Tesche, Matthias; Kolgotin, Alexei; Haarig, Moritz; Burton, Sharon P.; Ferrare, Richard A.; Hostetler, Chris A.; Müller, Detlef

The typical multiwavelength aerosol lidar data set for inversion of optical to microphysical parameters is composed of three backscatter coefficients (inline-formulaβ) at 355, 532, and 1064 nm and two extinction coefficients (inline-formulaα) at 355 and 532 nm. This data combination is referred to as a inline-formula3β+2α or inline-formula3+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 (inline-formulaδ) as a third input parameter for the inversion of lidar data. The inversion algorithm is generally not used if measurements show values of inline-formulaδ 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 inline-formulaδ at all three lidar wavelengths with an inversion routine that applies the spheroidal light-scattering model of citxref_text.1#bib1.bibx10Dubovik et al. (#bib1.bibx102006) with a fixed axis-ratio distribution to replicate scattering properties of non-spherical particles. The inversion gives the fraction of spheroids required to replicate the optical data as an additional output parameter. This is the first systematic test of the effect of using all theoretically possible combinations of inline-formulaδ taken at 355, 532, and 1064 nm as input in the lidar data inversion.

We find that depolarization information of at least one wavelength already provides useful information for the inversion of optical data that have been collected in the presence of non-spherical mineral dust particles. However, any choice of inline-formulaδλ will give lower values of the single-scattering albedo than the traditional inline-formula3+2 data set. We find that input data sets that include inline-formulaδ355 give a spheroid fraction that closely resembles the dust ratio we obtain from using inline-formulaβ532 and inline-formulaδ532 in a methodology applied in aerosol-type separation. The use of inline-formulaδ355 in data sets of two or three inline-formulaδλ reduces the spheroid fraction that is retrieved when using inline-formulaδ532 and inline-formulaδ1064. Use of the latter two parameters without accounting for inline-formulaδ355 generally leads to high spheroid fractions that we consider not trustworthy. The use of three inline-formulaδλ instead of two inline-formulaδλ, including the constraint that one of these is measured at 355 nm does not provide any advantage over using inline-formula M23inlinescrollmathml normal 3 + normal 2 + italic δ normal 355 57pt12ptsvg-formulamathimge1ad6e154e25dc9ac8b365bee39e44af amt-12-4421-2019-ie00004.svg57pt12ptamt-12-4421-2019-ie00004.png for the observations with varying contributions of mineral dust considered here. However, additional measurements at wavelengths different from 355 nm would be desirable for application to a wider range of aerosol scenarios that may include non-spherical smoke particles, which can have values of inline-formulaδ355 that are indistinguishable from those found for mineral dust. We therefore conclude that – depending on measurement capability – the future standard input for inversion of lidar data taken in the presence of mineral dust particles and using the spheroid model of citxref_text.2#bib1.bibx10Dubovik et al. (#bib1.bibx102006) might be inline-formula M25inlinescrollmathml normal 3 + normal 2 + italic δ normal 355 57pt12ptsvg-formulamathimg002a791052f30af7d77503a57a90d717 amt-12-4421-2019-ie00005.svg57pt12ptamt-12-4421-2019-ie00005.png or inline-formula M26inlinescrollmathml normal 3 + normal 2 + italic δ normal 355 + italic δ normal 532 88pt12ptsvg-formulamathimg9f571c36ca2327dd4bcb18d430f0038d amt-12-4421-2019-ie00006.svg88pt12ptamt-12-4421-2019-ie00006.png .

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Tesche, Matthias / Kolgotin, Alexei / Haarig, Moritz / et al: 3+2 + X: what is the most useful depolarization input for retrieving microphysical properties of non-spherical particles from lidar measurements using the spheroid model of Dubovik et al. (2006)?. 2019. Copernicus Publications.

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