Abstract:Abstract. The Saharan Air Layer (SAL) influences largescale environment from western Africa to eastern tropical Americas, by carrying large amounts of dust aerosols. However, the vertical distribution of the SAL is not well established due to a lack of systematic measurements away from the continents. This can be overcome by using the observations of the spaceborne lidar CALIOP onboard the satellite CALIPSO. By taking advantage of CALIOP's capability to distinguish dust aerosols from other types of aerosols th… Show more
“…Systematic studies of the east-to-west dust transport with the satellite lidar CALIOP (Cloud and Aerosol Lidar with Orthogonal Polarization) were then presented by Liu et al (2008a, b). Further Saharan dust studies over the tropical Atlantic based on CALIOP measurements can be found in Adams et al (2012) and Tsamalis et al (2013). The latter authors characterized the decay of the Saharan dust amount in terms of layer descent and deposition velocity.…”
Section: Saltrace R/v Meteor Cruise and Instrumentationmentioning
Abstract. We present final and quality-assured results of multiwavelength polarization/Raman lidar observations of the Saharan air layer (SAL) over the tropical Atlantic. Observations were performed aboard the German research vessel R/V Meteor during the 1-month transatlantic cruise from Guadeloupe to Cabo Verde over 4500 km from 61.5 to 20 • W at 14-15 • N in April-May 2013. First results of the shipborne lidar measurements, conducted in the framework of SALTRACE (Saharan Aerosol Long-range Transport and Aerosol-Cloud Interaction Experiment), were reported by Kanitz et al. (2014). Here, we present four observational cases representing key stages of the SAL evolution between Africa and the Caribbean in detail in terms of layering structures and optical properties of the mixture of predominantly dust and aged smoke in the SAL. We discuss to what extent the lidar results confirm the validity of the SAL conceptual model which describes the dust long-range transport and removal processes over the tropical Atlantic. Our observations of a clean marine aerosol layer (MAL, layer from the surface to the SAL base) confirm the conceptual model and suggest that the removal of dust from the MAL, below the SAL, is very efficient. However, the removal of dust from the SAL assumed in the conceptual model to be caused by gravitational settling in combination with large-scale subsidence is weaker than expected. To explain the observed homogenous (heightindependent) dust optical properties from the SAL base to the SAL top, from the African coast to the Caribbean, we have to assume that the particle sedimentation strength is reduced and dust vertical mixing and upward transport mechanisms must be active in the SAL. Based on lidar observations on 20 nights at different longitudes in May 2013, we found, on average, MAL and SAL layer mean values (at 532 nm) of the extinction-to-backscatter ratio (lidar ratio) of 17±5 sr (MAL) and 43 ± 8 sr (SAL), of the particle linear depolarization ratio of 0.025 ± 0.015 (MAL) and 0.19 ± 0.09 (SAL), and of the particle extinction coefficient of 67 ± 45 Mm −1 (MAL) and 68 ± 37 Mm −1 (SAL). The 532 nm optical depth of the lofted SAL was found to be, on average, 0.15 ± 0.13 during the ship cruise. The comparably low values of the SAL mean lidar ratio and depolarization ratio (compared to typical pure dust values of 50-60 sr and 0.3, respectively) in combination with backward trajectories indicate a smoke contribution to light extinction of the order of 20 % during May 2013, at the end of the burning season in central-western Africa.
“…Systematic studies of the east-to-west dust transport with the satellite lidar CALIOP (Cloud and Aerosol Lidar with Orthogonal Polarization) were then presented by Liu et al (2008a, b). Further Saharan dust studies over the tropical Atlantic based on CALIOP measurements can be found in Adams et al (2012) and Tsamalis et al (2013). The latter authors characterized the decay of the Saharan dust amount in terms of layer descent and deposition velocity.…”
Section: Saltrace R/v Meteor Cruise and Instrumentationmentioning
Abstract. We present final and quality-assured results of multiwavelength polarization/Raman lidar observations of the Saharan air layer (SAL) over the tropical Atlantic. Observations were performed aboard the German research vessel R/V Meteor during the 1-month transatlantic cruise from Guadeloupe to Cabo Verde over 4500 km from 61.5 to 20 • W at 14-15 • N in April-May 2013. First results of the shipborne lidar measurements, conducted in the framework of SALTRACE (Saharan Aerosol Long-range Transport and Aerosol-Cloud Interaction Experiment), were reported by Kanitz et al. (2014). Here, we present four observational cases representing key stages of the SAL evolution between Africa and the Caribbean in detail in terms of layering structures and optical properties of the mixture of predominantly dust and aged smoke in the SAL. We discuss to what extent the lidar results confirm the validity of the SAL conceptual model which describes the dust long-range transport and removal processes over the tropical Atlantic. Our observations of a clean marine aerosol layer (MAL, layer from the surface to the SAL base) confirm the conceptual model and suggest that the removal of dust from the MAL, below the SAL, is very efficient. However, the removal of dust from the SAL assumed in the conceptual model to be caused by gravitational settling in combination with large-scale subsidence is weaker than expected. To explain the observed homogenous (heightindependent) dust optical properties from the SAL base to the SAL top, from the African coast to the Caribbean, we have to assume that the particle sedimentation strength is reduced and dust vertical mixing and upward transport mechanisms must be active in the SAL. Based on lidar observations on 20 nights at different longitudes in May 2013, we found, on average, MAL and SAL layer mean values (at 532 nm) of the extinction-to-backscatter ratio (lidar ratio) of 17±5 sr (MAL) and 43 ± 8 sr (SAL), of the particle linear depolarization ratio of 0.025 ± 0.015 (MAL) and 0.19 ± 0.09 (SAL), and of the particle extinction coefficient of 67 ± 45 Mm −1 (MAL) and 68 ± 37 Mm −1 (SAL). The 532 nm optical depth of the lofted SAL was found to be, on average, 0.15 ± 0.13 during the ship cruise. The comparably low values of the SAL mean lidar ratio and depolarization ratio (compared to typical pure dust values of 50-60 sr and 0.3, respectively) in combination with backward trajectories indicate a smoke contribution to light extinction of the order of 20 % during May 2013, at the end of the burning season in central-western Africa.
