Statement of the Problem
Organic carbon (OC) and elemental carbon (EC) constitute large
fractions of the PM10 and PM2.5 mass in ambient air. EC, sometimes
termed “black carbon” or “soot,” is usually
produced by incomplete combustion of organic gases and particles.
It is not pure or highly structured carbon such as diamond and
graphite and rarely occurs as a pure particle unmixed with other
atmospheric constituents. OC, EC, and other fractions of the carbonaceous
aerosol are relevant to adverse health effects, urban and regional
haze, the Earth’s radiation balance, and the attribution
of ambient particle concentrations to their sources.
Total carbon is reliably measured by heating oxidation, conversion
to carbon dioxide or methane, and detection of the evolved gases.
The EC fraction has been determined by thermal evolution of particles
collected on a filter under different temperatures and atmospheres,
light reflectance and transmittance through particles collected
on a filter, sequential organic extraction of particles from a
filter, and the in-situ heating or particles in an air stream
with photoacoustic detection. Several methods report more refined
sub-fractions that can be combined to accommodate different definitions
of OC and EC. These carbon analysis methods do not consistently
provide comparable EC concentrations, as demonstrated in many
intermethod and interlaboratory comparisons. The degree of equivalence
depends on the nature of the sample (particle and filter) as well
as on the analysis method and protocol. Different protocols are
followed under the same method, but potentially important differences
that might affect the comparability are not adequately documented.
A more fundamental approach is needed to understand the reasons
why these discrepancies exist, how carbon sampling and analysis
methods should be documented, and how OC and EC concentrations
should be reported and validated.
Abstract and Workshop Goals
A. Clarification of Carbonaceous Aerosol Terminology
Elemental carbon aerosols have been studied extensively, but little
agreement exists as to whether the aerosol is only elemental carbon
material or a high molecular weight refractory organic species,
or a combination of both. Uncertainty exists in defining commonly
used terminology, such as elemental carbon, black carbon, total
carbon, soot, and black-elemental carbon. Furthermore, uncertainty
exists in defining the difference between what constitutes secondary
organic aerosols and what constitutes primary organic aerosols.
Goal 1: Reach consensus among leading researchers
in the field of carbonaceous aerosols on the definition of these
terms. In addition, new terms may be derived to more explicitly
characterize the molecular structure and/or classes of organic
aerosol species present in the atmosphere.
B. Identify and Explore Limitations of Current Organic
Aerosol Sampling Technologies
Due to the numerous types of chemical species comprising organic
aerosols, with their inherently different physical properties,
accurate sampling of organic aerosols is a very challenging task.
Two complications are positive and negative sampling artifacts.
Even though organic compounds are by far the most numerous class
of chemical compounds in the atmosphere, each individual species
generally is only a small proportion of the total mass of organic
carbon collected. Thus, adequate sample sizes must be collected
to ensure accurate quantification of trace compounds.[This is
simply not true that you need sampling times in excess of a day.
You just need to increase your sampling flow rate.]
Goal 2: Address sampling difficulties such as
the condensability (i.e., positive artifacts) and volatility of
organic species (i.e., negative artifacts), and develop a strategy
for sampling specific types of organic aerosols depending on their
physical and chemical attributes.
C. Identify and Explore Limitations of Current Technology
for Analyzing Carbonaceous Aerosols
The most difficult problem in the study of organic aerosols is
the lack of reliable chemical analysis methods to distinguish
the hundreds of compounds that may comprise a single aerosol sample.
Currently, several methods are available to process and chemically
analyze atmospheric organic aerosol samples including GC, GC/MS,
and liquid chromatography.[EM is used to determine physical characteristics.].
It should also be emphasized that there is no single accepted
methodology of distinguishing organic carbon aerosols from elemental
carbon aerosols. Furthermore, methods to distinguish secondary
organic carbon aerosol species from primary organic carbon aerosol
species have not been developed or tested.
Goal 3: Recommend standardized analysis method
to distinguish organic carbon aerosol species from elemental carbon.
Goal 4: Develop and recommend standardized
analysis methods to measure specific classes of organic aerosols.
These standard methods would allow inter-laboratory comparisons
of data, which will be necessary for the development of organic
aerosol formation simulation models.
Goal 5: Develop and recommend standardized
analysis methods to distinguish secondary organic carbon aerosol
species from primary organic carbon aerosol species.
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