76 research outputs found
Lidar-Messung der Extinktion des atmosphärischen Aerosols am Beispiel der Feldstudie SAMUM-1
Im Rahmen der vorliegenden Arbeit wurde ein spektral hochauflösendes Lidar (HSRL) aufgebaut und während des Feldexperiments SAMUM im Mai/Juni 2006 und im Januar/Februar 2008 an Bord des Forschungsflugzeugs Falcon betrieben.
Die Intensität von Lidar–Signalen wird maßgeblich durch die Rückstreuung und die Extinktion der atmosphärischen Teilchen beeinflusst. Dabei stehen die Rückstreuung und die Extinktion der Aerosole in keinem konstanten Verhältnis zueinander. Die Messgröße eines normalen Rückstreu–Lidars ist insofern mit zwei unbekannten Größen behaftet, weshalb die direkte Messung der Aerosolextinktion mit einem solchen Lidar nicht möglich ist.
Im Gegensatz dazu wird bei der Methode des HSRL neben der gesamten atmosphärischen Rückstreuung der Teil der molekularen Rückstreuung gesondert gemessen. Die Messung des molekularen Rückstreusignals wird durch die spektrale Filterung der atmosphärischen Rückstreuung mit einem schmalbandigen optischen Filter ermöglicht. Durch den Vergleich des gemessenen molekularen Signals mit dem zu erwartenden kann die Aerosolextinktion direkt bestimmt werden.
Zum Aufbau des Instruments wurde eine Joddampfabsorptionszelle konstruiert und in das Empfangsmodul des bestehenden flugzeuggetragenen Lidars des Deutschen Zentrums für Luft– und Raumfahrt integriert. Außerdem wurde der Lasertransmitter des Lidars mit einem neuartigen Verfahren der opto–akustischen Modulation frequenzstabilisiert.
Während SAMUM–1 wurden damit erstmalig die optischen Eigenschaften des reinen Saharastaubaerosols, insbesondere dessen Extinktion, das Verhältnis von Extinktion und
Rückstreuung sowie die Depolarisation, in der Nähe seiner Quellgebiete untersucht. Die Messungen des neuen HSRL wurden zur Qualitätssicherung mit Hilfe unabhängiger Instrumente validiert. Die Lidar–Verhältnis–Messungen wurden durch Trajektorienanalysen auf mögliche Abhängigkeiten von unterschiedlichen Quellgebieten untersucht. Die HSRL–Messungen der Aerosol–optischen Dicke wurden mit satellitengestützten Messungen verglichen.
Südlich des Hohen Atlas Gebirges wurden Aerosol–optische Dicken von 0,50 bis 0,60 gemessen. Es zeigte sich eine ausgeprägte laterale Variabilität der Aerosol–optischen Dicke, die bei homogenen Schichten allein auf deren unterschiedliche Dicke zurückgeführt werden
konnte. Die vertikalen Variationen des Lidar–Verhältnisses zwischen 38 sr und 50 sr wurden durch Trajektorienanalysen auf die Anströmung aus unterschiedlichen Quellgebieten
zurückgeführt. Im Depolarisationsverhältnis wurden jedoch keine vertikale Variationen beobachtet, was auf eine einheitliche Teilchenform schließen lässt. Die Aerosoldepolarisation betrug in den Staubaerosolschichten 0,30 ± 0,02. Dies bestärkt die Annahme, dass das
Lidar–Verhältnis in erster Linie durch die unterschiedliche chemische Zusammensetzung des Aerosols beeinflusst wird. Aufgrund der hohen natürlichen Variabilität erscheint die
Angabe eines mittleren Wertes als nicht sinnvoll. Der Vergleich mit MISR–Messungen der Aerosol–optischen Dicke zeigte größtenteils Übereinstimmung innerhalb der Fehlergrenzen.
