Retrieval of Atmospheric Parameters from Spectroscopic Observations using DOAS Instruments - RAPSODI

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The Earth s atmosphere is governed by complex chemical and dynamical processes, and a detailed knowledge of the vertical distribution of trace gases and aerosols is crucial for a thorough understanding of the atmospheric system. Remote sensing measurements are ideally suited for the determination of the vertical structure of the atmosphere. Ground-based, airborne and satellite borne remote sensing measurements provide a contact free method to measure the atmospheric composition from a distance. In particular, Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) represents versatile and cost effective method for the observation of the vertical distribution of a variety of trace gases and of aerosols. In the framework of the proposed project, a novel retrieval algorithm will be developed, which will allow for a thorough exploitation of the information contained in MAX-DOAS measurements of scattered and direct sunlight. In contrast to existing algorithms, DOAS analysis and profile retrieval will be performed in a single step. This simultaneous retrieval of vertical profiles of aerosols and trace gases directly from the measured spectral radiances is expected to lead to a significant increase in the overall information content. Additional information on the state of the atmosphere will be gained from a simulation of the broadband spectral structure, which has so far been ignored in traditional DOAS applications, and from an explicit modelling of rotational Raman scattering (Ring effect). A T-Matrix and/or Mie model coupled to a radiative transfer model will allow for a retrieval of aerosol microphysical properties, such as size distribution and complex refractive index, from solar almucantar measurements, similar to existing algorithms for Aeronet sun photometer measurements. This algorithm will serve as the basis for a thorough assessment of the overall information content of MAX-DOAS measurements with respect to trace gases, aerosol extinction and aerosol microphysical properties. Measurements by a novel Polarimetric MAX-DOAS (PMAX-DOAS) instrument, which will be designed and built in the scope of this project, will yield a further enhancement of the information content of scattered light measurements. Skylight measurements at different orientations of a rotatable polarising filter will allow for the reconstruction of the Stokes vector. Polarisation-dependent measurements of the intensity, trace gas column and Ring effect will significantly increase the accuracy of aerosol and trace gas retrievals. In order to fully exploit information content of the PMAX-DOAS, the retrieval algorithm will be based on a vectorised radiative transfer model, which enables the simulation of the full Stokes vector.