Jean-Noël Rouzaud

Comparison of quantification methods to measure fire‐derived (black/elemental) carbon in soils and sediments using reference materials from soil, water, sediment and the atmosphere

Black carbon (BC), the product of incomplete combustion of fossil fuels and biomass (called elemental carbon (EC) in atmospheric sciences), was quantified in 12 different materials by 17 laboratories from different disciplines, using seven different methods. The materials were divided into three classes: (1) potentially interfering materials, (2) laboratory-produced BC-rich materials, and (3) BC-containing environmental matrices (from soil, water, sediment, and atmosphere). This is the first comprehensive intercomparison of this type (multimethod, multilab, and multisample), focusing mainly on methods used for soil and sediment BC studies. Results for the potentially interfering materials (which by definition contained no fire-derived organic carbon) highlighted situations where individual methods may overestimate BC concentrations. Results for the BC-rich materials (one soot and two chars) showed that some of the methods identified most of the carbon in all three materials as BC, whereas other methods identified only soot carbon as BC. The different methods also gave widely different BC contents for the environmental matrices. However, these variations could be understood in the light of the findings for the other two groups of materials, i.e., that some methods incorrectly identify non-BC carbon as BC, and that the detection efficiency of each technique varies across the BC continuum. We found that atmospheric BC quantification methods are not ideal for soil and sediment studies as in their methodology these incorporate the definition of BC as light-absorbing material irrespective of its origin, leading to biases when applied to terrestrial and sedimentary materials. This study shows that any attempt to merge data generated via different methods must consider the different, operationally defined analytical windows of the BC continuum detected by each technique, as well as the limitations and potential biases of each technique. A major goal of this ring trial was to provide a basis on which to choose between the different BC quantification methods in soil and sediment studies. In this paper we summarize the advantages and disadvantages of each method. In future studies, we strongly recommend the evaluation of all methods analyzing for BC in soils and sediments against the set of BC reference materials analyzed here.

On the characterization of disordered and heterogeneous carbonaceous materials by Raman spectroscopy

The applicability of Raman spectroscopy to characterize disordered and heterogeneous carbonaceous materials (CM) is discussed, by considering both natural and synthetic CM. First, different analytical mismatches during the measurement are discussed and technical indications are provided in order to eliminate them. Second, the accuracy and relevance of the different parameters obtained by the decomposition of spectra by conventional fitting procedure, is reviewed. Lastly, a new Raman technique (Raman area mode microspectroscopy) giving an homogeneous repartition of power within a large laser beam is presented, this technique being powerful to study strongly heterogeneous CM and/or photosensitive samples.

Evaluation of a protocol for the quantification of black carbon in sediments

Formation of highly condensed black carbon (BC) from vegetation fires and wood fuel combustion presumably transfers otherwise rapidly cycling carbon from the atmosphere-biosphere cycle into a much slower cycling geological form. Recently reported BC fractions of total organic carbon (TOC) in surficial marine sediments span a wide range (2-90%), leaving it presently unclear whether this variation reflects natural processes or is largely due to method differences. In order to elucidate the importance of BC to carbon burial the specificity of applied methods needs to be constrained. Here the operating range and applicability of a commonly used chemothermal oxidation (CTO) method is evaluated using putative BC standards, potentially interfering substances, and natural matrix standards. Test results confirm the applicability of the method to marine sediments. Integrity tests with model substrates suggest applicability to low-carbon soils but only with a lower specificity to seawater particulate matter. The BC content of marine sediment samples in a set of studies employing the CTO method proved to be consistent with associated geochemical information. The radiocarbon content of the BC isolate in an environmental matrix standard was shown to be similar to the radiocarbon signature of pyrogenic polycyclic aromatic hydrocarbons (PAHs), here serving as molecular markers of combustion (fraction modern fM of BC was 0.065 ± 0.014 and of PAHs 0.056 ± 0.020), while being clearly distinct from the radiocarbon content of the bulk TOC (fM = 0.61 ± 0.08). Urgent questions such as the global accumulation rate of black carbon in soils and sediments may prove approachable with the chemothermal oxidation technique of BC quantification.

