OC/EC Workshop

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Main Topic & Follow Up Abstracts

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TOPIC #1 . . .

What is elemental carbon and how do definitions differ for different applications? What are the OC and EC properties that are of importance to human health, visibility, climate, and source attribution? To what extent can a single analytical method or protocol meet these different needs?

Robert Cary, President, Sunset Laboratory, Inc.

Compared with all the other elements, carbon has the ability to form an incredible number of different compounds with a likewise incredible number of characteristics. This is due to its unique bonding ability. When considering the pure element itself, this bonding allows for two unique substances. Diamond, which can be considered as the ultimate polycyclic aliphatic-bonded form of carbon; and graphitic carbon, which can be considered the ultimate polycyclic aromatic-bonded form. The later, unfortunately, seems to be the one found commonly as a component of atmospheric aerosols, and is the one of interest here when referring to elemental carbon, EC.

There are a number of physical and chemical properties that define this graphitic aerosol carbon. And there are a number of applications where some of these properties are important and others not so important.

From a chemist point of few, elemental carbon consists of an extended aromatic-ring structure where the carbon atoms are bonded by sp2 -bonds and delocalized pi-bonds. As the number of rings increase, these pi-bonding electron orbitals become degenerate across the entire structure and end up existing at a level with little or no gap between the valence bands and conduction bands. This results in elemental carbon being a fairly good conductor of electricity when influenced by an electric field and contributes to many of its chemical and physical properties.

The strengths of these bonds cause elemental carbon to be very thermally refractory, fairly chemically inert at temperatures less than several hundred degrees and insoluble in solvents.

From a physical-optics point of few, elemental carbon interacts with electro-magnetic radiation as a result of these conducting electrons very similarly to metals. In its bulk form as the mineral graphite, it in fact appears very “metallic”. As small aerosol particles, this interaction can be quantitated by its index of refraction consisting of a real part and imaginary part, which are measures of the ability to scatter and absorb electro-magnetic radiation. The values of this index of refraction are a function of not only the wavelength, but also the geometric “size” and morphology.

Since there are a fair number of electrons in the conductance bands with no energy gaps, there are no unique absorption frequencies and thus elemental carbon can absorb energy throughout the visible spectrum. This is what causes it to appear “black” as very small particles. Many applications such as climate and visibility studies are concerned with this interaction of EC with electro-magnetic energy.

Many applications are more interested in the chemical properties. Its inertness is one reason EC is used as a tracer for certain sources. Nearly all processes that form EC do so in a way that results in at least some occluded organics, and in many cases the EC is a small fraction of the aerosol with OC being the major mass fraction. However, when produced as the major mass fraction, such as in diesel engines, the large surface area causes EC to act as a very efficient adsorber of many compounds that diffuse to its surface. Thus, either alone or as a carrier of other compounds, there is a great deal of interest in its health effects. And it is this surface that is of interest as a location for other chemical process.

Most efforts at quantitating EC are focused on its chemical inertness, thermal stability or optical properties. The difficulty of properly speciating EC from other forms of carbon, such as organics, arises for a number of reasons. And the proper speciation becomes most difficult when the EC fraction is small compared to the other forms. Methods that depend upon the insolubility or chemical inertness of EC must contend with the fact that there are forms of OC that are also insoluble or inert. Optical methods must deal with optical properties of other particles as well as the morphology and optical properties of the EC particle itself. And thermal methods must deal with the likely possibility that some organic forms will be pyrolytically transformed into EC. And this is complicated by the fact that the EC fraction of the aerosol itself was originally produced by a pyrolitic process.

Topic 2 . . .
What options exist for fundamental and traceable OC and EC standards?

Lloyd Currie, Emeritus Fellow, National Institute of Standards and Technology, Gaithersburg, Maryland

Click Here for Dr. Currie's "draft reference material document" (in Microsoft Word format), in which he details his discussion on Topic 2.

Follow Up Abstract

On the Distribution of the Blank
L.A. Currie and J.M. Conny (NIST)

The distribution of the blank sets the ultimate limitation for low-level measurement. Its dispersion strongly influences the uncertainty of the net signal, and often it is the prime factor governing detection and quantification capabilities. If the blank distribution shows a positive skewness, it is strongly recommended that observations (sample, blank) be paired. The influence on the validity of detection decisions will be examined with examples from analytical blanks for sulfur (TIMS), 14C (AMS), OC (TOT), and EC (TOT).

