Instantaneous Photosynthetically Available Radiation (PAR) is derived from the irradiance above each water pixel, under a tabulated relationship. For a given illumination condition, i.e. the sun zenith angle, this irradiance is only driven by the transparency of the atmosphere, itself well represented by the main absorbing gases (ozone and water vapour) and aerosol scattering. Consequently, this step is applied to all clear-sky water pixels where aerosol identification did not fail.
The PAR value is retrieved by multi-linear interpolation within a look-up-table providing PAR for the sun at zenith as a function of aerosol optical thickness, aerosol Angstrom exponent, total column ozone and total column water vapour. It is then scaled using the cosine of the sun zenith angle at the considered pixel.