Enhancing MAX-DOAS atmospheric remote sensing by multispectral polarimetry

Abstract

Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a well-established remote sensing technique for the detection of atmospheric aerosol and trace gases. Ultra-violet and visible radiation spectra of skylight are analysed to obtain information on different atmospheric parameters. An appropriate set of spectra recorded under different viewing geometries ("Multi-Axis") allows to infer aerosol and trace gas vertical distributions as well as aerosol properties by applying numerical inversion methods. It is well known but not yet used in MAX-DOAS applications that, besides the spectra, the polarisation state of skylight provides additional information on the atmospheric conditions. The major aim of the presented work was to assess the potential of polarimetric MAX-DOAS observations. For this purpose, a novel polarization-sensitive MAX-DOAS instrument (PMAX-DOAS) and a corresponding inversion algorithm (RAPSODI) were developed, capable to record and process polarimetric information. Furthermore, RAPSODI is the first MAX-DOAS inversion algorithm allowing to retrieve aerosol microphysical properties. Compared to conventional non-polarimetric MAX-DOAS approaches, the information on the atmospheric state contained in polarimetric observations is strongly enhanced: assuming typical viewing geometries, the degrees of freedom of signal increase by about 50% and 70% for aerosol vertical distributions and aerosol properties, respectively, and by approximately 10% for trace gas vertical profiles. For an ideal atmosphere, the studies on synthetic data predict an improvement in the results’ accuracy (root-mean-square differences to the true values) of about 60%, 40% and 10% for aerosol vertical columns, aerosol properties and trace gas vertical columns, respectively