Follow Up Abstract (Mr. Klouda will not be attending, but requests inclusion in the Research Strategy)

A Filter-Based Fine Particulate Matter Reference Material (RM) 2784 [1]
George A. Klouda*, Helen J. Parish†, James J. Filliben* and Judith C. Chow‡

* National Institute of Standards and Technology, Gaithersburg, MD 20899
† SRI International, Menlo Park, CA 94025
‡ Desert Research Institute, Reno, NV 89512

To support national monitoring of fine particulate matter (PM 2.5) concentrations of organic carbon (OC), elemental carbon (EC) or black carbon, and total carbon (TC), NIST, in collaboration with SRI International and the Desert Research Institute, has produced a Filter-Based Fine Particulate Matter Reference Material (RM) 2784 using the Washington, D.C. Urban Dust Standard Reference Material (SRM) 1649a. SRM 1649a was resuspended in a purified airflow and size-segregated by a series of processes - exposure to air at sonic velocity, sedimentation and impaction – designed to pass only particles less than 2.5 mm aerodynamic diameter. This fine aerosol fraction was then collected on 320 quartz-fiber filters, simultaneously, under matched flow conditions. A total of 7 batches were obtained to complete production of 2240 filters [2]. The mass of fine PM on each filter was determined by gravimetric analysis before and after the resuspension and collection process. Process blanks were prepared for possible chemical analyses and weighing controls were obtained to estimate potential bias in the gravimetric measurement.

Each filter has a set of characteristics that provides a unique identification: filter identification number, PM-mass loading, batch, chamber, and chamber-position. The distribution of PM-mass loadings with respect to chamber-position, chamber and batch appears quite variable. After nine outliers with negative loadings are removed, the average mass and standard deviation for the remaining 2231 filters is 1064 ± 414 mg dust per filter (39% relative standard deviation) with a range of 2763 mg. Gravimetric analysis of control filters treated similarly as the loaded filters but without exposure to the dust shows that on average the difference between the initial weighing and the final weighing is 46 ± 63 mg (standard deviation, n = 140).

An inter-laboratory and -methods comparison is underway to obtain value assignments [3] and uncertainties [4, 5] for EC, OC and TC concentrations (in g/g dust) and for the EC/TC ratio of RM 2784 filters using two specific thermal-optical analysis (TOA) methods. The NIOSH (National Institute of Occupational Safety and Health) Method 5040 [6, 7], with a slightly modified temperature profile for the Environmental Protection Agency (EPA) PM 2.5 Chemical Speciation Trends Network (STN) [8], will be applied to thermal-optical transmission (TOT) instruments. The IMPROVE (Interagency Monitoring of Protected Visual Environments) Method will use thermal-optical reflectance (TOR) instruments [9].

Beyond providing value assignments of the analytes identified above for RM 2784, the results of the intercomparison are expected to show whether a mass-interval effect exists for the mass fraction of each analyte defined by the method and the EC/TC ratio, what is the within-filter homogeneity, and what is the filter-to-filter homogeneity. A secondary analysis of the data will examine factors that may have influenced the production of these filters, e.g., batch, sampling chamber, and chamber location, i.e. quadrant, column and row that are unique to each filter.

[1] ISO, “International Vocabulary of Basic and General Terms in Metrology (VIM),” 2nd Edition; BIPM/IEC/IFCC/ISO/IUPAC/IUPAP/OIML, International Organization for
Standardization (ISO), 1993.

[2] Watson, E.L., Irwin, K., and Parish, H.J., “ Preparation of SRM 2784 on quartz filters,” SRI International, Final Report, August 2002.

[3] May, W., Parris, R., Fassett, J., Greenberg, R., Guenther, F., Wise, S., Gills, T., Colbert, J., Gettings, R. and MacDonald, B., “Definitions of Terms and Modes Used at NIST for Value- Assignment of Reference Materials for Chemical Measurements,” Natl. Inst. Stand. Technol., Special Publication 260-136, January 2000.

[4] ISO, “Guide to the Expression of Uncertainty of Measurement: First edition 1993; ISBN 92-67- 10188-9; International Organization for Standardization (ISO), 1993.

[5] Taylor, B.N., and Kuyatt, C.E., “Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, Natl. Inst. Stand. Technol., Tech. Note 1297, (1994).

[6] Birch, M.E. and Cary, R.A., “Elemental carbon-based method for monitoring occupational exposures to particulate diesel exhaust,” Aerosol Sci. Technol., Vol. 25, pp. 221-241 (1996).

