by ~3.5 index points, pointing to a reflectance degradation of ~8% (between 340 and 380 nm) • Clear growing scan-angle dependence  scan-mirror is degrading (as in GOME-1) • Instrument decontamination periods in early years clearly visible A (SCIAMACHY) SCIAvisie meeting, SRON, 12-12-2008

  6. Case B: official in-flight calibration parameters (“m-factors”) applied • Use of m-factors reduces effects of degradation significantly, an increase of ~1 index point remains • Scan-angle dependence still there (m-factors are by definition scan-angle independent) • New (periodic) feature introduced (official degradation correction applied) B (SCIAMACHY) Feature is described by the seasonal variation of the elevation scan mirror (ESM) position that is used for solar irradiance monitoring measurements from which the radiance m-factors are deduced SCIAvisie meeting, SRON, 12-12-2008

  7. Case C: with additional degradation correction Self-calibration property: global mean residue (60°S–60°N) should be ~constant  we use this property to recalibrate the AAI (scan-angle dependent) C • Zonal behaviour appears to be OK • No remnants of effects or influence of instrument degradation visible • From now on: • AAI data = “case C” data, • i.e., fully corrected for instrument degradation SCIAvisie meeting, SRON, 12-12-2008

  8. 3 / Aerosol studies 3.1 / desert dust and biomass burning areas: Full agreement with time series from GOME-1 Areas 2,3,4,5: complicated interaction of clouds, aerosols, monsoon, ITCZ dynamics SCIAvisie meeting, SRON, 12-12-2008

  9. 3.2 / Seasonal variation due to SZA variation Ocean scenes with few aerosol events: Seasonal variations are not related to aerosol presence, but to SZA variations in combination with the non-Lambertian nature of clouds Variation of SZA in time series of regional means complicates interpretation SCIAvisie meeting, SRON, 12-12-2008

  10. 3.3 / Closer look at land & sea behaviour Black curves: maximum AAI value reached; no aerosols are present where < 0 Land/sea surface behave differently: (1) difference in spread (2) different “non-aerosol” levels Clouds? Surface albedo? Assumption that A340 = A380 breaks down? SCIAvisie meeting, SRON, 12-12-2008

  11. 4 / Conclusions • The AAI is a useful and sensitive tool for monitoring the radiometric calibration • SCIAMACHY shows a scan-angle dependent instrument degradation • The m-factor correction of the radiances introduce a seasonal variation • Effects of instrument degradation can be corrected for the AAI by assuming that the global mean residue is a constant • Interpretation of time series of regional means is complicated by the variation of the solar zenith angle • Regional analyses show that land and sea surfaces behave very differently • (speculation) perhaps due to assumption of wavelength independent albedo? SCIAvisie meeting, SRON, 12-12-2008

  12. Extra slides (R) SCIAvisie meeting, SRON, 12-12-2008

  13. R1: The “Global Dust Belt” SCIAvisie meeting, SRON, 12-12-2008

  14. R2: Zonal mean residue SCIAvisie meeting, SRON, 12-12-2008

  15. Replace measured solar irradiance values by theoretical solar irradiance values: (m-factor degradation correction but with theoretical solar irradiance values) (SCIAMACHY) No change in behaviour  m-factors introduce the feature in the Earth radiance AAI is able to detect relative calibration errors of 0.1% SCIAvisie meeting, SRON, 12-12-2008

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