Regularisation model study for the least-squares retrieval of aerosol extinction time series from UV/VIS MAX-DOAS observations for a ground layer profile parameterisation

[摘要] The retrieval of tropospheric aerosol extinctions from MAX-DOAS observationsof O₄ using a small number of three or four extinction profile parameterssuitable for boundary layer reconstruction is investigated with respect tothe following questions. First, to what extent does this nominallyover-constrained pure least-squares problem for the inversion of theradiative transfer equation require regularisation and how should parametersof the regularisation be chosen? Second, how can a lack of information in theunder-constrained case be best compensated by using the information containedin a sequence of observations and by explicitly including intensities intothe fit?

The forward model parameterises the optical properties of the boundary layeraerosol by its extinction profile, single-scattering albedo and aHenyey–Greenstein phase function. Forward calculations are carried outonline, i.e. without look-up tables. The retrieval uses a Tikhonovregularisation combined with an approximate L-curve criterion and empiricala priori information from the retrieval sequence based on previous validsolutions. The consistency of the approach is demonstrated in selected modelcase studies assuming a polluted urban scenario and westward viewingdirection of the instrument. It is shown that a dynamic choice of theregularisation parameter is crucial for high aerosol load and large diurnalvariations. The quality of the retrieval can be improved significantly ifthe retrieval sequence and thus the a priori is chosen according to theinformation content of the measurement series. Additional intensities improvethe solution for all solar angles if suitably weighted. This flexibleretrieval algorithm allows for reconstruction of aerosol profiles in the boundarylayer for a wide range of viewing directions and extinctions. It can thus beapplied to observational geometries scanning the sky in two angulardimensions and to retrieve further aerosol optical parameters in the boundarylayer.

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