[7] National Institute of Occupational Safety and Health (NIOSH), “Elemental Carbon (Diesel Particulate): Method 5040,” NIOSH Manual of Analytical Methods, 4th ed. (http://www.cdc.gov/niosh/nmam/pdfs/5040f3.pdf), Cincinnati, Ohio, 15 January 1997

[8] EPA Strategic Plan, “Development of the Particulate Matter (PM 2.5) Quality System for the Chemical Speciation Monitoring Trends Sites,” Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina (www.epa.gov/ttn/amtic/files/ambient/pm25/spec/strate1.pdf).

[9] Chow, J.C., Watson, J.G., Pritchett, L.C., Pierson, W.R., Frazier, C.A., and Purcell, R.G., “The DRI thermal/optical reflectance carbon analysis system: description, evaluation and applications in U.S. air quality studies,” Atmospheric Environment, Vol. 27A, No. 8, pp. 1185-1201 (1993).

For more information, contact: George A. Klouda, National Institute of Standards and Technology, 100 Bureau Dr., Mailstop 8375, Gaithersburg, MD 20899-8375; 301-975-3931; (Fax: 301-417-1321); george.klouda@nist.gov

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TOPIC #3 . . .
How does the sample affect the measurement of different carbon fractions? How do properties of particles on a filter differ from those in ambient air? How do different compounds react with heat and among themselves to create pyrolized carbon? How do different filter loadings affect optical measures of pyrolysis? Under what conditions might other carbon-containing components (e.g., carbonates) be detected as OC or EC? What additional information should be reported with OC and EC values to evaluate the precision and validity of an OC/EC split?

Judith Chow, Research Professor, Environmental Analysis Facility, Desert Research Institute

Click Here to view Dr. Chow's abstract in Microsoft Word

Follow Up Presentation Abstract 1

Effects of iron oxides on the determination of organic and elemental carbon using thermal optical techniques

Kochy Fung, AtmAA, Inc. Calabasas, CA, U.S.A.

Ambient particulate samples are routinely analyzed for organic and elemental carbon (OC/EC) using either thermal volatilization-pyrolysis correction methods, as in Interagency Monitoring of PROtected Visual Environments (IMPROVE) with correction by reflectance, or in the National Institute of Occupational Safety and Health (NIOSH) Method 5040 using transmittance (TOT). In both methods, the filter is heated stepwise with specified temperatures, rates of temperature increase, and residence times at each temperature, first in He to volatilize the OC, then in O2/He for EC combustion. The resulting CO2 is converted to methane for detection by a flame ionization detector. A recent comparison study (Chow et al, 2001) found that NIOSH EC was 107 + 77.5% lower than IMPROVE.

The difference was thought to be relating to the oxidation by metal oxides under the NIOSH 900oC-temperature step for OC. It is already known that MnO2 can oxidize EC at temperatures higher than 525oC. To study if other metal oxides bear similar property, soot samples containing iron oxides were created from combusting ethylene with iron pentacarbonyl vapor. Acetylene gas was added for soot formation. By varying the level of acetylene in the combustion mixture, different amount of soot to iron content can be obtained. Samples with only soot particles, iron oxides, and a mixture of both were analyzed according to the IMPROVE and NIOSH protocols.

The results of the analyses indicated that iron oxides attenuated the laser signals in both the OC and EC phase of the thermal optical methods. During the OC phase of the NIOSH protocol (helium atmosphere) EC or other light absorbing carbon was oxidized or volatilized in the samples containing iron oxides when the temperature reached 800oC and above. The orange-colored residue that remained on the filter disc after the IMPROVE analysis was not visible on NIOSH-analyzed disc. Thus the presence of significant amount of iron oxides in a sample can increase the apparent OC content due to their affects on increasing pyrolysis correction, as well as oxidation of EC during the OC phase of the NIOSH protocol.
Chow, J.C.; Watson, J.G.; Crow, D.; Lowenthal, D.H.; Merrifield, T. Comparison of IMPROVE and NIOSH carbon measurements; Aerosol Sci. Technol. 2001, 34(1), 23-34.

