There are two algorithms that will deliver total ozone column amounts to be retrieved from TROPOMI. The first algorithm "S5P_TO3_DOAS" is based on the "DOAS-style" GOME Data Processor (GDP) algorithm Version 4.x. The second "S5P_TO3_GODFIT" is based on GDP Version 5, which is a "direct-fitting" algorithm deployed for the GOME/ERS-2 16-year reprocessing in 2012. The "DOAS-style", because of its lower computational demand, is to be used for generation of near real-time products, while the "direct-fitting" algorithm will generate offline and reprocessed products.

NRT/DOAS Algorithm (S5P_TO3_DOAS)

The general method used for derivation of O₃ total column density is the two-step DOAS method. However, for O₃ total column retrieval, the calculation of the air mass factor uses an iterative AMF approach [Spurr et al., 2005] which uses the fitted slant column result N_s to adjust the AMF (and hence the VCD) that reflects the trace gas content as expressed in the value of N_s. This iterative approach circumvents the issue that can be encountered in a traditional DOAS algorithm (where N_s fitting and AMF calculation are decoupled) when the selected climatological input profile can sometimes be very different from the true profile, leading to an incorrect AMF computation.

OFFLINE/Direct Algorithm (S5P_TO3_GODFIT)

Unlike the NRT/DOAS algorithm, the OFFLINE algorithm is a one-step algorithm. There is no separation of slant-column fitting and AMF conversion. It is based on the GODFIT (GOme Direct FITting) algorithm [lerot et al., 2010 and 2013; van Roozendael et al., 2012]. The algorithm uses an iterative least-squares cost function minimisation method, based on the differences between satellite-measured and model-simulated reflectances. For the forward model component, GODFIT uses the LIDORT radiative transfer model [Spurr, 2008] for multiple-scatter RT simulation of satellite radiances and analytically calculated Jacobians with respect to the total O₃ column, the temperature shift and a number of surface albedo closure coefficients. In addition to these retrieval parameters, the retrieval state vector also has parameters for ring-effect and under-sampling corrections, and for a wavelength registration shift. The inversion module uses a special Levenberg-Marquardt algorithm with line-search.

Fitting Window

• The fitting window used for both NRT and OFFLINE algorithms is in the UV Huggins bands: 325-335 nm (spectral band 3).