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OFFLINE ABSORBING AEROSOL INDEX

Composite image of offline absorbing aerosol index
(Metop-B overpasses three days ago)

Absorbing aerosol index from Metop-B

Description: Absorbing aerosol index indicates the presence of elevated absorbing aerosols in the Earth's atmosphere. The aerosol types that are mostly seen are desert dust and aerosols from biomass burning and volcanic eruption events.

Type: Offline. The products are available to users in two weeks from sensing. Usual delay is 2-3 days.

Theoretical basis: Global Ozone Monitoring Experiment-2 (GOME-2) measures the radiance spectrum of sunlight scattered from the atmosphere. Absorbing aerosol index is derived from the reflectances measured by GOME-2 at 340 nm and 380 nm.

Processing: The AAI processor is integrated part of the AC SAF processing at the KNMI. The algorithm calculates a ”residue” by using the contrast between the two wavelengths. When the surface albedo is assumed to be constant over the spectral range of the two selected wavelengths, and when the total ozone column is given, all deviations from a fully Rayleigh scattering atmosphere are assumed to be caused by absorbing aerosols. Note that scattering aerosols will be accounted for via an increased surface albedo. The total ozone column needed for the calculation is derived from the ozone climatology or sonde profile, which is selected by the user in the configuration file.

Units and spatial resolution: Absorbing aerosol index is a unitless linear scale, with higher values indicating elevated amount of aerosols present in the atmosphere. Spatial resolution of the product is equal to the spatial resolution of the GOME-2 instrument: 40 km x 40 km for Metop-A and 80 km x 40 km for Metop-B.

Validation: This product is validated by KNMI. Validation is based on satellite-to-satellite comparison: GOME-2 AAI product is compared with OMI-TOMS (EOS Aura) and SCIAMACHY (Envisat) AAI products.

Archiving and dissemination: This product is archived at the FMI and available via offline data access page.

File format: This product is delivered in HDF5 format, each file containing one satellite orbit (sunlit portion). The size of a single product file is about 1.1 MB.

Algorithm version information:

Time period GOME-2 / Metop-A GOME-2 / Metop-B
until 30.05.2008 0.03 N.A.
30.05.2008 – 09.12.2008 0.04 N.A.
09.12.2008 – 09.11.2009 0.05 N.A.
09.11.2009 – 15.07.2013 0.06 N.A.
15.07.2013 - 23.02.2015 0.06 0.06
23.02.2015 - 28.05.2015 0.07 0.07
28.05.2015 - 16.11.2015 1.00 1.00
16.11.2015 onwards 1.01 1.01

Important algorithm changes in 1.01:

Tools: An IDL reader for this product has been released by KNMI. You can download the reader package here.

Documents: