scholarly journals Particle settling and convective mixing in the Saharan Air Layer as seen from an integrated model, lidar, and in-situ perspective

2016 ◽  
Author(s):  
Josef Gasteiger ◽  
Silke Groß ◽  
Bernadett Weinzierl ◽  
Daniel Sauer ◽  
Volker Freudenthaler
Keyword(s):  
Dust Particles ◽  
Depolarization Ratio ◽  
Western Atlantic ◽  
Particle Counter ◽  
Convective Mixing ◽  
Saharan Air Layer ◽  
Lidar Measurements ◽  
Linear Depolarization Ratio ◽  
Aerosol Properties

Abstract. Long-range transport of aerosol in the Saharan Air Layer (SAL) across the Atlantic plays an important role for weather, climate, and ocean fertilization. However, processes occurring within the SAL and their effects on aerosol properties are still unclear. In this work we study particle settling and convective mixing within the SAL based on measured and modeled vertical aerosol profiles in the upper 1 km of the transported SAL. We use ground-based POLIS lidar measurements and airborne particle counter measurements over the Western Atlantic, as well as space-based CALIOP lidar measurements from Africa to the Western Atlantic. In our model we take account of the optical properties and the Stokes gravitational settling of irregularly-shaped Saharan dust particles. We test two hypotheses about the occurrence of convective mixing within the SAL over the Atlantic to explain the aerosol properties observed by the lidars and the particle counter. Our first hypothesis (H1) assumes that no mixing occurs in the SAL leading to an altitude separation of super-micron dust particles as a result of settling. The second hypothesis (H2) assumes that convective mixing occurs in the SAL during the day allowing large super-micron dust particles to stay airborne longer than without convective mixing. In general, a decrease of the particle linear depolarization ratio towards the SAL top is found in the measured lidar data but the decrease is much weaker than modeled in case of H1. The in-situ data on particle number concentrations show a presence of large particles near the SAL top that is inconsistent with H1. Furthermore, the analysis of the CALIOP measurements reveals that the average vertical profile of the linear depolarization ratio of the aerosols in the upper 1 km of the SAL does not change along its transport path over the Atlantic. These findings indicate H2 to be much more likely than H1, giving evidence that convective mixing occurs within the SAL over the Atlantic with significant consequences for the evolution of the size distribution of the super-micron dust particles during transport.

scholarly journals Particle settling and vertical mixing in the Saharan Air Layer as seen from an integrated model, lidar, and in situ perspective

2017 ◽  
Vol 17 (1) ◽  
pp. 297-311 ◽  
Author(s):  
Josef Gasteiger ◽  
Silke Groß ◽  
Daniel Sauer ◽  
Moritz Haarig ◽  
Albert Ansmann ◽  
...  
Keyword(s):  
Vertical Mixing ◽  
Saharan Dust ◽  
Dust Particles ◽  
Particle Settling ◽  
Western Atlantic ◽  
Particle Counter ◽  
Saharan Air Layer ◽  
Lidar Measurements ◽  
Ground Based Lidar

Abstract. Long-range transport of aerosol in the Saharan Air Layer (SAL) across the Atlantic plays an important role for weather, climate, and ocean fertilization. However, processes occurring within the SAL and their effects on aerosol properties are still unclear. In this work we study particle settling and vertical mixing within the SAL based on measured and modeled vertical aerosol profiles in the upper 1 km of the transported SAL. We use ground-based lidar measurements and airborne particle counter measurements over the western Atlantic, collected during the SALTRACE campaign, as well as space-based CALIOP lidar measurements from Africa to the western Atlantic in the summer season. In our model we take account of the optical properties and the Stokes gravitational settling of irregularly shaped Saharan dust particles.We test two hypotheses about the occurrence of vertical mixing within the SAL over the Atlantic to explain the aerosol profiles observed by the lidars and the particle counter. Our first hypothesis (H1) assumes that no mixing occurs in the SAL leading to a settling-induced separation of particle sizes. The second hypothesis (H2) assumes that vertical mixing occurs in the SAL allowing large super-micron dust particles to stay airborne longer than without mixing. The uncertainties of the particle linear depolarization ratio (δl) profiles measured by the ground-based lidars are comparable to the modeled differences between H1 and H2 and do not allow us to conclude which hypothesis fits better. The SALTRACE in situ data on size-resolved particle number concentrations show a presence of large particles near the SAL top that is inconsistent with H1. The analysis of the CALIOP measurements also reveals that the average δl profile over the western Atlantic is inconsistent with H1. Furthermore, it was found that the average δl profile in the upper 1 km of the SAL does not change along its transport path over the Atlantic. These findings give evidence that vertical mixing within the SAL is a common phenomenon with significant consequences for the evolution of the size distribution of super-micron dust particles during transport over the Atlantic. Further research is needed to precisely characterize the processes that are relevant for this phenomenon.

scholarly journals The potential of elastic and polarization lidars to retrieve extinction profiles

2020 ◽  
Vol 13 (2) ◽  
pp. 893-905 ◽  
Author(s):  
Elina Giannakaki ◽  
Panos Kokkalis ◽  
Eleni Marinou ◽  
Nikolaos S. Bartsotas ◽  
Vassilis Amiridis ◽  
...  
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Dust Particles ◽  
Depolarization Ratio ◽  
Lidar System ◽  
Lidar Measurements ◽  
Lidar Ratio ◽  
Linear Depolarization Ratio ◽  
Aerosol Types ◽  
532 Nm

Abstract. A new method, called ElEx (elastic extinction), is proposed for the estimation of extinction coefficient lidar profiles using only the information provided by the elastic and polarization channels of a lidar system. The method is applicable to lidar measurements both during daytime and nighttime under well-defined aerosol mixtures. ElEx uses the particle backscatter profiles at 532 nm and the vertically resolved particle linear depolarization ratio measurements at the same wavelength. The particle linear depolarization ratio and the lidar ratio values of pure aerosol types are also taken from literature. The total extinction profile is then estimated and compared well with Raman retrievals. In this study, ElEx was applied in an aerosol mixture of marine and dust particles at Finokalia station during the CHARADMExp campaign. Any difference between ElEx and Raman extinction profiles indicates that the nondust component could be probably attributed to polluted marine or polluted continental aerosols. Comparison with sun photometer aerosol optical depth observations is performed as well during daytime. Differences in the total aerosol optical depth are varying between 1.2 % and 72 %, and these differences are attributed to the limited ability of the lidar to correctly represent the aerosol optical properties in the near range due to the overlap problem.

scholarly journals CCN concentration and INP-relevant aerosol profiles in the Saharan Air Layer over Barbados from polarization lidar and airborne in situ measurements

2019 ◽  
Author(s):  
Moritz Haarig ◽  
Adrian Walser ◽  
Albert Ansmann ◽  
Maximilian Dollner ◽  
Dietrich Althausen ◽  
...  
Keyword(s):  
Surface Area ◽  
Cloud Condensation Nuclei ◽  
Number Concentration ◽  
In Situ Measurements ◽  
Saharan Air Layer ◽  
Lidar Measurements ◽  
Research Aircraft ◽  
Lidar Observations ◽  
Aerosol Properties

Abstract. The present study aims to validate lidar retrievals of cloud-relevant aerosol properties by using polarization lidar and coincident airborne in situ measurements in the Saharan Air Layer over the Barbados region. Vertical profiles of the number concentration of cloud condensation nuclei (CCN), large particles (diameter d > 500 nm), surface area, and ice nucleating particles (INP) are estimated from the lidar measurements and compared with CCN concentrations and the INP-relevant aerosol properties in situ measured with aircraft in the framework of the Saharan Aerosol Long-range Transport and Aerosol–Cloudinteraction Experiment (SALTRACE) in summer 2013. The CCN number concentrations derived from lidar observations were up to a factor of two higher than the ones measured in situ on board the research aircraft Falcon. However, a reasonable agreement was obtained when taking the lidar uncertainty into account. The number concentration of particles with dry radius > 250 nm and the surface area concentration obtained from the lidar observations and used as input for the INP parameterizations agreed well (

scholarly journals Benefit of depolarization ratio at λ = 1064 nm for the retrieval of the aerosol microphysics from lidar measurements

2014 ◽  
Vol 7 (11) ◽  
pp. 3773-3781 ◽  
Author(s):  
J. Gasteiger ◽  
V. Freudenthaler
Keyword(s):  
Optical Properties ◽  
Mass Concentration ◽  
Volcanic Ash ◽  
Single Scattering ◽  
Depolarization Ratio ◽  
Complex Relation ◽  
Microphysical Properties ◽  
Lidar Measurements ◽  
Increased Sensitivity ◽  
Linear Depolarization Ratio

Abstract. A better quantification of aerosol properties is required for improving the modelling of aerosol effects on weather and climate. This task is methodologically demanding due to the diversity of the microphysical properties of aerosols and the complex relation between their microphysical and optical properties. Advanced lidar systems provide spatially and temporally resolved information on the aerosol optical properties that is sufficient for the retrieval of important aerosol microphysical properties. Recently, the mass concentration of transported volcanic ash, which is relevant for the flight safety of aeroplanes, was retrieved from measurements of such lidar systems in southern Germany. The relative uncertainty of the retrieved mass concentration was on the order of ±50%. The present study investigates improvements of the retrieval accuracy when the capability of measuring the linear depolarization ratio at 1064 nm is added to the lidar setup. The lidar setups under investigation are based on those of MULIS and POLIS of the Ludwig-Maximilians-Universität in Munich (Germany) which measure the linear depolarization ratio at 355 and 532 nm with high accuracy. The improvements are determined by comparing uncertainties from retrievals applied to simulated measurements of this lidar setup with uncertainties obtained when the depolarization at 1064 nm is added to this setup. The simulated measurements are based on real lidar measurements of transported Eyjafjallajökull volcano ash. It is found that additional 1064 nm depolarization measurements significantly reduce the uncertainty of the retrieved mass concentration and effective particle size. This significant improvement in accuracy is the result of the increased sensitivity of the lidar setup to larger particles. The size dependence of the depolarization does not vary strongly with refractive index, thus we expect similar benefits for the retrieval in case of measurements of other volcanic ash compositions and also for transported desert dust. For the retrieval of the single scattering albedo, which is relevant to the radiative transfer in aerosol layers, no significant improvements were found.

scholarly journals Benefit of depolarization ratio at λ = 1064 nm for the retrieval of the aerosol microphysics from lidar measurements

2014 ◽  
Vol 7 (5) ◽  
pp. 5095-5115
Author(s):  
J. Gasteiger ◽  
V. Freudenthaler
Keyword(s):  
Optical Properties ◽  
Mass Concentration ◽  
Single Scattering ◽  
Sufficient Information ◽  
Depolarization Ratio ◽  
Aerosol Composition ◽  
Microphysical Properties ◽  
Lidar Measurements ◽  
Increased Sensitivity ◽  
Linear Depolarization Ratio

Abstract. A better quantification of aerosol microphysical and optical properties is required to improve the modelling of aerosol effects on weather and climate. This task is methodologically demanding due to the huge diversity of aerosol composition and of their shape and size distribution, and due to the complexity of the relation between the microphysical and optical properties. Lidar remote sensing is a valuable tool to gain spatially and temporally resolved information on aerosol properties. Advanced lidar systems provide sufficient information on the aerosol optical properties for the retrieval of important aerosol microphysical properties. Recently, the mass concentration of transported volcanic ash, which is relevant for the flight safety of airplanes, was retrieved from measurements of such lidar systems in Southern Germany. The relative uncertainty of the retrieved mass concentration was on the order of ±50%. The present study investigates improvements of the retrieval accuracy when the capability of measuring the linear depolarization ratio at 1064 nm is added to the lidar setup. The lidar setups under investigation are based on the setup of MULIS and POLIS of the LMU in Munich which measure the linear depolarization ratio at 355 nm and 532 nm with high accuracy. By comparing results of retrievals applied to simulated lidar measurements with and without the depolarization at 1064 nm it is found that the availability of 1064 nm depolarization measurements reduces the uncertainty of the retrieved mass concentration and effective particle size by a factor of about 2–3. This significant improvement in accuracy is the result of the increased sensitivity of the lidar setup to larger particles. However, the retrieval of the single scattering albedo, which is relevant for the radiative transfer in aerosol layers, does hardly benefit from the availability of 1064 nm depolarization measurements.

