For the retrieval of trace gas amounts, the technique of Differential Optical Absorption Spectroscopy (DOAS) is a well established remote sensing method which has been developed and improved over the last decades. It was first used for measurements conducted with ground-based instruments, but can also be applied when observing the atmosphere from space. The DOAS method makes use of the individual absorption characteristics of molecules on the mathematical basis of Lambert-Beer’s absorption law. The technique enables to observe atmospheric trace gases such as ozone and NO2. For this the absorption in visible and near ultraviolet sunlight scattered in the zenith is measured. So called slant columns, that are the column densities along the light path can be derived. The fundamental difference between typical radiative transfer calculations and the point of view in the DOAS method is the way how the light is traced through the atmosphere. DOAS type measurements can be conducted either using an artificial light source (active DOAS, e.g. with xenon or mercury35 arc lamp) or the sun as natural light source (passive DOAS)36 .