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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 20 Mar 2019

Research article | 20 Mar 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

3 + 2 + X: What is the most useful depolarization input for retrieving microphysical properties of non-spherical particles from lidar measurements by assuming spheroidal particle shapes?

Matthias Tesche1,a, Alexei Kolgotin2, Moritz Haarig3, Sharon P. Burton4, Richard A. Ferrare4, Chris A. Hostetler4, and Detlef Müller1 Matthias Tesche et al.
  • 1School of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield, UK
  • 2A. M. Prokhorov General Physics Institute, Moscow, Russia
  • 3Leibniz Institute for Tropospheric Research (TROPOS), Leipzig, Germany
  • 4NASA Langley Research Center, Hampton, USA
  • anow at: Leipzig Institute for Meteorology (LIM), Leipzig University, Leipzig, Germany

Abstract. The typical multiwavelength aerosol lidar data set for inversion of optical to microphysical parameters is composed of three backscatter coefficients (β) at 355, 532, and 1064 nm and two extinction coefficients (α) at 355 and 532 nm. This data combination is referred to as 3β + 2α or 3 + 2 data set. This set of data is sufficient for retrieving some important microphysical particle parameters if the particles have spherical shape. Here, we investigate the effect of including the particle linear depolarization ratio (δ) as a third input parameter to the inversion of lidar data. The inversion algorithm is generally not used if measurements show values of δ that exceed 0.10 at 532 nm, i.e. in the presence of non-spherical particles such as desert dust, volcanic ash, and under special circumstances biomass-burning smoke. We use experimental data collected with instruments that are capable of measuring δ at all three lidar wavelengths with an inversion routine that uses the theory of light scattering by randomly oriented spheroids to replicate scattering properties of non-spherical particles. This is the first systematic test of the effect of using all theoretically possible combinations of δ taken at 355, 532, and 1064 nm as input in the lidar data inversion.

We find that depolarization information at least at one wavelength already provides useful information in the inversion of optical data that describe light-scattering by non-spherical particles. However, any choice of δλ will give lower values of the single-scattering albedo than the traditional 3 + 2 data set. We find that input data sets that include δ355 give a non-spherical fraction that closely resembles the dust ratio we obtain from using β532 and δ532 in a methodology applied in aerosol-type separation. The use of δ355 in data sets of two or three δλ reduces the fraction of non-spherical particles that is retrieved when using δ532 and δ1064. Use of the latter two without accounting for δ355 generally leads to high fractions of non-spherical particles that we consider not trustworthy. The use of three δλ instead of two δλ including the constraint that one of these is measured at 355 nm does not provide any advantage over using 3 + 2 + δ355. We conclude that — depending on measurement capability — the future standard input for inversion using spheroid kernels might be 3 + 2 + δ355 or 3 + 2 + δ355 + δ532.

Matthias Tesche et al.
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Matthias Tesche et al.
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Publications Copernicus
Short summary
Today, few lidars are capable of triple-wavelengths particle linear depolarisation ratio (PLDR) measurements. This study is the first systematic investigation of the effect of different choice of PLDR input on the inversion of lidar measurements of mineral dust and dusty mixtures using light-scattering by randomly oriented spheroids. We provide recommendations on the most suitable input parameters for use with the applied methodology based on a relational assessment of the inversion output.
Today, few lidars are capable of triple-wavelengths particle linear depolarisation ratio (PLDR)...