Direkt über dem Hohen Atlas traten signifikante Abweichungen auf, die durch die sich stark ändernde Topographie erklärt werden können
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Dust mobilization and aerosol transport from West Africa to Cape Verde - a meteorological overview of SAMUM-2
The second field campaign of the SAharan Mineral dUst experiMent (SAMUM-2) was performed between 15 January and 14 February 2008 at the airport of Praia, Cape Verde, and provided valuable information to study the westward transport of Saharan dust and the mixing with biomass-burning smoke and sea-salt aerosol. Here lidar, meteorological, and particle measurements at Praia, together with operational analyses, trajectories, and satellite and synoptic station data are used to give an overview of the meteorological conditions and to place other SAMUM-2 measurements into a large-scale context. It is demonstrated that wintertime dust conditions at Cape Verde are closely related to the movement and intensification of mid-latitude high-pressure systems and the associated pressure gradients at their southern flanks. These cause dust emission over Mauritania, Mali, and Niger, and subsequent westward transport to Cape Verde within about 1–5 d. Dust emissions often peak around midday, suggesting a relation to daytime mixing of momentum from nocturnal low-level jets to the surface. The dust layer over Cape Verde is usually restricted to the lowest 1.5 km of the atmosphere. During periods with near-surface wind speeds about 5.5 ms−1, a maritime aerosol layer develops which often mixes with dust from above. On most days, the middle levels up to about 5 km additionally contain smoke that can be traced back to sources in southernWest Africa. Above this layer, clean air masses are transported to Cape Verde with the westerly flow at the southern side of the subtropical jet. The penetration of extra-tropical disturbances to low latitudes can bring troposphere-deep westerly flow and unusually clean conditions to the region
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Vertically resolved dust optical properties during SAMUM: Tinfou compared to Ouarzazate
Vertical profiles of dust key optical properties are presented from measurements during the Saharan Mineral Dust Experiment (SAMUM) by Raman and depolarization lidar at two ground-based sites and by airborne high spectral resolution lidar. One of the sites, Tinfou, is located close to the border of the Sahara in Southern Morocco and was the main in situ site during SAMUM. The other site was Ouarzazate airport, the main lidar site. From the lidar measurements the spatial distribution of the dust between Tinfou and Ouarzazate was derived for 1 d. The retrieved profiles of backscatter and extinction coefficients and particle depolarization ratios show comparable dust optical properties, a similar vertical structure of the dust layer, and a height of about 4 km asl at both sites. The airborne cross-section of the extinction coefficient at the two sites confirms the low variability in dust properties. Although the general picture of the dust layer was similar, the lidar measurements reveal a higher dust load closer to the dust source. Nevertheless, the observed intensive optical properties were the same. These results indicate that the lidar measurements at two sites close to the dust source are both representative for the SAMUM dust conditions
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Spatial distribution and optical properties of Saharan dust observed by airborne high spectral resolution lidar during SAMUM 2006
Airborne measurements of pure Saharan dust extinction and backscatter coefficients, the corresponding lidar ratio and the aerosol optical thickness (AOT) have been performed during the Saharan Mineral Dust Experiment 2006, with a high spectral resolution lidar. Dust layers were found to range from ground up to 4–6 km above sea level (asl). Maximum AOT values at 532 nm, encountered within these layers during the DLR Falcon research flights were 0.50–0.55. A significant horizontal variability of the AOT south of the High Atlas mountain range was observed even in cases of a well-mixed dust layer. High vertical variations of the dust lidar ratio of 38–50 sr were observed in cases of stratified dust layers. The variability of the lidar ratio was attributed to dust advection from different source regions. The aerosol depolarization ratio was about 30% at 532 nm during all measurements and showed only marginal vertical variations
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Solar radiative effects of a Saharan dust plume observed during SAMUM assuming spheroidal model particles
