Three different Auxiliary Data Files (ADFs) are used in the common processing steps at the beginning of OLCI Level-2 processing:

1. The OLCI Level-2 Pre-Processing data file (OL_2_PPP_AX) contains the parameters used in the pixel classification step. The data is divided into three groups:

• Classification 1 provides parameters dedicated to cloud/snow/ice classification
•  Classification 2 provides consolidated land/water classification parameters
• Gas correction provides parameters used in the gas correction section

2. The OLCI Level-2 Water Vapour data file (OL_2_WVP_AX) contains the parameters used in the water vapour retrieval step. Two different algorithms are applied depending on the brightness of the underlying surface and are based on a differential absorption method. Both algorithms are implemented using LUTs to speed up the processing. This ADF provides, for ocean and land cases, the scattering correction factor and several parameters related to surface albedo and optical thickness coefficients.

3. The OLCI Level-2 Climatology data file (OL_2_CLP_AX) contains several climatology maps used in different steps of the processing chain. The different LUTs included in this ADF provide climatology, giving the mean water-leaving reflectance at 510 nm or the water-leaving reflectance variability at 510 nm. NO2 concentration, sea surface salinity and sea surface temperature maps are also included.