“…In boreal summer, SAL is characterized by hot, dry, very dust-laden air, and it is located between 10 and 25 • N (Dunion and Marron, 2008;Tsamalis et al, 2013). This SAL is marked by very strong potential temperatures up to 40 • C and a radon presence ( 222 radon) indicating the desert origin of air masses (Carlson and Prospero, 1972).…”
Section: Introductionmentioning
confidence: 99%
“…In winter, the SAL is characterized by the transport of dust containing chemical elements such as aluminum (Al), silicon (Si), iron (Fe), titanium (Ti), and manganese (Mn; e.g., Formenti et al, 2001;Ben-Ami et al, 2010) and is located between 5 and 10 • N (e.g., Tsamalis et al, 2013). Some studies relating aerosols to their transport are generally a simple description of the vertical distribution of aerosols in the SAL (Generoso et al, 2008;Liu et al, 2008;Ben-Ami et al, 2009;Braun, 2010;Yu et al, 2010;Adams et al, 2012;Ridley et al, 2012;Yang et al, 2012) or a description of the seasonality of the SAL in connection with large-scale dynamics (Liu et al, 2012;Tsamalis et al, 2013).…”
“…During the summer months, the westward moving SAL typically occurs 5 at relatively high altitudes (1-6km) in the Cabo Verde area (Tsamalis et al, 2013), and the warm, dry, dust-laden air creates a stable layer that is separated from the lower levels by a strong temperature inversion (Wong et al, 2009). …”
Section: Potential Source Areas and Transport Historymentioning
Abstract. The mineralogy and mixing state of dust particles originating from the African continent influences climate and marine ecosystems in the North Atlantic due to its effect on radiation, cloud properties and biogeochemical cycling. Single-particle mineralogy and mixing state is particularly important in many processes but is difficult to predict because of large temporal and spatial variability and the lack of in-situ measurements of dust properties during emission, transport and deposition. This lack of measurements is in part due to the remoteness of potential source areas (PSA) and transport pathways, but also because of 5 the lack of an efficient method to report the mineralogy and mixing state of single particles with a time resolution comparable to atmospheric processes.In this work, the mineralogy and mixing state of the fine fraction (< 2.5µm) in laboratory suspended dust from the Sahara and Sahel were made using novel techniques with on-line single-particle mass spectrometry (SPMS) and traditional off-line scanning electron microscopy (SEM). A regional difference in mineralogy was detected, with material sourced from Morocco 10 contained a high number fraction of illite like particles in contrast to Sahelian material which contains potassium and sodium depleted clay minerals like kaolinite. Applying the same methods to ambient measurement of transported dust in the marine boundary layer at Cabo Verde in the remote North Atlantic enabled the number fractions of illite/smectite clay mineral (ISCM), non-ISCM, and calcium containing particles to be reported at a 1 hour time resolution over a 20 day period alongside internal mixing with nitrate, sulphate and organic/biological material. The ISCM and nitrate content was found to change significantly 15 between distinct dust events, indicating a shift in source and transport pathways which may not be captured in off-line composition analysis or remote sensing techniques.The results show SPMS and SEM techniques are complimentary and demonstrate that SPMS can provide a meaningful high resolution measurement of single-particle mineralogy and mixing state in laboratory and ambient conditions. In most cases, the mineralogy varies continuously between particles rather than a collection of discrete mineral phases. These techniques 20 will be useful in resolving the complexity of mineral dust transport and in obtaining atmospherically relevant test material for laboratory experiments of dust properties.
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