Abstract
Ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a widely-used measurement technique for the remote detection of atmospheric aerosol and trace gases. The technique relies on the analysis ultra-violet and visible radiation spectra of scattered sunlight (skylight) to obtain information on different atmospheric parameters. From an appropriate set of spectra recorded under different viewing directions (typically a group of observations at different elevation angles) the retrieval of aerosol and trace gas vertical distributions is achieved through numerical inversion methods. It is well known that the polarisation state of skylight is particularly sensitive to atmospheric aerosol content as well as aerosol properties, and that polarimetric measurement could therefore provide additional information for MAX-DOAS profile retrievals; however such measurement have not yet been used for this purpose. To address this issue, we have developed the RAPSODI (Retrieval of Atmospheric Parameters from Spectroscopic Observations using DOAS Instruments) algorithm. In contrast to existing MAX-DOAS algorithms, it can process polarimetric information, and it can retrieve simultaneously profiles of aerosols and various trace gases at multiple wavelengths in a single retrieval step; a further advantage is that it contains a Mie scattering model, allowing for the retrieval aerosol microphysical properties. The forward model component in RAPSODI is based on a linearized vector radiative transfer model with Jacobian facilities, and we have used this model to create a data base of synthetic measurements in order to carry out sensitivity analyses aimed at assessing the potential of polarimetric MAX-DOAS observations. We find that multispectral polarimetry significantly enhances the sensitivity, particularly to aerosol related quantities. Assuming typical viewing geometries, the degree of freedom for signal (DFS) increases by about 50 % and 70 % for aerosol vertical distributions and aerosol properties, respectively, and by approximately 10 % for trace gas vertical profiles. For an idealised scenario with a horizontally homogeneous atmosphere, our findings predict an improvement in the inversions results' accuracy (root-mean-square deviations to the true values) of about 60 % for aerosol VCDs as well as for aerosol surface concentrations, and by 40 % for aerosol properties. For trace gas VCDs, very little improvement is apparent, although the accuracy of trace gas surface concentrations improves by about 50 %.
Highlights
Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) (e.g. Hönninger and Platt, 2002; Hönninger et al, 25 2004; Wagner et al, 2004; Heckel et al, 2005; Frieß et al, 2006; Platt and Stutz, 2008; Irie et al, 2008; Clémer et al, 2010; Wagner et al, 2011; Vlemmix et al, 2015) is a versatile passive remote sensing technique for the simultaneous detection of aerosol and trace gases
We have developed and tested a novel retrieval algorithm (RAPSODI), capable of processing polarimetric MAXDOAS observations; the algorithm utilises the corresponding information to retrieve vertical distributions of aerosol and trace 595 gases as well as aerosol properties
In contrast to earlier MAX-DOAS algorithms, RAPSODI retrieves all species of interest simultaneously in a shared model atmosphere, enabling us to infer aerosol microphysical properties through the use of a Mie scattering model
Summary
Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) (e.g. Hönninger and Platt, 2002; Hönninger et al, 25 2004; Wagner et al, 2004; Heckel et al, 2005; Frieß et al, 2006; Platt and Stutz, 2008; Irie et al, 2008; Clémer et al, 2010; Wagner et al, 2011; Vlemmix et al, 2015) is a versatile passive remote sensing technique for the simultaneous detection of aerosol and trace gases. The spectra are analysed using Differential Optical Absorption Spectroscopy (DOAS, Platt and Stutz, 30 2008), to obtain information on different atmospheric parameters, integrated over the effective light path from the top of the atmosphere (TOA) to the instrument From these spectra, one can retrieve tropospheric aerosol and trace gas vertical distributions as well as aerosol properties by applying inverse modelling approaches, using dedicated retrieval algorithms. Polarimetry has been extensively utilized in recent decades for aerosol detection with radiometers/polarimeters, most prominently in the "Aerosol Robotic Network" (AERONET Holben et al, 1998; Li et al, 2009), and the "POLarization and Directionality of Earth’s Reflectances" (POLDER) satellite instrument (Deschamps et al, 1994) Such polarimeters derive aerosol abundances 45 and properties from measurements of the polarisation state and absolute radiance of direct and scattered solar radiation at low spectral resolution (applying narrow-band optical bandpass filters at few individual wavelengths, typically covering a spectral range from approximately 400 to 1000 nm)
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