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Abstract. Retrievals of trace gas concentrations from satellite observations are mostly performed for clear regions or regions with lowcloud coverage. However, even fully clear pixels can be affected byclouds in the vicinity, either by shadowing or by scattering ofradiation from clouds in the clear region. Quantifying the error ofretrieved trace gas concentrations due to cloud scattering is adifficult task. One possibility is to generate synthetic data bythree-dimensional (3D) radiative transfer simulations usingrealistic 3D atmospheric input data, including 3D cloudstructures. Retrieval algorithms may be applied on the syntheticdata, and comparison to the known input trace gas concentrationsyields the retrieval error due to cloud scattering. In this paper we present a comprehensive synthetic dataset which hasbeen generated using the Monte Carlo radiative transfer modelMYSTIC (Monte Carlo code for the phYSicallycorrect Tracing of photons In Cloudy atmospheres). The dataset includes simulated spectra in two spectral ranges (400–500 nm and the O2A-band from 755–775 nm). Moreoverit includes layer air mass factors (layer-AMFs) calculated at460 nm. All simulations are performed for a fixed backgroundatmosphere for various sun positions, viewing directions and surfacealbedos. Two cloud setups are considered: the first includes simplebox clouds with various geometrical and optical thicknesses. Thiscan be used to systematically investigate the sensitivity of theretrieval error on solar zenith angle, surface albedo and cloudparameters. Corresponding 1D simulations are also provided. Thesecond includes realistic three-dimensional clouds from an ICONlarge eddy simulation (LES) for a region covering Germany and partsof surrounding countries. The scene includes cloud types typical ofcentral Europe such as shallow cumulus, convective cloud cells,cirrus and stratocumulus. This large dataset can be used toquantify the trace gas concentration retrieval error statistically. Along with the dataset, the impact of horizontal photon transport onreflectance spectra and layer-AMFs is analysed for the box-cloudscenarios. Moreover, the impact of 3D cloud scattering on the NO2vertical column density (VCD) retrieval is presented for a specificLES case. We find that the retrieval error is largest in cloudshadow regions, where the NO2 VCD is underestimated by more than20 %. The dataset is available for the scientific community to assess thebehaviour of trace gas retrieval algorithms and cloud correctionschemes in cloud conditions with 3D structure.

atmospheric measurement techniques

Impact of 3D cloud structures on the atmospheric trace gas products from UV–Vis sounders – Part 1: Synthetic dataset for validation of trace gas retrieval algorithms