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Land Inversion

‘Procedure A’ for dark surfaces

If following Procedure A (for dark surfaces), the QA confidence (QAC) is initially set to a value between 0 (‘bad quality’) and 3 (‘good quality’), depending on the number of dark pixels remaining. In Procedure A, the algorithm assigns the fine aerosol model, based on the location and time (Levy et al., 2013). From the lookup table, the atmospheric path reflectance (ρa), atmospheric transmission (T), normalized downward flux (F), and atmospheric backscattering ratio (s) (for the fine model and coarse model separately) are interpolated for angle, resulting in six values for each parameter, corresponding to aerosol loading (indexed by τ at 0.55 mm).

The 2.12 µm path reflectance is a non-negligible function of the τ, so that the surface reflectance is therefore also a function of the τ. For discrete values of η between -0.1 and 1.1 (intervals of 0.1), the algorithm attempts to find the τ at 0.55 µm and the surface reflectance at 2.12 µm that exactly matches the MODIS measured reflectance at 0.47 µm. There will be some error, ε, at 0.65 µm.  The solution is the one where the error at 0.65 µm is minimized. The primary products are τ (τ0.55), η (η 0.55 ), and the surface reflectance (ρs2.12). The error ε is also noted.

‘Procedure B’, alternative for brighter surfaces

The derivation of aerosol properties is possible when the 2.12 µm reflectance is brighter than 0.25, but is expected to be less accurate, due to increasing errors in the surface reflectance assumptions. The upper limit of the ρ 2.12 value is permitted to increase as a function of the slant path until a final upper bound of ρ 2.12 = 0.40 is reached. When the sun is at zenith and the satellite view nadir, path B collapses back to path A requirements. However, as the photon path increases, more and more signal originates from the atmosphere, and the contribution from the surface reflectance becomes less and less important. This is especially true at the 0.47 µm channel where atmospheric signal is highest and the surface usually darkest. For this reason, the alternative path B retrieves aerosol only in the 0.47 µm channel. At least 12 pixels must again meet the path B criteria, otherwise the procedure ends with no retrieval made, and fill values are placed in the output fields. Path B is considered to be less accurate than path A, and the quality control (QC) is set to 0, representing “poor quality.” Because of the greater uncertainty over these brighter surfaces and because we retrieve in only one wavelength, only the continental model is used in the retrieval. The aerosol optical thickness is derived from the LUT for 0.55 µm, and η is assumed to be 1. The primary products for Procedure B are τ (τ 0.55),  and the surface reflectance (ρs2.12). The ‘land fitting error’ ε is also saved.