OC/EC Workshop - Durango, Colorado

Statement of the Problem

(Revised 1/13/03)

SOA Presentation 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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