Abstract
Abstract. Triple-wavelength polarization lidar measurements in Saharan dust layers were performed at Barbados (13.1° N, 59.6° W), 5000–8000 km west of the Saharan dust sources, in the framework of the Saharan Aerosol Long-range Transport and Aerosol-Cloud-Interaction Experiment (SALTRACE-1, June–July 2013, SALTRACE-3, June–July 2014). Three case studies are discussed. High quality was achieved by comparing the dust linear depolarization ratio profiles measured at 355, 532, and 1064 nm with respective dual-wavelength (355, 532 nm) depolarization ratio profiles measured with a reference lidar. A unique case of long-range transported dust over more than 12 000 km is presented. Saharan dust plumes crossing Barbados were measured with an airborne triple-wavelength polarization lidar over Missouri in the midwestern United States 7 days later. Similar dust optical properties and depolarization features were observed over both sites indicating almost unchanged dust properties within this 1 week of travel from the Caribbean to the United States. The main results of the triple-wavelength polarization lidar observations in the Caribbean in the summer seasons of 2013 and 2014 are summarized. On average, the particle linear depolarization ratios for aged Saharan dust were found to be 0.252 ± 0.030 at 355 nm, 0.280 ± 0.020 at 532 nm, and 0.225 ± 0.022 at 1064 nm after approximately 1 week of transport over the tropical Atlantic. Based on published simulation studies we present an attempt to explain the spectral features of the depolarization ratio of irregularly shaped mineral dust particles, and conclude that most of the irregularly shaped coarse-mode dust particles (particles with diameters > 1 µm) have sizes around 1.5–2 µm. The SALTRACE results are also set into the context of the SAMUM-1 (Morocco, 2006) and SAMUM-2 (Cabo Verde, 2008) depolarization ratio studies. Again, only minor changes in the dust depolarization characteristics were observed on the way from the Saharan dust sources towards the Caribbean.
Highlights
Mineral dust belongs to the major natural atmospheric aerosol components and influences weather and climate, visibility, air quality, and human health
The main goal of the paper is to present for the first time ground-based triple-wavelength polarization/Raman lidar observations of the depolarization ratio of Saharan dust after long-range transport and to provide a high-quality statistical data set of dust
We found good agreement between the two depolarization data sets collected with particle depolarization ratios were 0.28 (POLIS) (Groß et al, 2015) and BERTHA for July 2013
Summary
Mineral dust belongs to the major natural atmospheric aerosol components and influences weather and climate, visibility, air quality, and human health. A variety of features of the impact of dust on (climate-relevant) atmospheric processes are not well understood or represented in atmospheric models, and need to be explored, preferably in comprehensive field campaigns such as Aerosol Characterization Experiment ACE-Asia (Huebert et al, 2003; Shimizu et al, 2004), the Puerto Rico Dust Experiment PRIDE (Colarco et al, 2003; Reid et al, 2003), the Saharan Dust Experiment SHADE (Tanré et al, 2003), the Saharan Mineral Dust Experiments SAMUM-1 (Heintzenberg, 2009) and SAMUM-2 (Ansmann et al, 2011), the Dust and Biomass-burning Experiment DABEX (Haywood et al, 2008), the Dust Outflow and Deposition to the Ocean project DODO (McConnell et al, 2008), the Geostationary Earth Radiation Budget Intercomparisons of Long-wave and Short-wave radiation GERBILS (Johnson and Osborne, 2011), Fennec (Ryder et al, 2013), the Saharan Aerosol Long-Range Transport and Aerosol-Cloud-Interaction Experiment SALTRACE (Weinzierl et al, 2017), and the Study of Saharan Dust Over West Africa SHADOW (Veselovskii et al, 2016). Based on SALTRACE triple-wavelength polarization measurements, an investigation is made into which of the three wavelengths this new method works best with and results in the lowest uncertainties (Mamouri and Ansmann, 2017)
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