Information content and aerosol property retrieval potential for different types of in situ polar nephelometer data
Authors: Moallemi Alireza , Modini Rob L. , Lapyonok Tatyana , Lopatin Anton , Fuertes David , Dubovik Oleg , Giaccari Philippe , Gysel-Beer Martin 
Published: October 2022

Abstract. Polar nephelometers are in situ instruments used to measure the angular
distribution of light scattered by aerosol particles. These types of
measurements contain substantial information about the properties of the
aerosol being probed (e.g. concentrations, sizes, refractive indices, shape
parameters), which can be retrieved through inversion algorithms. The
aerosol property retrieval potential (i.e. information content) of a given
set of measurements depends on the spectral, polarimetric, and angular
characteristics of the polar nephelometer that was used to acquire the measurements. To
explore this issue quantitatively, we applied Bayesian information content
analysis and calculated the metric degrees of freedom for signal (DOFS) for a range of simulated polar nephelometer instrument configurations, aerosol models and test cases, and
assumed levels of prior knowledge about the variances of specific aerosol
properties. Assuming a low level of prior knowledge consistent with an
unconstrained ambient/field measurement setting, we demonstrate that even
very basic polar nephelometers (single wavelength, no polarization
capability) will provide informative measurements with a very high retrieval
potential for the size distribution and refractive index state parameters
describing simple unimodal, spherical test aerosols. As expected, assuming a
higher level of prior knowledge consistent with well-constrained laboratory
applications leads to a reduction in potential for information gain via
performing the polarimetric measurement. Nevertheless, we show that in this
situation polar nephelometers can still provide informative measurements:
e.g. it can be possible to retrieve the imaginary part of the refractive
index with high accuracy if the laboratory setting makes it possible to
keep the probed aerosol sample simple. The analysis based on a high level of
prior knowledge also allows us to better assess the impact of different
polar nephelometer instrument design features in a consistent manner for
retrieved aerosol parameters. The results indicate that the addition of
multi-wavelength and/or polarimetric measurement capabilities always leads
to an increase in information content, although in some cases the increase
is negligible, e.g. when adding a fourth, near-IR measurement wavelength for
the retrieval of unimodal size distribution parameters or if the added
polarization component has high measurement uncertainty. By considering a
more complex bimodal, non-spherical-aerosol model, we demonstrate that
performing more comprehensive spectral and/or polarimetric measurements
leads to very large benefits in terms of the achieved information content.
We also investigated the impact of angular truncation (i.e. the loss of
measurement information at certain scattering angles) on information
content. Truncation at extreme angles (i.e. in the near-forward or
near-backward directions) results in substantial decreases in information
content for coarse-aerosol test cases. However for fine-aerosol test cases,
the sensitivity of DOFS to extreme-angle truncation is noticeably smaller and
can be further reduced by performing more comprehensive measurements.
Side angle truncation has very little effect on information content for both
the fine and coarse test cases. Furthermore, we demonstrate that increasing
the number of angular measurements generally increases the information
content. However, above a certain number of angular measurements
(∼20–40) the observed increases in DOFS plateau out. Finally, we
demonstrate that the specific placement of angular measurements within a
nephelometer can have a large impact on information content. As a
proof of concept, we show that a reductive greedy algorithm based on the
DOFS metric can be used to find optimal angular configurations for given target
aerosols and applications.