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- ItemSize distribution and optical properties of mineral dust aerosols transported in the western Mediterranean(München : European Geopyhsical Union, 2016) Denjean, C.; Cassola, F.; Mazzino, A.; Triquet, S.; Chevaillier, S.; Grand, N.; Bourrianne, T.; Momboisse, G.; Sellegri, K.; Schwarzenbock, A.; Freney, E.; Mallet, M.; Formenti, P.This study presents in situ aircraft measurements of Saharan mineral dust transported over the western Mediterranean basin in June–July 2013 during the ChArMEx/ADRIMED (the Chemistry-Aerosol Mediterranean Experiment/Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region) airborne campaign. Dust events differing in terms of source region (Algeria, Tunisia and Morocco), time of transport (1–5 days) and height of transport were sampled. Mineral dust were transported above the marine boundary layer, which conversely was dominated by pollution and marine aerosols. The dust vertical structure was extremely variable and characterized by either a single layer or a more complex and stratified structure with layers originating from different source regions. Mixing of mineral dust with pollution particles was observed depending on the height of transport of the dust layers. Dust layers carried a higher concentration of pollution particles below 3 km above sea level (a.s.l.) than above 3 km a.s.l., resulting in a scattering Ångström exponent up to 2.2 below 3 km a.s.l. However, the optical properties of the dust plumes remained practically unchanged with respect to values previously measured over source regions, regardless of the altitude. Moderate absorption of light by the dust plumes was observed with values of aerosol single scattering albedo at 530 nm ranging from 0.90 to 1.00. Concurrent calculations from the aerosol chemical composition revealed a negligible contribution of pollution particles to the absorption properties of the dust plumes that was due to a low contribution of refractory black carbon in regards to the fraction of dust and sulfate particles. This suggests that, even in the presence of moderate pollution, likely a persistent feature in the Mediterranean, the optical properties of the dust plumes could be assumed similar to those of native dust in radiative transfer simulations, modelling studies and satellite retrievals over the Mediterranean. Measurements also showed that the coarse mode of mineral dust was conserved even after 5 days of transport in the Mediterranean, which contrasts with the gravitational depletion of large particles observed during the transport of dust plumes over the Atlantic. Simulations with the WRF mesoscale meteorological model highlighted a strong vertical turbulence within the dust layers that could prevent deposition of large particles during their atmospheric transport. This has important implications for the dust radiative effects due to surface dimming, atmospheric heating and cloud formation. The results presented here add to the observational data set necessary for evaluating the role of mineral dust on the regional climate and rainfall patterns in the western Mediterranean basin and understanding their atmospheric transport at global scale.
- ItemIn situ measurements of optical properties at Tinfou (Morocco) during the Saharan Mineral Dust Experiment SAMUM 2006(Milton Park : Taylor & Francis, 2017) Schladitz, A.; Müller, T.; Kaaden, N.; Massling, A.; Kandler, K.; Ebert, M.; Weinbruch, S.; Deutscher, C.; Wiedensohler, A.In situ measurements of optical and physical properties of mineral dust were performed at the outskirts of the Saharan Desert in the framework of the Saharan Mineral Dust Experiment part 1 (SAMUM-1). Goals of the field study were to achieve information on the extent and composition of the dust particle size distribution and the optical properties of dust at the ground. For the particle number size distribution, measured with a DMPS/APS, a size dependent dynamic shape factor was considered. The mean refractive index of the particles in this field study is 1.53–4.1 × 10-3i at 537 nm wavelength and 1.53–3.1 × 10-3i at 637 nm wavelength derived from measurements of scattering and absorption coefficients, as well as the particle size distribution. Whereas the real part of the refractive index is rather constant, the imaginary part varies depending on the mineral dust concentrations. For high dust concentration the single scattering albedo is primarily influenced by iron oxide and is 0.96 ± 0.02 and 0.98 ± 0.01 at 537 nm and 637 nm wavelength, respectively. During low dust concentration the single scattering albedo is more influenced by a soot-type absorber and is 0.89 ± 0.02 and 0.93 ± 0.01 for the same wavelengths.
