Saharan dust contribution to the Caribbean summertime boundary layer – a lidar study during SALTRACE

Silke Groß,Thomas Müller,Daniel Sauer,Albert Ansmann,Volker Freudenthaler,Carlos Toledano,Josef Gasteiger

doi:10.5194/acp-16-11535-2016

Abstract

Abstract. Dual-wavelength lidar measurements with the small lidar system POLIS of the Ludwig-Maximilians-Universität München were performed during the SALTRACE experiment at Barbados in June and July 2013. Based on high-accuracy measurements of the linear depolarization ratio down to about 200 m above ground level, the dust volume fraction and the dust mass concentration within the convective marine boundary layer can be derived. Additional information from radiosonde launches at the ground-based measurement site provide independent information on the convective marine boundary layer height and the meteorological situation within the convective marine boundary layer. We investigate the lidar-derived optical properties, the lidar ratio and the particle linear depolarization ratio at 355 and 532 nm and find mean values of 0.04 (SD 0.03) and 0.05 (SD 0.04) at 355 and 532 nm, respectively, for the particle linear depolarization ratio, and (26 ± 5) sr for the lidar ratio at 355 and 532 nm. For the concentration of dust in the convective marine boundary layer we find that most values were between 20 and 50 µgm−3. On most days the dust contribution to total aerosol volume was about 30–40 %. Comparing the dust contribution to the column-integrated sun-photometer measurements we see a correlation between high dust contribution, high total aerosol optical depth and a low Angström exponent, and of low dust contribution with low total aerosol optical depth.

Highlights

Saharan dust is one of the primary components of the global aerosol load (Forster and et al, 2007; Haywood and Boucher, 2000) with an estimated annual emission of more than 1000 Mt (Duce et al, 1991)
During this time the aerosol situation above Barbados was characterized by an aerosol optical depth (AOD) which mainly ranged between about 0.2 and 0.4 almost wavelength-independent for the CIMEL measurements at 340, 500 and 1020 nm (Fig. 1)
With the method applied in this study it is possible to characterize the different layers within the atmospheric column, and to derive the contributions of the different aerosol types to aerosol mixtures

Summary

Introduction

Saharan dust is one of the primary components of the global aerosol load (Forster and et al, 2007; Haywood and Boucher, 2000) with an estimated annual emission of more than 1000 Mt (Duce et al, 1991). Turbulent downward mixing of dust in the convective marine boundary layer (CMBL) over the tropical Atlantic is assumed to be an efficient dust removal process. To support modeling efforts to simulate dust long-range transport and removal processes accurately and to validate the model output, a high-quality vertically resolved characterization of the optical and microphysical particle properties in the convective marine boundary layer and in the main dust layer (Saharan Air Layer, SAL) is essential. SAL properties found during SALTRACE have already been presented by Groß et al (2015), here we present the properties for the CMBL. Further papers will be presented in the framework of this SALTRACE special issue that will partly deal with the removal of dust by turbulent downward mixing (Rittmeister et al, 2016; Marinou et al, 2016)

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Conclusion