[1] 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

[1] Sequestration in sediments of black carbon (BC) from vegetation fires and fuel combustion may constitute a significant sink of otherwise rapidly cycling carbon from the atmosphere-biosphere cycle. It also has the potential to provide a historical record of atmospheric BC loadings. Previous treatments of BC as one homogeneous entity are being replaced with the growing awareness of a BC combustion continuum, a range spanning from slightly charred biomass to soot and graphite. Here the relative recalcitrance of different BC forms is evaluated, and implications for both BC quantification and environmental stability are considered. The stabilities of four BC reference materials against thermal oxidation in air were quite distinct with T 50%BC values (i.e., the temperature where 50% BC remained in the residue) of 444°C (diesel soot-BC), 388°C (n-hexane soot-BC), 338°C (wood char-BC), and 266°C (grass char-BC). The implications for BC quantification have been illustrated for a thermal oxidation (the CTO-375) method commonly applied to study BC in sediments. This technique measured BC:TOC ratios of 78.3 ± 1.3% for the diesel soot-BC and 45.3 ± 6.1% for n-hexane soot-BC, whereas no CTO375-BC was detected for the two analyzed char-BC materials. The greater lability of char-BC compared to soot-BC likely reflects higher accessibility to internal microporosity in char-BC, facilitating internal O 2 transfer. Decreasing the temperature cutoff below 375°C to also include char-BC is not possible as thermograms of nonpyrogenic reference materials indicated that such material would then be artifactually quantified as BC. The presence of mineral oxides in the sediment matrix may lead to a catalytically mediated lowering of the activation energy for soot-BC oxidation but not for char-BC or nonpyrogenic organic material. Several recent studies combine to challenge the proposition of complete recalcitrance of BC. Particularly, the thermal lability of char-BC from grassland fires deserves further attention in order to improve the understanding of BC in the global carbon cycle.Citation: Elmquist, M., G. Cornelissen, Z. Kukulska, and Ö . Gustafsson (2006), Distinct oxidative stabilities of char versus soot black carbon: Implications for quantification and environmental recalcitrance, Global Biogeochem. Cycles, 20, GB2009,

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In view of poor constraints on historical combustion emissions, past environmental loadings of black carbon (BC) and polycyclic aromatic hydrocarbon (PAH) were reconstructed from dated lake sediment cores collected 70 km south of Stockholm, Sweden. Compared to several dramatic variations over the recent 150 years, the preindustrial loading were steady within +/-50% through the entire medieval with BC fluxes of 0.071 g m(-2) yr(-1) and PAH fluxes of 6 microg m(-2) yr(-1). In the wood-burning dominated century leading up to the industrial revolution around 1850, increasing BC fluxes were leading PAH fluxes. BC fluxes reached their millennial-scale maximum around 1920, whereas PAH fluxes increased exponentially to its record maximum around 1960, 50-fold above preindustrial values. For 1920-1950, BC fluxes consistently decreased as PAH fluxes kept increasing. Coal and coke represented >50% of the Swedish energy market in the 1930s. Combined with sharply decreasing (1,7-)/(1,7-+2,6-dimethylphenanthrene), indicative of diminishing wood combustion, and decreasing methylphenanthrenes/phenanthrene, indicative of higher-temperature combustion (coal instead of wood), the sediment archive suggests that the relative BC/PAH emission factors thus are lower for coal than for wood combustion. For the first time, both BC and PAH fluxes decreased after 1960. This trend break is a testament to the positive effects of decreasing reliance on petroleum fuels and a number of legislative actions aimed at curbing emissions and by 1990, the loading of BC was back at preindustrial levels, whereas that of PAH were the lowest since the 1910s. However, for the most recent period (1990-2004) the BC and PAH fluxes are no longer decreasing, putatively reflecting a slight increase in diesel consumption and a doubling of softwood-pellet burners for home heating.

Distal tephra horizons, particularly within lacustrine sequences, are increasingly being used as time-synchronous marker horizons within palaeoclimatic and palaeoenvironmental investigations. As sedimentary features marking the presence of these so-called cryptotephras are absent to the naked eye, it is of some importance that the stratigraphic position representing primary airfall, and likewise the timing of the volcanic event, is accurately and consistently recorded amongst these deposits. Often tephra shards from a cryptotephra deposit can be found spanning several centimetres of sediments within lacustrine and peat sequences. Very few studies, however, have looked in detail at the sedimentation and vertical distribution of cryptotephra deposits within such sequences and, more importantly, the criteria for defining the correct stratigraphic position of the volcanic event. Two sediment cores from Lake Spåime and Lake Getvaltjärnen, west central Sweden are employed to investigate in detail the vertical distribution of the tephra shards derived from the AD 1875 eruption of the Askja volcano in Iceland. Detailed geochemical analysis of shards from both records indicate that products of the Askja eruption are present for at least 120 years and thus emphasize the importance of carefully identifying the correct horizon that marks the timing of the volcanic event rather than shards resulting from a period of reworking or downward migration. Both sites yield contrasting shard concentration profiles and thus raise a number of questions regarding the influence of site-specific processes on cryptotephra sedimentation, particularly the role of snow-beds acting as tephra traps, the possibility of reworking, and downward migration of shards in soft sediment. A second tephra is also identified at Lake Getvaltjärnen and is believed to originate from the AD 1477 Veidivötn eruption and represents the first occurrence of this tephra outside of Iceland.