- ItemSize distribution, mass concentration, chemical and mineralogical composition and derived optical parameters of the boundary layer aerosol at Tinfou, Morocco, during SAMUM 2006(Milton Park : Taylor & Francis, 2017) Kandler, K.; Schütz, L.; Deutscher, C.; Ebert, M.; Hofmann, H.; Jäckel, S.; Jaenicke, R.; Knippertz, P.; Lieke, K.; Massling, A.; Petzold, A.; Schladitz, A.; Weinzierl, B.; Wiedensohler, A.; Zorn, S.; Weinbruch, S.During the SAMUM 2006 field campaign in southern Morocco, physical and chemical properties of desert aerosols were measured. Mass concentrations ranging from 30μgm−3 for PM2.5 under desert background conditions up to 300 000μgm−3 for total suspended particles (TSP) during moderate dust storms were measured. TSP dust concentrations are correlated with the local wind speed, whereasPM10 andPM2.5 concentrations are determined by advection from distant sources. Size distributions were measured for particles with diameter between 20 nm and 500μm (parametrizations are given). Two major regimes of the size spectrum can be distinguished. For particles smaller than 500 nm diameter, the distributions show maxima around 80 nm, widely unaffected of varying meteorological and dust emission conditions. For particles larger than 500 nm, the range of variation may be up to one order of magnitude and up to three orders of magnitude for particles larger than 10μm. The mineralogical composition of aerosol bulk samples was measured by X-ray powder diffraction. Major constituents of the aerosol are quartz, potassium feldspar, plagioclase, calcite, hematite and the clay minerals illite, kaolinite and chlorite. A small temporal variability of the bulk mineralogical composition was encountered. The chemical composition of approximately 74 000 particles was determined by electron microscopic single particle analysis. Three size regimes are identified: for smaller than 500 nm in diameter, the aerosol consists of sulphates and mineral dust. For larger than 500 nm up to 50μm, mineral dust dominates, consisting mainly of silicates, and—to a lesser extent—carbonates and quartz. For diameters larger than 50μm, approximately half of the particles consist of quartz. Time series of the elemental composition show a moderate temporal variability of the major compounds. Calcium-dominated particles are enhanced during advection from a prominent dust source in Northern Africa (Chott El Djerid and surroundings). The particle aspect ratio was measured for all analysed particles. Its size dependence reflects that of the chemical composition. For larger than 500 nm particle diameter, a median aspect ratio of 1.6 is measured. Towards smaller particles, it decreases to about 1.3 (parametrizations are given). From the chemical/mineralogical composition, the aerosol complex refractive index was determined for several wavelengths from ultraviolet to near-infrared. Both real and imaginary parts show lower values for particles smaller than 500 nm in diameter (1.55–2.8 × 10−3i at 530 nm) and slightly higher values for larger particles (1.57–3.7 × 10−3i at 530 nm).
