ALBERT

Description: The climate change and environmental literature, including that on aerosols, is replete with mention of black carbon (BC), but neither reliable samples nor standards exist. Thus, there is uncertainty about its exact nature. That confusion can be avoided if terms are defined and widely understood. Here we discuss an ambiguity between BC and soot and propose a more precise definition for soot as a specific material, which we call ns-soot, where "ns" refers to carbon nanospheres. We define ns-soot as particles that consist of nanospheres, typically with diameters 〈 100 nm, that possess distinct structures of concentrically wrapped, graphene-like layers of carbon and with grape-like (acinoform) morphologies. We additionally propose that, because of their importance for climate modeling and health issues, distinctions are made among bare, coated, and embedded ns-soot. BC, on the other hand, is not a well-defined material. We propose that the term should be restricted to light-absorbing refractory carbonaceous matter of uncertain character and that the uncertainty is stated explicitly.

Description: Atmospheric tar balls are particles of special morphology and composition that are abundant in the plumes of biomass smoke. These particles form a specific subset of brown carbon (BrC) which has been shown to play a significant role in atmospheric shortwave absorption and thus climate forcing. Formerly tar balls were hypothesized to be formed in secondary processes in the atmosphere from lignin pyrolysis products. Based on their typical size distributions, morphology, chemical characteristics and other features we now suggest that tar balls are initially produced by the emission of primary tar droplets upon biomass burning. To verify our hypothesis tar balls were produced under laboratory conditions with the total exclusion of flame processes. An all-glass apparatus was designed and tar ball particles were generated from liquid tar obtained previously by dry distillation of wood. The size range, morphology and the chemical composition of the laboratory-generated tar ball particles were similar to those observed in biomass smoke plumes or elsewhere in the atmosphere. Based on our results and the chemical and physical characteristics of tar we suggest that tar balls can be formed by the chemical transformation of emitted primary tar droplets.

Description: Atmospheric tar balls are particles of special morphology and composition that are fairly abundant in the plumes of biomass smoke. These particles form a specific subset of brown carbon (BrC) which has been shown to play a significant role in atmospheric shortwave absorption and, by extension, climate forcing. Here we suggest that tar balls are produced by the direct emission of liquid tar droplets followed by heat transformation upon biomass burning. For the first time in atmospheric chemistry we generated tar-ball particles from liquid tar obtained previously by dry distillation of wood in an all-glass apparatus in the laboratory with the total exclusion of flame processes. The particles were perfectly spherical with a mean optical diameter of 300 nm, refractory, externally mixed, and homogeneous in the contrast of the transmission electron microscopy (TEM) images. They lacked any graphene-like microstructure and exhibited a mean carbon-to-oxygen ratio of 10. All of the observed characteristics of laboratory-generated particles were very similar to those reported for atmospheric tar-ball particles in the literature, strongly supporting our hypothesis regarding the formation mechanism of atmospheric tar-ball particles.

Description: Nanocrystalline iron sulfides form in diverse anoxic environments. The initial precipitate is commonly referred to as nanocrystalline mackinawite (FeS) or amorphous FeS. In order to better understand the structure of the initial precipitate and its conversion to mackinawite and greigite (Fe3S4), we studied synthetic iron sulfide samples that were precipitated from hydrous solutions near room temperature. The transformation of precipitated FeS was followed in both aqueous and dry aging experiments using X-ray powder diffraction (XRD) and scanning and transmission electron microscopy (SEM/TEM) and selected-area electron diffraction (SAED). Under tightly controlled anoxic conditions the first precipitate was nanocrystalline mackinawite. In contrast, when anaerobic conditions during synthesis were not completely ensured, freshly precipitated iron sulfide was typically X-ray amorphous (FeSam), and showed only one broad Bragg-peak at 2 circle minus = 16.5 degrees (5.4 angstrom). A distribution of interatomic distances calculated from pair-distribution function analysis of SAED patterns of FeSam showed that only short-range (〈7 angstrom) order was present in the bulk of the material, with Fe mainly present in tetrahedral coordination. SEM and. TEM images confirmed the poorly ordered structure and showed that FeSam formed aggregates of curved, amorphous sheets that contained 3-8 structurally ordered layers at their cores. Such layers are generally assumed to be structurally similar to the tetrahedral iron sulfide layers in mackinawite. However, both inter- and intralayer spacings measured in high-resolution TEM images (similar to 5.3 to 6.3 and similar to 3.0 to 3.1 angstrom, respectively) were significantly larger than the corresponding spacings in crystalline mackinawite (5.03 and 2.6 angstrom. respectively), suggesting that short-range structural order within the semi-ordered layers of FeSam was not mackinawite-like. In aqueous aging experiments at room temperature, FeSam transformed into a mixture of mackinawite and greigite in similar to 2 months, and completely converted to platy greigite crystals after similar to 10 months. These aqueous transformations were likely driven by excess sulfur in the reacting solutions. We also studied the conversions of nanocrystalline mackinawite. In order to accelerate phase transitions, the initial FeS precipitate was heated to 120 degrees C, resulting in the formation of crystalline mackinawite within 2 h; at 150 degrees C, the material converted directly to pyrrhotite. Finally, when stored in a dry state at room temperature, crystalline mackinawite converted to greigite in 3 months, much faster than in the equivalent experiments in the aqueous solution, probably as a result of a more oxidative environment. The distinction between FeSam and nanocrystalline mackinawite is significant, since conditions for the formation of both phases are present in natural settings. Our experiments in a well-sealed anaerobic chamber simulate iron sulfide formation under anoxic conditions, whereas the samples that were prepared under less tightly controlled conditions can be regarded as representative of the oxic-anoxic transition zone in sediments. Our observations of the structural and morphological features of precipitated FeSam and the details of its aqueous conversion to greigite at ambient conditions are relevant to problems related to the biogeochemical cycling of elements in anoxic and suboxic marine sediments. An additional important finding is that even at moderately high temperatures (up to 170 degrees C), the conversions of iron monosulfides follow different pathways than at ambient conditions. (C) 2011 Elsevier B.V. All rights reserved.