A pan‐arctic geospatial picture of black carbon (BC) characteristics was obtained from the seven largest arctic rivers by combining with molecular combustion markers (polycyclic aromatic hydrocarbons) and radiocarbon (14C) analysis. The results suggested that the contribution from modern biomass burning to BC ranged from low in the Yukon (8%) and Lena (5%) Rivers to high in the Yenisey River (88%). The Mackenzie River contributed almost half of the total arctic fluvial BC export of 202 kton a−1 (kton = 109 g), with the five Russian‐Arctic rivers contributing 10–36 kton a−1 each. The 14C‐based source estimate of fluvially exported BC to the Arctic Ocean, weighted by the riverine BC fluxes, amount to about 20% from vegetation/biofuel burning and 80% from 14C‐extinct sources such as fossil fuel combustion and relict BC in uplifted source rocks. Combining these pan‐arctic data with available estimates of BC export from other rivers gave a revised estimate of global riverine BC export flux of 26 × 103 kton a−1. This is twice higher than a single previous estimate and confirms that river export of BC is a more important pathway of BC to the oceans than direct atmospheric deposition.

The highly condensed products and residues of incomplete combustion of biomass and fossil fuel termed black carbon (BC) partake in a multitude of important geochemical processes. However, ambiguity persists because different quantification methods give highly variable results, leaving it unclear whether this reflects method difficulties or that different methods simply mirror different parts of the BC spectrum. Introducing additional wet chemical ex situ pretreatment steps (Gélinas et al. 2001b), to a commonly applied chemothermal oxidation (CTO) method led to large handling and transfer losses of hydrophobic diesel-soot BC (NIST SRM-2975). The resulting yield of soot BC spiked to a coastal marine and a freshwater sediment was only 6 ± 1% and 70 ± 4%, respectively, for the entire ex situ procedure. Instead, a standard addition approach yielded statistically significant linear returns of incremental soot BC additions. The slopes suggested that 51 ± 6% and 78 ± 6% of the added soot BC was accounted for in the marine and freshwater sediment, respectively. The lower recovery of soot BC in the marine sediment is likely caused by chloride enhancing the contact between BC surfaces and mineral oxides, catalyzing the BC oxidation. A consequential lowering of the oxidation energy from 375°C to 360°C resulted in increased soot BC yields of 75 ± 7% and 97 ± 3% in the marine and freshwater sediments, respectively. The standard addition approach suggests a native "soot BC-like" concentration of 0.48 mg BC/g dry weight (soot BC/total organic carbon = 0.03) in the bioturbated marine sediment from Kosterfjord, Skagerrak and 6.2 mg BC/g dry weight (soot BC/TOC = 0.10) in the heavily polluted Rhine River sediment.

Soot and charcoal, collectively termed "black carbon" or BC, can exhibit extremely strong sorption of many hydrophobic organic compounds. In order to include BC sorption in fate models, it is important to know BC nanopore surface areas. In addition, it is useful to know for which compounds BC sorption can be expected to be important. By nitrogen adsorption measurements at ultralow pressures on sediment that was strongly enriched in BC by HF treatment and/or chemothermal oxidation at 375 degrees C, we found that environmental BC has nanoporosity in the <4-10 A size range. The nanopore surface area (<50 A) of BC in Lake Ketelmeer (The Netherlands) sediment was approximately 58 m2/g. We measured sorption isotherms over a wide concentration interval for four compounds with the same Kow (10(46+/-0.1): planar anthracene (ANT), phenanthrene (PHE), and 4-chlorobiphenyl (4-PCB) along with nonplanar 2,2'-dichlorobiphenyl (2,2'-PCB). The environmental BC sorption coefficients of these iso-Kow compounds decreased in the order ANT > PHE approximately 4-PCB >> 2,2'-PCB and spanned a factor of 50-200, depending on concentration. Nonplanar 2,2'-PCB showed much more linear BC sorption (nF = 0.92) than the planar compounds (nF = 0.54-0.70). This shows that steric hindrance strongly attenuates BC-sorbate interactions for a nonplanar PCB. Thus, BC is more important for environmental sorption of planar compounds (>50% sorbed to BC in the nanogram per liter range) than for nonplanar ones (<10-20%). Using the measured BC nanopore surface area, a close agreement between modeled and measured BC sorption data could be found.