Follow-up Presentation Abstract 2

Measurement of Carbonate Minerals in Aerosol Samples

Johann Engelbrecht, Associate Research Professor, Desert Research Institute

Carbonate minerals such as calcite, dolomite and magnesite commonly occur in aerosols from arid regions of the western USA. Screened as well as PM10 and PM2.5 filter samples of ten re-suspended soils were analyzed for their mineral contents by X-ray Diffraction (XRD). Mineral species identified in these samples include quartz, calcite, mica, feldspar, dolomite, rutile, gypsum, and clay minerals. These samples were also chemically analyzed for their elements, ions, OC and EC, as well as carbonate contents. The mineralogical and chemical results are compared, with emphasis on the measurement of carbonate minerals.

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TOPIC #4 . . .What are the important parameters that need to be defined for a carbonaceous aerosol analysis and how should these be documented for different analysis protocols?
Hélène Cachier, LSCE/CFR, laboratoire mixte CEA-CNRS, France

Intercomparisons have shown for a long time important discrepancies among EC/OC ratio results produced with different analytical protocols. Only very recently have efforts been deployed to understand the reasons. Although some answers clearly reveal which things to avoid, answers for what to do are still pending.

Keeping in mind firstly, the chemical complexity and the diversity of carbonaceous aerosols and, secondly, the fact that there is no clear separation between high molecular weight OC and EC, the “thermal” definition of EC is obviously operationally-dependent. The aim is however to uniformize results and ultimately to reconcile both thermal and optical definitions of BC and EC.

1) Indeed thermal-optical analyses rely on both properties of BC/EC, the light absorption and the refractory behaviour. This is an additional argument to have only one name for this component: BC, whereas EC can induce some conceptual errors in the chemical nature of the particles.

2) There are two main classes of analyses:

  • analyses under oxidative atmosphere, producing a gradual combustion (oxidation) of the carbonaceous particles;
  • 2-step analyses with the first step under an inert gas (helium or nitrogen), producing a pyrolysis of the OC component.

Applying such thermal treatments modifies the particles differently. This is particularly crucial for EC particles. In conclusion, any mix of both treatments in the analytical protocol must be considered with caution.

3) The analysis by itself brings artefacts: charring of OC and untimely departure of BC.

In all cases, it is better to minimize artefacts than to try to quantify them. This is primarily due to the fact that the different fractions of EC have different optical properties (absorption coefficient) and that their time (temperature) evolution may overlap.

When entering the problem of carbon analysis, some questions arise:

a) The choice of method: under oxygen/or mixed atmosphere?

  • Why choose the second method, which apparently brings more artefacts?
  • Does the EGA/O2 bring a satisfactory split between OC and BC?
  • For this methodology, why has the importance of the plateau duration (almost) never been discussed?

b) Do we actually need the split of OC and BC into several fractions?

  • Hasn’t it a historical origin?
  • Have these fractions any environmental significance?

c) How do dust particles affect the signal (laser signal, evolution of EC)?

d) How do we treat the problem of carbonates?

  • In other words, is it possible to avoid removal of carbonates prior to thermal analysis? If so, may carbonate peak be clearly identified?
  • Does acidic treatment affect the carbon content and the split EC/BC?

e) What is the limitation of the analysis in terms of EC concentration per area unit?

  • Indeed, this parameter may affect the gas penetration in the particle layer, and additionally may affect the laser sensitivity for pyrolysis correction.

f) If we chose the EGA/He/O2 analysis:

  • How to minimize charring; although some fractions char more than others, it may be seen that charring occurs very early in the temperature program.

g) Finally, a side problem is the absorption of VOC (or CO2?) during storing.

  • Should we treat the sample prior to analysis?
  • How does the cleaning of the filter affect its capability of positive (passive and active) artefacts?

In light of the above-mentioned considerations, there is a need to delineate the main determinants which have the potential to create discrepancies in the determination of EC/TC ratios.

Specific parameters to be defined for EGA/O2 analysis:

- decarbonatation: indeed, due to a matrix effect, carbonates evolve at lower temperature than assessed for pure material, and the generally adopted temperature for the evolution of EC (720 to 750°c) actually creates some decomposition of carbonate.
- The temperature increase rate which is especially critical between 300°c to 400°c where there is some overlap of the OC and EC fractions.
- optimum concentration range of the filter loading.
- beginning and end temperature.

Specific parameters to be defined for EGA/He/O2 analysis:

- number of plateaux, duration, temperature.
- should we link the plateau duration to background recovery?
- is there any special procedure in the transition zone? (lower increase of temperature)
- laser monitoring wavelength, type of optical measurement: T or R?
- description of flash heating if any.

Finally in the discussion 2 other items will appear:

- the role of the nature (origin, age) of the aerosol will be introduced which probably leads to the discussion that some methods are “better” for a given type of aerosol.
- How to analyze cascade impactor samples?