Structure, microtexture, and optical properties of anthracene and saccharose-based carbons

Abstract Graphitizing anthracene-based (AC) and nongraphitizing saccharose-based carbons (SC) were heat treated up to 2900°C. Their structure and microtexture were studied by TEM and compared with some of their properties. The data obtained show that graphitization of AC is prepared by an annealing of the aromatic layer distortions occurring in four stages. In the first stage, elemental basic structural units (BSU) ( 1 μm). However, BSU are separated by tilt and twist boundaries where defects and heteroatoms are gathered. Stage 2 begins at the end of heteroatom release. BSU coalesce face to face into distorted columns; the lateral coalescence is inhibited by misoriented single BSU. This stage ends when misoriented BSU disappear. Stage 3 corresponds to lateral coalescence of columns into distorted continuous layer stacks. Stage 4 begins when all in-plane defects are wiped out, entirely annealing the distortions. Graphitization occurs from the beginning of stage 4. The end of stage 1 corresponds to a reflectance equal to that of graphite and to an electrical resistivity almost at its minimum. The end of stage 2 corresponds to a fast increase of diamagnetic susceptibility. The beginning of stage 4 is the plateau of the diamagnetic susceptibility, the maximum of Hall effect, the minimum of the magnetoresistance, and the final decrease of resistivity. SC follows the same stages as AC, even though SC are nongraphitizing carbons. This is due to the very small extent of LMO (5–10 nm).

Carbon films: structure and microtexture (optical and electron microscopy, Raman spectroscopy)

Abstract Nearly pure carbon films (150 and 500 A thick) were prepared by condensing carbon vapour in vacuum. They were then heat treated under an inert gas flow from 25 to 2700 °C. The samples were studied comparatively with (1) a conventional transmission electron microscope (structure and microtexture), (2) a Raman microprobe (for quantification of the different types of defect by Raman spectroscopy) and (3) an optical microscope (for determination of the optical indices). The carbon films were found to graphitize in five stages. Each stage is characterized by the release of a given type of defect. Crystal growth occurs first in thickness and then in diameter. Perfect aromatic layers are formed progressively and the three-dimensional order improves. Polycrystalline graphite is then obtained. The graphitization is faster and more complete for thick films (about 500 A thick) than for thin films (about 150 A).

Surface functionality and porosity of activated carbons obtained from chemical activation of wood

a `´ ´ ´ ´ Abstract Mixtures of wood with either phosphoric acid or diammonium hydrogen phosphate ((NH ) HPO ) were heated under 42 4 nitrogen or steam / nitrogen flows at temperatures between 300 and 500 8C. As a result of these processes, activated carbons with various pore size distribution and surface properties were obtained. The samples were characterised using potentiometric titration and sorption of nitrogen at its boiling point. The results showed the significant influence of the nature of the activating agent and the atmosphere on the final properties of the materials. It was demonstrated that steam inhibits the incorporation of heteroatoms into the carbon matrix. Activation of wood in the presence of phosphoric acid together with 2 steam gives carbon of high surface area (|1800 m / g) with well-developed mesoporosity and an almost neutral surface.

Chemical structure and sources of the macromolecular, resistant, organic fraction isolated from a forest soil (Lacadée, south-west France)

The insoluble, non-hydrolyzable, macromolecular material isolated from a forest soil from Lacadee (south-west France) was examined via a combination of various methods: FTIR spectroscopy, elemental analysis, “off-line” pyrolysis and high resolution transmission electron microscopy. Such a resistant material, which accounts for ca. 25% of total humin, was shown to be chiefly composed of melanoidins and black carbon. Two types of black carbon particles were identified by dark field and lattice fringe electron microscopy. Contrary to previous observations, based on solid state 13C NMR spectroscopy and Curie point Py/GC/MS, highly aliphatic moieties only afford a minor contribution to the refractory material of the Lacadee soil. Additional studies, using mixtures of model compounds, were carried out to examine the origin of this conspicuous overestimation of the level of aliphaticity in such heterogeneous material when the latter two methods are used.