scholarly journals Dual-wavelength linear depolarization ratio of volcanic aerosols: Lidar measurements of the Eyjafjallajökull plume over Maisach, Germany

2012 ◽  
Vol 48 ◽  
pp. 85-96 ◽  
Author(s):  
Silke Groß ◽  
Volker Freudenthaler ◽  
Matthias Wiegner ◽  
Josef Gasteiger ◽  
Alexander Geiß ◽  
...  
Keyword(s):  
Dual Wavelength ◽  
Depolarization Ratio ◽  
Lidar Measurements ◽  
Volcanic Aerosols ◽  
Linear Depolarization Ratio

scholarly journals Monitoring atmospheric particulate matters using vertically resolved measurements of a polarization lidar, in-situ recordings and satellite data over Tehran, Iran

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Hossein Panahifar ◽  
Ruhollah Moradhaseli ◽  
Hamid Reza Khalesifard
Keyword(s):  
Boundary Layer ◽  
Atmospheric Aerosols ◽  
Urban Pollution ◽  
Ground Level ◽  
Dust Particles ◽  
Depolarization Ratio ◽  
Backscatter Coefficient ◽  
Dust Events ◽  
The City

AbstractThe highly polluted atmosphere above Tehran has been investigated by using a polarization lidar operating at 532 nm, in-situ particulate matter suites distributed over the city, and meteorological observations. The measurement campaign is conducted from Nov. 2014 to Jan. 2016. Three typical cases are studied in detail where, the atmosphere is polluted with urban pollution, mixture of urban pollution and dust particles from local sources, and long range transported dust from the Arabian Peninsula. For these cases, vertical profiles of the lidar backscatter coefficient, extinction coefficients, particle depolarization ratio ($$\delta _{\text {p}}$$ δ p ) and mass concentrations of atmospheric aerosols (separated into dust and non-dust particles) are presented. Using the lidar recordings, variations of the planetary boundary layer height above the city are investigated along the year. During November to February, lidar profiles frequently show polluted boundary layers that are reaching up to 1 km above the ground level. The depolarization ratio ($$\delta _{\text {p}}$$ δ p ) varies between 0.04 and 0.08 in the polluted boundary layer. During the campaign, for 103 days the urban pollution was dominant, 45 recorded dust events ($$0.15<\delta _{\text {p}}<0.20$$ 0.15 < δ p < 0.20 ) were originated from the dry regions in the south of Tehran and 15 dust events ($$0.20<\delta _{\text {p}}<0.35$$ 0.20 < δ p < 0.35 ) impacted the city that were originated from the Arabian Desert and Mesopotamia.

scholarly journals Triple-wavelength depolarization-ratio profiling of Saharan dust over Barbados during SALTRACE in 2013 and 2014

2017 ◽  
Vol 17 (17) ◽  
pp. 10767-10794 ◽  
Author(s):  
Moritz Haarig ◽  
Albert Ansmann ◽  
Dietrich Althausen ◽  
André Klepel ◽  
Silke Groß ◽  
...  
Keyword(s):  
United States ◽  
Long Range ◽  
The United States ◽  
Saharan Dust ◽  
Dust Particles ◽  
Depolarization Ratio ◽  
Dust Sources ◽  
The Caribbean ◽  
Linear Depolarization Ratio ◽  
June July