The solar optical properties of Saharan mineral dust observed during the Saharan Mineral Dust Experiment (SAMUM) were explored based on measured size-number distributions and chemical composition. The size-resolved complex refractive index of the dust was derived with real parts of 1.51–1.55 and imaginary parts of 0.0008–0.006 at 550 nm wavelength. At this spectral range a single scattering albedo ωo and an asymmetry parameter g of about 0.8 were derived. These values were largely determined by the presence of coarse particles. Backscatter coefficients and lidar ratios calculated with Mie theory (spherical particles) were not found to be in agreement with independently measured lidar data. Obviously the measured Saharan mineral dust particles were of non-spherical shape. With the help of these lidar and sun photometer measurements the particle shape as well as the spherical equivalence were estimated. It turned out that volume equivalent oblate spheroids with an effective axis ratio of 1:1.6 matched these data best. This aspect ratio was also confirmed by independent single particle analyses using a scanning electron microscope. In order to perform the non-spherical computations, a database of single particle optical properties was assembled for oblate and prolate spheroidal particles. These data were also the basis for simulating the non-sphericity effects on the dust optical properties: ωo is influenced by up to a magnitude of only 1% and g is diminished by up to 4% assuming volume equivalent oblate spheroids with an axis ratio of 1:1.6 instead of spheres. Changes in the extinction optical depth are within 3.5%. Non-spherical particles affect the downwelling radiative transfer close to the bottom of the atmosphere, however, they significantly enhance the backscattering towards the top of the atmosphere: Compared to Mie theory the particle non-sphericity leads to forced cooling of the Earth-atmosphere system in the solar spectral range for both dust over ocean and desert
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Vertical profiling of Saharan dust with Raman lidars and airborne HSRL in southern Morocco during SAMUM
Three ground-based Raman lidars and an airborne high-spectral-resolution lidar (HSRL) were operated duringSAMUM 2006 in southern Morocco to measure height profiles of the volume extinction coefficient, the extinction-to-backscatter ratio and the depolarization ratio of dust particles in the Saharan dust layer at several wavelengths. Aerosol Robotic Network (AERONET) Sun photometer observations and radiosoundings of meteorological parameters complemented the ground-based activities at the SAMUM station of Ouarzazate. Four case studies are presented. Two case studies deal with the comparison of observations of the three ground-based lidars during a heavy dust outbreak and of the ground-based lidars with the airborne lidar. Two further cases show profile observations during satellite overpasses on 19 May and 4 June 2006. The height resolved statistical analysis reveals that the dust layer top typically reaches 4–6 km height above sea level (a.s.l.), sometimes even 7 km a.s.l.. Usually, a vertically inhomogeneous dust plume with internal dust layers was observed in the morning before the evolution of the boundary layer started. The Saharan dust layer was well mixed in the early evening. The 500 nm dust optical depth ranged from 0.2–0.8 at the field site south of the High Atlas mountains, Ångström exponents derived from photometer and lidar data were between 0–0.4. The volume extinction coefficients (355, 532 nm) varied from 30–300Mm−1 with a mean value of 100Mm−1 in the lowest 4 km a.s.l.. On average, extinction-to-backscatter ratios of 53–55 sr (±7–13 sr) were obtained at 355, 532 and 1064 nm
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Properties of dust aerosol particles transported to Portugal from the Sahara desert
Aerosol properties of mineral particles in the far field of an African desert dust outbreak were investigated that brought Saharan dust over the Mediterranean in different layers to Portugal. The measurements were performed inside the project Desert Aerosols over Portugal (DARPO) which was linked to the Saharan Mineral Dust Experiment (SAMUM). The maximum particle mass concentration was about 150μgm−3 and the corresponding scattering coefficient was 130Mm−1 which results in a mass scattering efficiency of 0.87m2 g−1. The aerosol optical depth reached values up to 0.53 and the lidar ratio was between 45 and 50 in the whole dust loaded column. A comparison between particle size distributions and refractive indices derived from different instruments and models showed a general good agreement but some minor differences could also be observed. Measurements as well as calculations with a particle transport model suggest that there is a relatively higher concentration of very large particles in the upper region of the dust layer than on the surface which is likely connected with meteorological conditions at the observational site ( Évora, Portugal)