Mechanisms of graphite formation from kerogen: experimental evidence

Abstract To resolve the role of strain in the formation of natural graphite, a ‘hard’ carbon-based anthracite and a ‘soft’ carbon-based high volatile bituminous coal were deformed in hydrostatic, coaxial and simple shear configurations at temperatures up to 900°C and confining pressures up to 1 GPa. Additional tests were carried out at ambient pressures at temperatures up to 2800°C. In simple shear, graphite appears, with an anthracite starting material at temperatures as low as 600°C; samples tested at 900°C are predominately graphitized, as is evident from optical microscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). In tests on high volatile bituminous coal, graphite first appears in simple shear tests at temperatures of 800°C and is common at 900°C. In TEM observations graphite particles are lamellar, have punctual hkl reflections or Debye-Scherrer hkl rings (triperiodic order) and long, stiff and stacked lattice fringes typical of well crystallized graphite. No graphite was formed in either the hydrostatic or coaxial tests (they remain porous and turbostratic). Micro-Raman spectroscopy of deformed samples indicates the presence of defects (Band at 1350 cm −1 ) even in samples that prove to be mainly graphite by XRD and TEM. With increasing experimental temperatures there is an overall increase in maximum reflectance and bireflectance. Samples deformed in simple shear locally have reflectance values typical of graphite. In anthracite the highest reflectance and bireflectance values occur in zones of kink banding or cataclasis, indicating the importance of localized areas of high strain on graphitization. In high volatile bituminous coal localization of graphite appears to reflect compositional heterogeneity as well as strain partitioning during the experiments. The occurrence of low reflectance zones and mesoporous-turbostratic particles in samples otherwise composed of graphite is interpreted as reflecting localized areas of low strain (strain shadows) during deformation. Comparison of anthracite and high volatile bituminous coal samples tested under the same general conditions indicate that anthracite is more graphitizable under all conditions. The importance of simple shear experiments is that, because of their geometry, a significant component of strain is imparted to the samples. Strain energy has facilitated additional flattening of existing pores, with likely mechanical rotation of stacks of basic structural units (BSUs) and rupturing of pore walls. Thus, strain facilitates coalescence of pores, parallelism of BSUs and, therefore, the growth of aromatic sheets (by coalescence of neighbouring pores), leading to the formation of graphite. We propose that a major component of the activation energy required for graphitization in our experiments and, by analogy, in nature, is provided by strain energy.

Extreme Deuterium Excesses in Ultracarbonaceous Micrometeorites from Central Antarctic Snow

Dust to Dust Interplanetary dust particles are thought to sample the most primitive materials in the solar system. Because of their large deuterium enrichments, they are thought to have formed in interstellar molecular clouds—the birthplaces of stars—and to predate the solar system. Duprat et al. (p. 742; see Perspective by Nittler) describe two large interplanetary dust particles collected from Antarctic snow. The particles contain large zones of organic matter with deuterium excesses 10 to 30 times the terrestrial value. Because the organic matter is associated with crystalline silicates similar to those formed within the solar accretion disk, it is expected that the particles themselves formed in the Suns protoplanetary disk, contradicting the idea that all organics with deuterium excesses are of interstellar origin. Interplanetary dust particles recovered from Antarctic snow may provide a sample of the early solar system. Primitive interplanetary dust is expected to contain the earliest solar system components, including minerals and organic matter. We have recovered, from central Antarctic snow, ultracarbonaceous micrometeorites whose organic matter contains extreme deuterium (D) excesses (10 to 30 times terrestrial values), extending over hundreds of square micrometers. We identified crystalline minerals embedded in the micrometeorite organic matter, which suggests that this organic matter reservoir could have formed within the solar system itself rather than having direct interstellar heritage. The high D/H ratios, the high organic matter content, and the associated minerals favor an origin from the cold regions of the protoplanetary disk. The masses of the particles range from a few tenths of a microgram to a few micrograms, exceeding by more than an order of magnitude those of the dust fragments from comet 81P/Wild 2 returned by the Stardust mission.

Carbon nanoparticles from laser pyrolysis

Carbon nanoparticles synthesised by laser pyrolysis of hydrocarbons in a flow reactor have been investigated as a function of laser power. Samples are cross-characterised by high resolution transmission electron microscopy (HRTEM) and infrared (IR) spectroscopy. Nanoparticles appear highly aromatic in character in all the experimental conditions explored here. As the flame temperature in the interaction zone increases, the nanoparticles evolve drastically from poorly organised, highly hydrogenated samples toward turbostratic concentric particles of carbon. The multiscale organisation of the samples and its evolution with the synthesis parameters are quantitatively determined and correlated to IR properties through an original development of HRTEM image analysis. The multiscale organisation does not reduce to the classical view of assemblies of basic structural units (BSU) of aromatic bricks. More refined models are proposed where non-stacked aromatic layers play a noticeable role and lead to a better understanding of the samples optical properties. Possible contribution to a better understanding of carbon cosmic dust is discussed from an astrophysical point of view.