## Abstract

Properly interpreting lidar (light detection and ranging) signal for
characterizing particle distribution relies on a key parameter, ${\chi _p}(\pi
)$, which relates the particulate volume
scattering function (VSF) at 180° (${\beta _p}(\pi
)$) that a lidar measures to the
particulate backscattering coefficient (${b_\textit{bp}}$). However, ${\chi _p}(\pi
)$ has been seldom studied due to
challenges in accurately measuring ${\beta _p}(\pi
)$ and ${b_\textit{bp}}$ concurrently in the field. In this
study, ${\chi _p}(\pi
)$, as well as its spectral dependence,
was re-examined using the VSFs measured *in
situ* at high angular resolution in a wide range of waters. ${\beta _p}(\pi
)$, while not measured directly, was
inferred using a physically sound, well-validated VSF-inversion
method. The effects of particle shape and internal structure on the
inversion were tested using three inversion kernels consisting of
phase functions computed for particles that are assumed as homogenous
sphere, homogenous asymmetric hexahedra, or coated sphere. The
reconstructed VSFs using any of the three kernels agreed well with the
measured VSFs with a mean percentage difference $ \lt {5}\% $ at scattering angles $ \lt {170}^\circ
$. At angles immediately near or equal
to 180°, the reconstructed ${\beta _p}(\pi
)$ depends strongly on the inversion
kernel. ${\chi _p}(\pi
)$ derived with the sphere kernels was
smaller than those derived with the hexahedra kernel but consistent
with ${\chi _p}(\pi
)$ estimated directly from
high-spectral-resolution lidar and *in
situ* backscattering sensor. The possible explanation was that
the sphere kernels are able to capture the backscattering enhancement
feature near 180° that has been observed for marine particles. ${\chi _p}(\pi
)$ derived using the coated sphere
kernel was generally lower than those derived with the homogenous
sphere kernel. Our result suggests that ${\chi _p}(\pi
)$ is sensitive to the shape and
internal structure of particles and significant error could be induced
if a fixed value of ${\chi _p}(\pi
)$ is to be used to interpret lidar
signal collected in different waters. On the other hand, ${\chi _p}(\pi
)$ showed little spectral
dependence.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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