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Microphysical and optical properties of dust and tropical biomass burning aerosol layers in the Cape Verde region - an overview of the airborne in situ and lidar measurements during SAMUM-2
In the framework of the Saharan Mineral Dust Experiment (SAMUM) airborne High Spectral Resolution Lidar and in situ measurements of the particle size, aerosol mixing state and absorption coefficient were conducted. Here, the properties of mineral dust and tropical biomass burning layers in the Cape Verde region in January/February 2008 are investigated and compared with the properties of fresh dust observed in May/June 2006 close the Sahara. In the Cape Verde area, we found a complex stratification with dust layers covering the altitude range below 2 km and biomass burning layers aloft. The aerosol type of the individual layers was classified based on depolarization and lidar ratios and, in addition, on in situ measured Ångström exponents of absorption åap. The dust layers had a depth of 1.3 ± 0.4 km and showed a median åap of 3.95. The median effective diameter Deff was 2.5 μm and the dust layers over Cape Verde yielded clear signals of aging: large particles were depleted due to gravitational settling and the accumulation mode diameter was shifted towards larger sizes as a result of coagulation. The tropical biomass layers had a depth of 2.0 ± 1.1 km and were characterized by a median åap of 1.34. They always contained a certain amount of large dust particles and showed a median Deff of 1.1 μm and a fine mode Deff,fine of 0.33. The dust and biomass burning layers had a median aerosol optical depth (AOD) of 0.23 and 0.09, respectively. The median contributions to the AOD of the total atmospheric column below 10 km were 75 and 37%, respectively
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Desert dust aerosol air mass mapping in the western Sahara, using particle properties derived from space-based multi-angle imaging
Coincident observations made over the Moroccan desert during the Sahara mineral dust experiment (SAMUM) 2006
field campaign are used both to validate aerosol amount and type retrieved from multi-angle imaging spectroradiometer
(MISR) observations, and to place the suborbital aerosol measurements into the satellite’s larger regional context.
On three moderately dusty days during which coincident observations were made, MISR mid-visible aerosol optical
thickness (AOT) agrees with field measurements point-by-point to within 0.05–0.1. This is about as well as can be
expected given spatial sampling differences; the space-based observations capture AOT trends and variability over an
extended region. The field data also validate MISR’s ability to distinguish and to map aerosol air masses, from the
combination of retrieved constraints on particle size, shape and single-scattering albedo. For the three study days, the
satellite observations (1) highlight regional gradients in the mix of dust and background spherical particles, (2) identify
a dust plume most likely part of a density flow and (3) show an aerosol air mass containing a higher proportion of
small, spherical particles than the surroundings, that appears to be aerosol pollution transported from several thousand
kilometres away
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Regional modelling of Saharan dust and biomass-burning smoke, Part I: Model description and evaluation
The spatio-temporal evolution of the Saharan dust and biomass-burning plume during the SAMUM-2 field campaign
in January and February 2008 is simulated at 28 km horizontal resolution with the regional model-system COSMOMUSCAT.
The model performance is thoroughly tested using routine ground-based and space-borne remote sensing
and local field measurements. Good agreement with the observations is found in many cases regarding transport
patterns, aerosol optical thicknesses and the ratio of dust to smoke aerosol. The model also captures major features
of the complex aerosol layering. Nevertheless, discrepancies in the modelled aerosol distribution occur, which are
analysed in detail. The dry synoptic dynamics controlling dust uplift and transport during the dry season are well
described by the model, but surface wind peaks associated with the breakdown of nocturnal low-level jets are not
always reproduced. Thus, a strong dust outbreak is underestimated. While dust emission modelling is a priori more
challenging, since strength and placement of dust sources depend on on-line computed winds, considerable inaccuracies
also arise in observation-based estimates of biomass-burning emissions. They are caused by cloud and spatial errors of
satellite fire products and uncertainties in fire emission parameters, and can lead to unrealistic model results of smoke
transport
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