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. We present a new MAX-DOAS profiling algorithm for aerosols and trace gases,BOREAS, which utilizes an iterative solution method including Tikhonovregularization and the optimal estimation technique. The aerosol profileretrieval is based on a novel approach in which the absorption depth of O4 is directly used in order to retrieve extinction coefficientprofiles instead of the commonly used perturbation theory method. Theretrieval of trace gases is done with the frequently used optimal estimationmethod but significant improvements are presented on how to deal with wronglyweighted a priori constraints and for scenarios in which the a priori profileis inaccurate. Performance tests are separated into two parts. First, we address the generalsensitivity of the retrieval to the example of synthetic data calculated withthe radiative transfer model SCIATRAN. In the second part of the study, wedemonstrate BOREAS profiling accuracy by validating the results with the help ofancillary measurements carried out during the CINDI-2 campaign in Cabauw, theNetherlands, in 2016. The synthetic sensitivity tests indicate that the regularization betweenmeasurement and a priori constraints is insufficient when knowledge of thetrue state of the atmosphere is poor. We demonstrate a priori pre-scaling andextensive regularization tests as a tool for the optimization of retrievedprofiles. The comparison of retrieval results with in situ, ceilometer, NO2 lidar, sonde and long-path DOAS measurements during the CINDI-2campaign always shows high correlations with coefficients greater than 0.75.The largest differences can be found in the morning hours, when the planetaryboundary layer is not yet fully developed and the concentration of tracegases and aerosol, as a result of a low night-time boundary layer havingformed, is focused in a shallow, near-surface layer.

BOREAS – a new MAX-DOAS profile retrieval algorithm for aerosols and trace gases