Abstract

Abstract. Measurements performed in western Africa (Senegal) during the SHADOW field campaign are analyzed to show that spectral dependence of the imaginary part of the complex refractive index (CRI) of dust can be revealed by lidar-measured particle parameters. Observations in April 2015 provide good opportunity for such study, because, due to high optical depth of the dust, exceeding 0.5, the extinction coefficient could be derived from lidar measurements with high accuracy and the contribution of other aerosol types, such as biomass burning, was negligible. For instance, in the second half of April 2015, AERONET observations demonstrated a temporal decrease in the imaginary part of the CRI at 440 nm from approximately 0.0045 to 0.0025. This decrease is in line with a change in the relationship between the lidar ratios (the extinction-to-backscattering ratio) at 355 and 532 nm (S355 and S532). For instance in the first half of April, S355∕S532 is as high as 1.5 and the backscatter Ångström exponent, Aβ, is as low as −0.75, while after 15 April S355/S532=1.0 and Aβ is close to zero. The aerosol depolarization ratio δ532 for the whole of April exceeded 30 % in the height range considered, implying that no other aerosol, except dust, occurred. The performed modeling confirmed that the observed S355∕S532 and Aβ values match the spectrally dependent imaginary part of the refractive index as can be expected for mineral dust containing iron oxides. The second phase of the SHADOW campaign was focused on evaluation of the lidar ratio of smoke and estimates of its dependence on relative humidity (RH). For five studied smoke episodes the lidar ratio increases from 44±5 to 66±7 sr at 532 nm and from 62±6 to 80±8 sr at 355 nm, when RH varied from 25 % to 85 %. Performed numerical simulations demonstrate that observed ratio S355∕S532, exceeding 1.0 in the smoke plumes, can indicate an increase in the imaginary part of the smoke particles in the ultraviolet (UV) range.

Highlights

  • Atmospheric dust has significant impact on the Earth’s climate system, and this impact remains highly uncertain (IPCC, 2013)

  • The backscattering coefficient and so the lidar ratio are sensitive to the imaginary part of the complex refractive index (CRI)

  • The S355/S532 ratio can be an indicator of the imaginary refractive index enhancement in the UV

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Summary

Introduction

Atmospheric dust has significant impact on the Earth’s climate system, and this impact remains highly uncertain (IPCC, 2013). The desert dust in source regions is sometimes qualified as “pure dust”, it is always a mixture of various elements, e.g., iron oxides, clays, quartz and calciumrich species, whose proportions can vary (Sokolik and Toon, 1999; Kandler et al, 2011; Wagner et al, 2012; Di Biagio et al, 2017, 2019, and references therein). The dust optical properties, and the lidar ratio (S), can vary, depending on the relative abundance of various minerals in emission sources. The imaginary part of the complex refractive index (CRI) of different minerals can vary spectrally and often exhibits an increase in the UV spectral region for dust containing iron oxides. The retrieval of the dust extinction profiles from elastic backscatter lidar observations should account for the spectral variation in the lidar ratio

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