- ItemGround-based off-line aerosol measurements at Praia, Cape Verde, during the Saharan Mineral Dust Experiment: Microphysical properties and mineralogy(Milton Park : Taylor & Francis, 2017) Kandler, K.; Schütz, L.; Jäckel, S.; Lieke, K.; Emmel, C.; Müller-Ebert, D.; Ebert, M.; Scheuvens, D.; Schladitz, A.; Šegvić, B.; Wiedensohler, A.; Weinbruch, S.A large field experiment of the Saharan Mineral Dust Experiment (SAMUM) was performed in Praia, Cape Verde, in January and February 2008. This work reports on the aerosol mass concentrations, size distributions and mineralogical composition of the aerosol arriving at Praia. Three dust periods were recorded during the measurements, divided by transitional periods and embedded in maritime-influenced situations. The total suspended particle mass/PM10/PM2.5 were 250/180/74μg/m3 on average for the first dust period (17–21 January) and 250/230/83μg/m3 for the second (24–26 January). The third period (28 January to 2 February) was the most intensive with 410/340/130 μg/m3. Four modes were identified in the size distribution. The first mode (50–70 nm) and partly the second (700–1100 nm) can be regarded as of marine origin, but some dust contributes to the latter. The third mode (2–4 μm) is dominated by advected dust, while the intermittently occurring fourth mode (15–70 μm) may have a local contribution. The dust consisted of kaolinite (dust/maritime period: 35%wt./25%wt.),K-feldspar (20%wt./25%wt.), illite (14%wt./10%wt.), quartz (11%wt./8%wt.), smectites (6%wt./4%wt.), plagioclase (6%wt./1%wt.), gypsum (4%wt./7%wt.), halite (2%wt./17%wt.) and calcite (2%wt./3%wt.).
- ItemDesert dust aerosol air mass mapping in the western Sahara, using particle properties derived from space-based multi-angle imaging(Milton Park : Taylor & Francis, 2017) Kahn, Ralph; Petzold, Andreas; Wendisch, Manfred; Bierwirth, Eike; Dinter, Tilman; Esselborn, Michael; Fiebig, Marcus; Heese, Birgit; Knippertz, Peter; Müller, Detlef; Schladitz, Alexander; Von Hoyningen-HUENE, WolfgangCoincident 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.
- ItemSolar radiative effects of a Saharan dust plume observed during SAMUM assuming spheroidal model particles(Milton Park : Taylor & Francis, 2017) Otto, Sebastian; Bierwirth, Eike; Weinzierl, Bernadett; Kandler, Konrad; Esselborn, Michael; Tesche, Matthias; Schladitz, Alexander; Wendisch, Manfred; Trautmann, ThomasThe 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.
- ItemVertical profiling of Saharan dust with Raman lidars and airborne HSRL in southern Morocco during SAMUM(Milton Park : Taylor & Francis, 2017) Tesche, Matthias; Ansmann, Albert; MüLLER, Detlef; Althausen, Dietrich; Mattis, Ina; Heese, Birgit; Freudenthaler, Volker; Wiegner, Matthias; Esselborn, Michael; Pisani, Gianluca; Knippertz, PeterThree 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.
- ItemDepolarization ratio profiling at several wavelengths in pure Saharan dust during SAMUM 2006(Milton Park : Taylor & Francis, 2017) Freudenthaler, Volker; Esselborn, Michael; Wiegner, Matthias; Heese, Birgit; Tesche, Matthias; Ansmann, Albert; Müller, Detlef; Althausen, Dietrich; Wirth, Martin; Fix, Andreas; Ehret, Gerhard; Knippertz, Peter; Toledano, Carlos; Gasteiger, Josef; Garhammer, Markus; Seefeldner, MeinhardVertical profiles of the linear particle depolarization ratio of pure dust clouds were measured during the Saharan Mineral Dust Experiment (SAMUM) at Ouarzazate, Morocco (30.9◦N, –6.9◦E), close to source regions in May–June 2006, with four lidar systems at four wavelengths (355, 532, 710 and 1064 nm). The intercomparison of the lidar systems is accompanied by a discussion of the different calibration methods, including a new, advanced method, and a detailed error analysis. Over the whole SAMUM periode pure dust layers show a mean linear particle depolarization ratio at 532 nm of 0.31, in the range between 0.27 and 0.35, with a mean Ångström exponent (AE, 440–870 nm) of 0.18 (range 0.04–0.34) and still high mean linear particle depolarization ratio between 0.21 and 0.25 during periods with aerosol optical thickness less than 0.1, with a mean AE of 0.76 (range 0.65–1.00), which represents a negative correlation of the linear particle depolarization ratio with the AE. A slight decrease of the linear particle depolarization ratio with wavelength was found between 532 and 1064 nm from 0.31 ± 0.03 to 0.27 ± 0.04.
