The baseline operation flow of the scheme is based on a DOAS retrieval algorithm and is identical to that implemented in the retrieval algorithm for HCHO (also developed by BIRA-IASB, see S5P HCHO ATBD [RD12]). The main output of the algorithm are SO2 vertical column density, slant column density, air mass factor, Averaging Kernels (AK), and error estimates. Here, we will first briefly discuss the principle of the DOAS VCD retrieval before discussing the separate steps of the process in more detail.

First, the radiance and irradiance data are read from an S5P L1b file, along with geolocation data such as pixel coordinates and observation geometry (sun and viewing angles). At this stage also cloud cover information is obtained from the S5P cloud L2 data, as required for the calculation of the AMF, later in the scheme. Then relevant absorption cross-section data (SO2), as well as characteristics of the instrument (e.g., slit functions) are used as input for the SO2 slant column density determination. As a baseline, the slant column fit is done in a sensitive window from 312 to 326 nm. For pixels with a strong SO2 signal, results from alternative windows, where the SO2 absorption is weaker, can be used instead. An empirical offset correction (dependent on the fitting window used) is then applied to the SCD. The latter correction accounts for systematic biases in the SCDs. Following the SCD determination, the AMF is estimated. For computational efficiency, the algorithm makes no ‘on the fly’ calculation but uses a pre-calculated box air mass factor look-up table (LUT). This lookup-table is generated using the LIDORT radiative transfer code and has several entries: cloud cover data, topographic information, observation geometry, surface albedo, effective wavelength (representative of the fitting window used), total ozone column, and the shape of the vertical SO2 profile. The algorithm also includes an error calculation and retrieval characterization module that computes the so-called DOAS-type averaging kernels (Eskes & Boersma, 2003), which characterize the vertical sensitivity of the measurement and which are required for comparison with other types of data (Veefkind et al., 2012). For more information please look at the ATBD document on the TROPOMI website.