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.

scholarly journals Measurement report: Balloon-borne in-situ profiling of Saharan dust over Cyprus with the UCASS optical particle counter

2020 ◽  
Author(s):  
Maria Kezoudi ◽  
Matthias Tesche ◽  
Helen Smith ◽  
Alexandra Tsekeri ◽  
Holger Baars ◽  
...  
Keyword(s):  
Remote Sensing ◽  
Extinction Coefficient ◽  
Particle Number ◽  
Saharan Dust ◽  
Dust Particles ◽  
Particle Counter ◽  
Diameter Range ◽  
Optical Particle Counter ◽  
Total Particle

Abstract. This paper presents measurements of mineral dust concentration in the diameter range from 0.4 to 14.0 μm with a novel balloon-borne optical particle counter, the Universal Cloud and Aerosol Sounding System (UCASS). The balloon launches were coordinated with ground-based active and passive remote-sensing observations and airborne in-situ measurements with a research aircraft during a Saharan dust outbreak over Cyprus from 20 to 23 April 2017. The aerosol optical depth at 500 nm reached values up to 0.5 during that event over Cyprus and particle number concentrations were as high as 50 cm−3 for the diameter range between 0.8 and 13.9 μm. Comparisons of the total particle number concentration and the particle size distribution from two cases of balloon-borne measurements with aircraft observations show reasonable agreement in magnitude and shape despite slight mismatches in time and space. While column-integrated size distributions from balloon-borne measurements and ground-based remote sensing show similar coarse-mode peak concentrations and diameters, they illustrate the ambiguity related to the missing vertical information in passive sun photometer observations. Extinction coefficient inferred from the balloon-borne measurements agrees with those derived from coinciding Raman lidar observations at height levels with particle number concentrations smaller than 10 cm−3 for the diameter range from 0.8 to 13.9 μm. An overestimation of the extinction coefficient of a factor of two was found for layers with particle number concentrations that exceed 25 cm−3. This is likely the result of a variation in the refractive index, the shape- and size-dependency of the extinction efficiency of dust particles along the UCASS measurements.

scholarly journals First triple-wavelength lidar observations of depolarization and extinction-to-backscatter ratios of Saharan dust

2021 ◽  
Author(s):  
Moritz Haarig ◽  
Albert Ansmann ◽  
Ronny Engelmann ◽  
Holger Baars ◽  
Dietrich Althausen ◽  
...  
Keyword(s):  
Saharan Dust ◽  
Depolarization Ratio ◽  
Spectral Slope ◽  
Desert Dust ◽  
Lidar Measurements ◽  
Lidar Ratio ◽  
Linear Depolarization Ratio ◽  
First Time ◽  
Dust Case ◽  
Lidar Observations

Abstract. Two Saharan dust layers observed over Leipzig in February and March 2021 were used to provide the first ever lidar measurements of the extinction coefficient at 1064 nm for desert dust. The advanced multiwavelength Raman polarization lidar was able to provide, for the first time, the lidar ratio (extinction-to-backscatter ratio) and particle linear depolarization ratio at all three classical lidar wavelengths (355, 532 and 1064 nm). The pure dust conditions during the first event exhibit lidar ratios of 47±8, 50±5 and 63±13 sr and particle linear depolarization ratios of 0.260±0.026, 0.298±0.017 and 0.214±0.025 at the wavelengths of 355, 532 and 1064 nm, respectively. The second, slightly polluted dust case shows a similar spectral behavior with values of the lidar ratio of 52±8, 47±5 and 61±10 sr and the depolarization ratio of 0.188±0.053, 0.270±0.017 and 0.242±0.007 at 355, 532 and 1064 nm, respectively. The results were compared to AERONET v3 inversions and GRASP retrievals at six and seven wavelengths, which could reproduce the spectral slope of the lidar ratio from 532 to 1064 nm. The spectral slope of the particle linear depolarization ratio could not be reproduced by the AERONET inversions, especially at 1064 nm.

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