- ItemEvaluation of long-range transport and deposition of desert dust with the CTM MOCAGE(Milton Park : Taylor & Francis, 2017) Martet, M.; Peuch, V-H.; Laurent, B.; Marticorena, B.; Bergametti, G.Desert dust modelling and forecasting attract growing interest, due to the numerous impacts of dusts on climate, numerical weather prediction, health, ecosystems, transportation, as well as on many industrial activities. The validation of numerical tools is a very important activity in this context, and we present here an example of such an effort, combining in situ (horizontal visibility in SYNOP messages, IMPROVE database) and remote-sensing data (satellite imagery, AERONET aerosol optical thickness data). Interestingly, these measurements are available routinely, and not only in the context of dedicated measurements campaign; thus, they can be used in an operational context to monitor the performances of operational forecasting systems. MOCAGE is the chemistry-transport model of Météo-France, used operationally to forecast the three-dimensional transport of dusts and their deposition. Two very long-range transport episodes of dust have been studied: one case of Saharan dust transported to East America through Asia and Pacific observed in November 2004 and one case of Saharan dust transported from West Africa to Caribbean Islands in May 2007. Episodes of geographical extension had seldom been studied, and they provide a very selective reference to compare the modelled desert dusts with. The representation of dusts in MOCAGE appears to be realistic in these two very different cases. In turn, the model simulations are used to make the link between the complementary information provided by the different measurements tools, providing a fully consistent picture of the entire episodes. The evolution of the aerosol size distribution during the episodes has also been studied. With no surprise, our study underlines that deposition processes are very sensitive to the size of dust particles. If the atmospheric cycle, in terms of mass, is very much under the influence of larger particles (some micrometres and above), only the finer particles actually travel over thousands of kilometres. This illustrates the need for an accurate representation of size distributions for this aerosol component in numerical models and advocates for using a size-resolved (bin) approach as sinks, and particularly, deposition do not affect the emitted log-normal distributions symmetrically on both sides of the median diameter. Overall, the results presented in this study provide an evaluation of Météo-France operational dust forecasting system MOCAGE.
- ItemOptical and microphysical properties of smoke over Cape Verde inferred from multiwavelength lidar measurements(Milton Park : Taylor & Francis, 2017) Tesche, Matthias; Müller, Detlef; Gross, Silke; Ansmann, Albert; Althausen, Dietrich; Freudenthaler, Volker; Weinzierl, Bernadett; Veira, Andreas; Petzold, AndreasLidar measurements of mixed dust/smoke plumes over the tropical Atlantic ocean were carried out during the winter campaign of SAMUM-2 at Cape Verde. Profiles of backscatter and extinction coefficients, lidar ratios, and Ångstr¨om exponents related to pure biomass-burning aerosol from southern West Africa were extracted from these observations. Furthermore, these findings were used as input for an inversion algorithm to retrieve microphysical properties of pure smoke. Seven measurement days were found suitable for the procedure of aerosol-type separation and successive inversion of optical data that describe biomass-burning smoke. We inferred high smoke lidar ratios of 87 ± 17 sr at 355 nm and 79 ± 17 sr at 532 nm. Smoke lidar ratios and Ångstr¨om exponents are higher compared to the ones for the dust/smoke mixture. These numbers indicate higher absorption and smaller sizes for pure smoke particles compared to the dust/smoke mixture. Inversion of the smoke data set results in mean effective radii of 0.22 ± 0.08 μm with individual results varying between 0.10 and 0.36 μm. The single-scattering albedo for pure biomass-burning smoke was found to vary between 0.63 and 0.89 with a very low mean value of 0.75 ± 0.07. This is in good agreement with findings of airborne in situ measurements which showed values of 0.77 ± 0.03. Effective radii from the inversion were similar to the ones found for the fine mode of the in situ size distributions.