This document, identified as S4-L2-KNMI-ATBD-2017 is identified as the Algorithm Theoretical Basis Document (ATBD) for the Sentinel 4 UV Aerosol Index. This is a part of a series of ATBDs describing the technical basis of the products from the Sentinel 4/UVN Instrument. It describes the algorithm that will deliver the UV aerosol index for the Sentinel 4 UVN instrument. The purpose of the ATBD is to provide detailed mathematical and physical descriptions of this algorithm, along with discussions of algorithm inputs and outputs, data products, algorithm validation and error analysis.