N-hexane soot
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.
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).
References
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.