Chemical characterization of the black carbon

Chemical characterization of the black carbon reference materials – validating appropriateness based on elemental, spectral and isotopic analyses
K. Hammes (1), J.O. Skjemstad (2), R.J. Smernik (2) and M.W.I. Schmidt (1)
(1) Department of Geography, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich,
Switzerland (2) CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia
(hammes@geo.unizh.ch / Fax: +41 1 635 6848 / Phone: +41 1 635 5183)

Black carbon (BC) is globally present as an inert residue resulting from vegetation
and fossil fuel burning. BC has a diverse nature, and there are various methods with
which to measure it (thermal, chemical, optical, indirect). An international ring trial
was initiated to compare these different methods of analysis. For consistency, twelve
reference materials were chosen as standard reference materials for BC measurement.
To validate if the reference materials were chosen appropriately to be representative
of natural materials, we chemically characterised the twelve reference materials using
elemental analysis, DRIFT, 13C CP and BD NMR and 13C. The materials chosen for
the ring trial can be split into two groups. The first group is expected to contain actual
BC. It includes soot, charcoal (wood and rice), aerosol, two soils (chernozem and vertisol)
and marine sediment. The second group (shale, melanoidin, natural organic matter
from river water and two coals) was included to detect methodological artefacts.
These materials are considered to contain little or no BC, but can possibly produce
artefacts during application of some of the methods. Most of the reference materials
were obtained through (standard material) suppliers, however, the wood char, rice
char and melanoidin were produced by us. The wood char was produced by pyrolysis
to simulate natural conditions in a burning log. No commercial standards for these
materials are available. The starting materials for the wood and rice char were also
chemically analysed.
The reference materials come from diverse environments, with large variations in C
values, ranging from 2 % for the chernozem to 90 % for the soot. The clay percentage of the vertisol (37 %) is twice as high as the chernozem (19 %), which plays a
significant role in the protection of organic matter and possible BC too. Soot had the
lowest H/C and O/C ratios, indicating the presence of condensed ring structures. Both
char materials had relatively low H/C ratios (thus hydrogen-poor) but enough oxygen
functional groups to have a moderate O/C ratio. The uncharred wood and rice had high
H/C and O/C ratios and plotted in the cellulose region as expected. The other standard
materials had H/C and O/C ratios within the typical range for these materials given in
literature. This elemental data is supported by the CP and BD NMR analysis done on
the materials. Thermal treatment of the wood and rice material resulted in a loss of Oalkyl
structures and an increase in aromatic carbon. The 13C CP an BD MAS spectra
for the vertisol and chernozem display two to three times more aryl C than any other
structure, indicating a large presence of aromatic carbon. The 13C values of the materials
range from –29.4 per mille for the shale to –16.4 per mille for the melanoidin.
The 13C values of the uncharred wood (-26.6 per mille and uncharred rice (-26.4 per
mille decreased only slightly (-0.3 per mille and -0.1 per mille respectively) during
charring and might not be significant. Summarising, from the data presented it can be
concluded that the reference materials represent typical natural samples and underscore
characterisation data from literature, rendering them appropriately chosen for
the black carbon ring trial.

Geophysical Research Abstracts, Vol. 7, 02205, 2005
SRef-ID: 1607-7962/gra/EGU05-A-02205
© European Geosciences Union 2005