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Black carbon (1,2) soot particles are nucleated, grow and are deposited from the gas phase during combustion. They have surface, structural and compositional characteristics demonstrably different from those of commonly-used surrogates for atmospheric BC. Some of these surrogates are carbon blacks which have been post-treated with NOx (thus increasing adsorption), or graphite, for example. These materials differ in significant ways from environmentally produced BC. Major differences include soot-BC particle reactivity, hydration, radiative properties, C,H,O content, surface functionalities, unpaired electron spin density, surface area, and porosity. The properties of commercial carbons are sufficiently different from those of environmental BC and we recommend against their use in BC studies. Instead, we recommend using a soot standard produced in the laboratory from saturated hydrocarbons. N-hexane is recommended as the fuel, both because of a large amount of soot characteristic and reactivity data already in the scientific literature (e.g. 2-6) and because of variabilities due to fuel and combustion conditions. Careful control of air/fuel ratio in premixed flames recently has resulted in the establishment of quantitative relationships between several key particle properties (6). Test quantities of standard soot can be prepared from an established protocol in individual laboratories, or batches contracted from such commercial carbon producers as Cabot, Degussa, Columbian, etc., following a specified protocol.
Samples of n-hexane soot, as a surrogate for hydrocarbon combustion soot, are prepared as described in such publications as (3). Reagent grade n-hexane is burned under least turbulent conditions, the soot depositing on the inner surface of a 5" x 9" Pyrex funnel. The accumulated soot is removed, collected in a glass vial, mixed using a mechanical shaker, and stored in a desiccator. The soot prepared in this way has a surface area of 89 -/+2 m2 g-1, elemental composition ranging from 87% C, 1.6% H and 11% O to 92% C, 1.2% H, and 6%O. It consists of agglomerated spheroids of geometric diameter 0.05-0.10 micrometers, and has an aromaticity of at least 0.9. Representations of average particle structure have been published widely, e.g. (3).
1) Novakov, T. 1984. The role of soot and primary oxidants in atmospheric chemistry. Science of the Total Environment 36, 1-10.
2) Goldberg, E.D. 1985. Black Carbon in the Environment, Wiley, New York.
3) Akhter, M.S., Chughtai, A.R., Smith, D.M. 1985. The structure of hexane soot I: Spectroscopic studies. Applied Spectroscopy 39(1), 143-153.
4) Cachier, H., 1998. Carbonaceous combustion aerosols. In: Atmospheric Particles, R.M. Harrison and R.E. van Grieken, eds., Wiley, Chichester, New York.
5) Chughtai, A.R., Williams, G.R., Atteya, M.M.O. et al. 1999. Carbonaceous particle hydration. Atmospheric Environment 33, 2679-2687.
6) Chughtai, A.R., Kim, J., Smith, D.M. 2002. The effect of air/fuel ratio on properties and reactivity of combustion soots. Journal of Atmospheric Chemistry 45, 231-243.
Commercial charcoals and other lignocellulosic charcoals represent the carbonized residues of initial starting plant material that has been subjected to incomplete combustion and/or pyrolysis. The chemical and physical properties of charcoal prepared from lignocellulosic material in the laboratory vary significantly and are dependent on multiple factors, including starting material, temperature, ramping rates, atmosphere, and oxygen mass transfer conditions (1). In this context, we propose a large quantity of synthetic charcoal obtained using reproducible techniques from a single source. Moreover, because it still can be difficult to ensure reproducible results, we produced two large batches of each of two well-homogenized charred materials, one of which is representative of wood charcoal and one of which is representative of grass charcoal (2).
Wood and grass charcoal were produced from well-defined biomass sources under standardized conditions in a pilot scale pyrolyis oven (300 mm diameter, 700 mm length) at the Swiss Federal Laboratories for Materials Testing and Research, Ceramics Division. The temperature programme was as follows: room temperature increased at the rate of 300 degrees K per hour up to 200 °C (no hold), then 50 degrees K per hour up to 450 °C (held for 5 hours). Nitrogen flow rate was 500 litres per hour. The wood originates from Chestnut (Castanea sativa) trees and was made into pieces of established size, charred and homogenized. Rice (Oryza sativa) straw was taken as are a reasonable starting material for grass charcoal, since it was easy to obtain, inexpensive, and high in silica (a major chemical difference between grass and wood). Rice straw originated from Southern Switzerland (Magadino Plain). Production conditions and properties of the charcoals are described in detail in (2,3).
The charcoal standards are well homogenized, stored under standard lab conditions, and can be shipped on request. Subsamples (4 g each) can be ordered from Michael W. I. Schmidt, Univ. Zurich, Dept. Geography, 8057 Zurich, Switzerland (michael.schmidt@geo.uzh.ch).
Cost 40 EUR for each charcoal sample.
(1) Mackay D. M., Roberts, P. V. 1982. The dependence of char and carbon yield on lignocellulosic precursor composition. Carbon, 20 (2), 87-94.
(2) Hammes K., Smernik R.J., Skjemstad J.O., Herzog A., Vogt U.F., Schmidt M.W.I. 2006. Synthesis and characterisation of laboratory-charred grass straw (Oryza sativa) and chestnut wood (Castanea sativa) as reference materials for black carbon quantification. Organic Geochemistry 37, 1629-1633.
(3) Hammes K., Smernik R.J., Skjemstad J.O., Schmidt M.W.I. 2008. Characterization and evaluation of reference materials for black carbon analysis using elemental composition, colour, BET surface area and 13C NMR spectroscopy. Applied Geochemistry 23, 2113-2122.
