SU_AATSR v4.2 
    - Version providing first global 17 year processing for ATSR-2 and AATSR. Extensive update from pervious versions, principally:
	- Per retrieval uncertainty estimate using propagation of errors from instrument, atmospheric radiative transfer, surface model, and retrieval fit
	- Ocean BRDF & glint using Cox and Munk BRDF, surface foam  (Koepke 1984) and Morel (1988) ocean colour models
	- Various minor improvements in LUT representation of atmospheric radiative transfer, improved cloud and glint screening


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More detailed explanation of sub-directories L2 and L3.

Level 2:

Directory L2:
Resolution - 10x10 km2 sinusoidal
Format NetCDF version 3

All retrieved variables are stored as vectors depending only on the unlimited dimension "pixel_number".
For each reported successful retrieval the following variables are available in the level 2 NetCDF files:

 - Latitude, Longitude 
 - AOD - at 550, 659, 865 and 1610 nm  
 - Ångström exponent (555nm - 659nm);
 - Per-pixel uncertainty estimates in AOD at 550, 659, 865, 1610nm; 
 - Aerosol mixture fractions (fine/coarse, absorbing/non-absorbing, dust/sea salt), 
 - atmospherically corrected surface reflectance 555, 659, 865, 1610nm (nadir only) 
 - cloud fraction in the 10x10km superpixel
 - land / ocean flag (a 10x10km pixel can only have one surface type)
 - time of measurement
 - viewing geometry (solar zenith, view zenith and relative azimuth angle) for forward and nadir

The aerosol type information is only given as mixture fractions of 
the 4 AerosolCCI base components:
   2 coarse mode - non-spherical dust and spherical "sea-salt"
   2 fine mode   - absorbing and weak-absorbing
Reported are the retrieved fraction of fine mode and the prescribed fraction of
the weak absorbing mode of the fine mode and the dust fraction of the coarse mode.



Level 3: 

Level 3, directory L3_DAILY:
daily files
Resolution - 1 degree x 1 degree global grids
Format NetCDF version 3
                     
Level 3 data is stored as global 2 dimensional grids depending on lat / lon.                     
Latitude, Longitude (Vectors specifying the grid box centre locations)
Mean and standard deviation of the following variables:
 - AOD (550, 659, 865, 1610nm)
 - AOD per pixel uncertainty (550, 659, 865, 1610nm)
 - Ångström (555nm - 659nm)
 - surface reflectance (550, 659, 865, 1610nm)
 - fine mode AOD (AOD550 * fine_mode_fraction)
 - dust AOD (AOD550 * (1 - fine_mode_fraction) * dust_fraction)
 - absorbing AOD (AOD550 * fine_mode_fraction * absorbing_fraction) 
   WARNING: this is NOT the commonly used definition of absorbing AOD
   The quantity here is only the absorbing AOD fraction of the fine mode.
   It can approximate the fine mode absorbing AOD by multiplying with (1-SSA) for the fine mode,
   where the fine mode SSA(550nm) = 0.7960.
 - cloud_fraction
 - view geometry
 - surface type (for mixed land/sea grid boxes, the mean surface type gives the fraction of land retrievals 
   within that grid box for that day or month)

Level 3, directory L3_MONTHLY:
same as L3_DAILY, but monthly averages and s.d.