MERIS aerosol L2 product

1

• MERIS aerosol L2 product
• over land
• over ocean
• L2 aerosol product derived from L1 product, L1
  cloud mask
• (more cloud screening is implicitly done by the
  algorithms, maximum AOT over ocean, maximum ARVI
  over land)

2

• MERIS ATBD aerosol L2 product over land
• (MERIS ATBD 2_15 and 2_19)
• Input L1, auxiliary data
• Outputs AOT Aerosol Optical Thickness) at 443 nm
  and AC (Ängstrom Coefficient)
• Method Use land surface reflectance model over
  extended DDV pixels and Junge aerosol models
• Land surface reflectance model
• Use MODIS albedo
• Use biome for BRDF (Bidirectional Reflectance
  Function) and to set the value of the ARVI
  Atmospheric Resistant Vegetation Index) of DDV
  (Dark Dense Vegetation)
• Extended DDV analysis of MODIS albedo in 1x1
  -gt DLARS f(DARVI)
• Aerosol models Junge model(s), parameter g
  varies the size distribution
• AOTs and g are retrieved from best fit at 443 and
  865 nm, however, g is not accurately retrieved,
  use default g 3 -gtuse AC 1

3
Extended DDV
4
Validation AERONET (Santer et al., 2007, IJRS)
5

• MERIS ATBD aerosol L2 product over ocean
• (MERIS ATBD 2_7)
• Input L1 and auxiliary data
• Outputs AOT (Aerosol Optical Thickness) at 865
  nm and AC (Angstrom Coefficient), absorbing
  aerosol flag (desert dust)
• Method Assume very low marine reflectance of
  black pixels in NIR (778 and 865 nm) and use a
  set of aerosol models (18 with AC from 0 to 3,
  plus desert dust models)

6

• Aerosol models
• Boundary layer (0-2 km) Maritime or coastal
  or rural aerosol model (Shettle and Fenn, 1979
  Gordon and Wang, 1994), for RH 50 or 70 or 90
  or 99. (non-absorbing aerosols, ? about
  0.98-0.99. 6 additional desert dust models
  (Moulin et al. (2001)) are used when the flag at
  510 nm has been activated.
• Free troposphere (2-12 km) Continental
  aerosol (WCRP, 1986) (slightly absorbing
  aerosol 0.95 lt ? lt 0.98. Optical thickness is
  constant, and equal to 0.025 at 550 nm.
• Stratosphere (12-50 km) A 75 solution of
  sulphuric acid in water (WCRP, 1986)
  (non-absorbing aerosol, ? about 0.99)
  Optical thickness is constant, and equal to 0.005
  at 550 nm.
• The algorithm uses pre-computed LUTs of the TOA
  radiances, at different wavelengths, different
  geometries, different aerosol optical
  thicknesses, to bracket the MERIS measurements
  at 775 and 865 nm by 2 aerosol models.

7
Validation of L2 MERIS AOT over the oceans using
AERONET data (from GLOBCOLOUR project, Antoine,
personal communication)
8
Validation of L2 MERIS AOT over the oceans using
SIMBADA data (MAVT 2006)
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Summary MERIS Standard product

• Over land
• AOT is retrieved with an accuracy better than /-
  0.2 at 443 nm
• AC is not retrieved accurately
• Over the oceans
• AOT is retrieved at 865 nm with an accuracy of
  about /- 0.05
• AC is retrieved with a poor accuracy, above /-
  0.5

10
Possible improvement of the MERIS L2 aerosol
product

• Give additional parameters
• Aerosol Index (AI) AI AOT AC, more
  accurately described the small mode
• Absorption (ABS) when applicable ABS (1-??)
  AOT, where ?? is the single scattering albedo of
  the retrieved aerosol model
• Over land
• Use a better land surface reflectance, surface
  albedo and BRDF from MODIS or MERIS/GLOBCOVER
• Over ocean
• Extend the range of retrieved AOTs the
  processing of AOT is presently limited at a low
  value by the atmospheric correction requirement
• Give the information about the retrieved aerosol
  model in the L2 product give the bracketing
  models
• improve the accuracy of the retrieval by
  alternate algorithms like POLMER (see next Fig.).
• Increase the spatial coverage, presently reduced
  by the glitter by using newly developed
  algorithms (current ESA studies, HYGEOS study)

11
POLMER retrieval of aerosol over ocean

• POLMER algorithm uses 665 and 865 nm instead of
  765 and 865 nm to decrease the arm level and to
  reduce errors (instrument noise, adjacency effect
  to the coast)
• 12 SeaWiFS like aerosol models (Shettle and Fenn,
  1979, Gordon and Wang, 1994)
• Maritime H 98 95 90 80
• Coastal 90 80 70
• Terrestrial 99 98 90 80 70
• The algorithm uses pre-computed LUTs od the TOA
  radiances to select the 2 models that best
  bracket the MERIS measurements at 670 and 865
  nm
• Status developed for atmospheric correction of
  ocean colour data, would need minor adaptation to
  process L1-gt L2 MERIS data by the GLOBAER project

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POLMER retrieval of aerosol over ocean