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

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