Stephan Weinbruch

Transmission electron microscopical and aerosol dynamical characterization of soot aerosols

Abstract Size, morphology and microstructure of Palas soot, Diesel soot and of Diesel soot/ammonium sulfate mixtures were studied by transmission electron microscopy (TEM). The diameter of the primary particles derived from TEM is 6.6±1.7 nm for Palas soot and 22.6±6.0 nm for Diesel soot. Palas soot predominantly consists of amorphous carbon. In a few cases, nanocrystalline graphite with domain sizes on the order of 1 nm were observed. Primary particles of Diesel soot always show an onion-shell structure of nanocrystalline graphite with domain sizes between 2– 3 nm . Fractal properties of 37 Diesel soot agglomerates were determined from TEM images by two different techniques. The average fractal dimension of Diesel soot derived from TEM is 1.70±0.13. TEM further showed that the initially external mixture of Diesel soot and ammonium sulfate developed with time in a significant degree of internal mixing. A second independent approach to determine the fractal properties of soot is based on computer simulations of the aerosol dynamics. A good reproduction of the time evolution of mass and number concentrations and of the mobility size distribution was achieved. The primary particle diameters obtained from the computer simulations ( 7.3±0.8 nm for Palas soot, 25±3 nm for Diesel soot) are in excellent agreement with the TEM results. The fractal dimension of Diesel soot received from the COSIMA algorithm of 1.9±0.2 (overlap of primary particles was taken into consideration) is consistent with the value obtained from TEM image analysis. For Palas soot, the computer simulation yielded a fractal dimension of 2.0±0.1 (overlap was not corrected, as the overlap coefficient is not known).

Size distribution, mass concentration, chemical and mineralogical composition and derived optical parameters of the boundary layer aerosol at Tinfou, Morocco, during SAMUM 2006

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).

Modifiers and coatings in graphite furnace atomic absorption spectrometry—mechanisms of action (A tutorial review)

Abstract A multitude of different and often contradictory mechanisms for the effects of modifiers and coatings have been proposed. Many of these proposals lack sufficient experimental evidence. Therefore, a series of statements based on our own investigations is given as ‘facts’. Another series of statements is made as ‘fictions’ related to erroneous proposals on the functioning of modifiers and coatings in the pertinent literature. Two basic concepts are developed for the sequence of processes leading to analyte stabilization for the two most important groups of modifiers: refractory carbide forming elements of the IVa–VIa groups of the periodic system on the one hand and Pt-group metals on the other hand. These concepts are based on the main reactions of graphite with elements and compounds: carbide formation and intercalation. Most important experimental results leading to this understanding are described: Penetration measurements for modifiers and analytes indicated the subsurface zone down to approximately 10 μm as the essential place for graphite–analyte–modifier interactions. The reason for this phenomenon is an open porosity of the pyrocarbon coating of 5–10% (v/v) into which liquids penetrate upon sample application. This also indicates that modifiers are best applied by impregnation or electrolysis whereas dense coatings are not advantageous. It is also shown that graphite tube assemblies are dynamic systems with a limited lifetime and carbon losses are an essential feature of tube corrosion. Most frequently found erroneous statements are discussed: (a) Particles on the tube surface are responsible for analyte stabilization and retention during pyrolysis. (b) Analyte stabilization is taking place by formation of intermetallic compounds or thermally stable alloys. (c) Experiments are performed with unrealistic concentrations of analytes and/or modifiers. (d) Dense coatings are advantageous. Finally, a functional schedule is given for the three steps of graphite furnace atomic absorption spectrometry (GFAAS): sample application and drying; pyrolysis; atomization. Contrary to the vast amount of literature on this topic it tried to provide the analyst working with GFAAS and in an increasing number working with Solid Sampling-GFAAS with a set of most important statements. This might spare the experimentalist a lot of useless optimization procedures but should lead him to a basic understanding of the complex phenomena taking place in his instrument and during his analytical work.

Electron microscopy of particles collected at Praia, Cape Verde, during the Saharan Mineral Dust Experiment: particle chemistry, shape, mixing state and complex refractive index

A large field experiment of the Saharan Mineral Dust Experiment (SAMUM) was performed in Praia, Cape Verde, in January and February 2008. The aerosol at Praia is a superposition of mineral dust, sea-salt, sulphates and soot. Particles smaller than 500 nm are mainly mineral dust, mineral dust–sulphate mixtures, sulphates and soot–sulphate mixtures. Particles larger then 2.5μm consist of mineral dust, sea-salt and few mineral dust–sulphate mixtures. A transition range exists in between. The major internal mixtures are mineral dust–sulphate and soot–sulphate. Mineral dust–sea-salt mixtures occur occasionally, mineral dust–soot mixtures were not observed. The aspect ratio was 1.3–1.4 for dry particles smaller than 500 nm and 1.6–1.7 for larger ones. Parameterizations are given for dry and humid state. Although the real part of the refractive index showed low variation (1.55–1.58 at 532 nm), a multi-modal imaginary part was detected as function of particle size, reflecting the complex composition. Soot mainly influences the absorption for wavelengths longer than the haematite absorption edge, whereas for shorter wavelengths dust is dominating. The refractive index of the aerosol depends on the source region of the mineral dust and on the presence/absence of a marine component.

