The Aerosol Index (AI) is a qualitative index indicating the presence of elevated layers of aerosols with significant absorption. The main aerosol types that cause signals detected in the AI are desert dust, biomass burning and volcanic ash plumes. An advantage of the AI is that it can be derived for clear as well as (partly) cloudy ground pixels.
The relatively simple calculation of the AI is based on wavelength-dependent changes in Rayleigh scattering in the UV spectral range where ozone absorption is very small. For a given wavelength pair a ratio is calculated from measured Top Of Atmosphere (TOA) reflectance and calculated theoretical reflectance for a Rayleigh scattering-only atmosphere and results in a residual value. Positive values of this residual indicate the presence of UV-absorbing aerosol.
Under the assumption that the surface albedo is constant for the two wavelengths used, the AI is given by:
AI = 100.log10[Rmeas(?2)/Rmeas(?1)] – 100.log10[Rcalc(?2,ALER(?2)) / Rcalc(?1,ALER(?2)) ]
- Rmeas(?2) and Rmeas(?1) are the measured reflectances at two wavelengths (?2 > ?1)
- Rcalc(?2,ALER(?2)) and Rcalc(?1,ALER(?2)) are the corresponding calculated reflectances from an atmosphere with only Rayleigh scattering, stored in a pre-calculated LUT.
- ALER(?2) is the Lambert-equivalent reflectivity (LER) fitted for the reflectance Rmeas(?2).
The TROPOMI aerosol index retrieval retrieves AI for two pairs of wavelengths in spectral band 3: 340/380 nm which is the pair of wavelengths used by most instruments retrieving AI, and 354/388 nm which is the pair used by the OMI instrument. AI340/380 and AI354/388 are both written in the Level-2 AI product.