In situ measurements of optical properties at Tinfou (Morocco) during the Saharan Mineral Dust Experiment SAMUM 2006

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.

Speciation of iron in atmospheric aerosol samples

Abstract The influence of iron in the atmosphere is manifold and a function of its concentration, chemical environment, and solubility. Several analytical methods were applied for the characterization of iron in aerosol samples: for the analysis of solid aerosol samples, instrumental neutron activation analysis, energy- and wavelength-dispersive X-ray fluorescence, Mossbauer spectrometry and electron probe microanalysis were used. For the analysis of the digestion or leaching solutions, total-reflection X-ray fluorescence, atomic absorption spectrometry with flame or graphite furnace atomization and ion chromatography were applied. The bulk iron content of some urban aerosol sample was determined to be about 7% w/w, predominantly occurring as oxides with goethite as the major phase. The major fraction of the investigated aerosol originates from anthropogenic sources. Only 2% of the total iron content is soluble in an aqueous phase.

Counterflow Virtual Impactor Based Collection of Small Ice Particles in Mixed-Phase Clouds for the Physico-Chemical Characterization of Tropospheric Ice Nuclei: Sampler Description and First Case Study

A ground-based sampling system named Ice-CVI is introduced that is able to extract small ice particles with sizes between 5 and 20 μ m out of mixed-phase clouds. The instrument is based on a counterflow virtual impactor (CVI) removing interstitial particles and is supplemented by additional modules that pre-segregate other constituents of mixed-phase clouds. Ice particles of 20 μ m and smaller are expected to grow only by water vapor diffusion and there is a negligible probability that they scavenge aerosol particles by impaction and riming. Thus, their residuals which are released by the Ice-CVI can be interpreted as the original ice nuclei (IN). In a first field test within the Cloud and Aerosol Characterization Experiment (CLACE-3) at the high alpine research station Jungfraujoch, the collection behavior of the single components and the complete system was evaluated under atmospheric sampling conditions. By comparing parameters measured by the Ice-CVI with corresponding results obtained from other inlets or with in-situ instrumentation it is verified that the small ice particles are representatively collected whereas all other mixed phase cloud constituents are effectively suppressed. In a case study it is observed that super-micrometer particles preferentially serve as IN although in absolute terms the IN concentration is dominated by sub-micrometer particles. Mineral dust (Si), non-volatile organic matter and black carbon could be identified as IN components by means of different chemical analyses. The latter suggests an anthropogenic influence on the heterogeneous ice nucleation in supercooled, tropospheric clouds.

Ground-based off-line aerosol measurements at Praia, Cape Verde, during the Saharan Mineral Dust Experiment: microphysical properties and mineralogy

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.).

Particle chemical properties in the vertical column based on aircraft observations in the vicinity of Cape Verde Islands

During the second Saharan Mineral Dust Experiment (SAMUM-2) field campaign, particles with geometric diameters (d) between ∼0.1 and 25 μm were collected on board of the Deutsches Zentrum f¨ur Luft- und Raumfahrt (German Aerospace Center, DLR) Falcon aircraft. Size, chemical composition and mixing state of aerosols sampled (spatially and vertically resolved) along theWest African coastline and in the Cape Verde Islands region were determined by electron microscopy. A pronounced layer structure of biomass-burning aerosol and desert dust was present for all days during the sampling period from 23 January to 6 February. The aerosol composition of the small particles (d < 0.5 μm) was highly variable and in cases of biomass burning strongly dominated by soot with up to 90% relative number abundance. Internal mixtures of soot particles with mineral dust were not detected. Soot was only observed to mix with secondary sulphate. The coarse particles (d > 0.5 μm) were dominated by silicates. In the Cape Verde Islands region mineral dust is well mixed. The determination of source regions by elemental or mineralogical composition was generally not possible, except for air masses which were transported over the Gulf of Guinea. The real part of the refractive index showed little variation. In contrast, the imaginary part strongly depended on the abundance of soot (biomass-burning aerosol) and haematite (mineral dust).

Ultrafine particles at workplaces of a primary aluminium smelter

The number concentration and size distribution of ultrafine particles in a Søderberg and a prebake potroom of an aluminium primary smelter have been measured using a scanning mobility particle spectrometer. The particle morphology was studied by transmission electron microscopy (TEM). The study shows the existence of elevated number concentrations of ultrafine particles in both potrooms. The main source of these particles is likely to be the process of anode changing. The ultrafine particles were measured directly at the source but could also be identified as episodes of high number concentrations in the general background air. Unlike the larger particles belonging to the 50-100 nm mode, the nanoparticle mode could not be detected in the TEM indicating that they may not be stable under the applied sampling conditions and/or the high vacuum in the instrument.