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TOPIC #5 . . .
What specific compounds are likely to evolve during different temperature fractions of thermal evolution methods used to analyze carbonaceous aerosols?
Joellen Lewtas, US EPA/Office of Research & Dev’t/Nat’l Exposure Research Lab

Follow Up Presentation Abstracts

Organic Carbon Concentration and Composition in Fine Particulate Matter Collected During ARIES Study

Barbara Zielinska, Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512

Fine particulate matter (PM2.5) was measured daily from August 1998 to December 1999 over a 24-hr sampling period in an Atlanta residential area to provide data for an epidemiological study. Samples were collected using the DRI Sequential Fine Particulate/Semi-Volatile Organic Compounds (PSVOC) sampler, which allows unattended collection of up to four samples. To account for the semi-volatile organic compounds associated with particles, a quartz filter of 10 cm diameter was backed by polyurethane foam plugs (PUF) in combination with the polystyrene-divinylbenzene resin XAD-4.

All quartz filters were analyzed for organic and elemental carbon (OC/EC) by thermal/optical reflectance method (TOR) prior to extraction. Filters and PUF/XAD cartridges were extracted together, using three solvents of different polarity (dichloromethane followed by acetone, followed by water). The mass of the extracts was determined by weighing the residue of 20 microliter aliquots of the extracts deposited on a pre-fired quartz filter punch. The organic carbon content of the extracts was determined by subsequent analysis of the punch by the TOR method. Select punches were also analyzed by gas chromatography/mass spectrometry (GC/MS) using a thermal desorption method. Thermal desporption was conducted in a sequential fashion with increasing temperatures that try to mimic the temperature ramps used with the TOR method (1: 120 ºC; 2: 250 ºC; 3: 380 ºC).

This presentation will discuss the major compounds identified during sequential desorption analyses of the extracts deposited on quartz punches. The desorption method can provide some insight into the composition of the organic fractions observed during the TOR temperature ramps.

Topic 5 Follow Up Abstract 2Insights from Thermal Analysis of Individual Organic Compounds, Mixtures, Black Carbon Surrogates, Airborne Particulate Matter and Extracts.

L. A. Gundel, Environmental Energy Technologies Division, Lawrence Berkeley National Lab

L.A. Gundel, T.W. Kirchstetter, R.L. Dod, Y. Pang, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720; C.S. Claiborn, Department of Civil and Environmental Engineering, 101 Sloan Hall, Washington State University, Pullman, WA 99164-2910.

This presentation compares thermograms of organic compounds, ambient, source and 'surrogate' particulate matter. Here 'surrogate PM' refers to well-characterized mixtures of fine activated carbon (AC) particles and known compounds. In addition, thermograms of extracts and extracted ambient PM are compared to 'reconstructed' PM (extracts + fine AC particles) and the original ambient PM. The goal is to shed light on the chemical characteristics of the temperature-defined carbon fractions in relation to what is currently known from detailed speciation efforts for organic compounds in source and ambient PM.

Thermograms of individual compounds suggested that volatility and MW, rather than class or functional group, controlled the evolution of OC in the fractions through 250 C. For compounds that evolved between 250 and 500 C, formation of light absorbing carbon in situ was more likely for polyfunctional molecules with at least one aromatic ring. Complex molecules with saturated rings also pyrolyzed easily.

When adsorbed onto clean fine AC particles, individual compounds showed thermograms with more complexity: in surrogate PM the compounds could distribute over more than one temperature fraction, for example (not counting the EC from AC). However, adding extracts of ambient PM to fine AC did not necessarily yield the original ambient PM OC-EC pattern. Reconstructed PM did not contain the high-molecular weight unextractable OC that was usually associated with ambient PM.

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TOPIC # 6 . . .
How does carbonaceous particle composition, shape, and size affect optical properties in the air and when sampled on a filter?
Kirk Fuller, Research Scientist, National Space Science and Technology Center, Univ. of Alabama

Organic carbon aerosols free of any graphitic (elemental) carbon may be both spherical and hygroscopic. To the extent that this is so, their in situ optical properties are amenable to analysis with Lorenz-Mie theory, with the greatest unknown being the complex refractive index (i.e., the exact composition) of the particles. Soot is typically comprised of graphitic carbon mixed with incompletely oxidized fuel. Rather than being spherical, fresh soot particles are loose (ramiform), fractal-like agglomerations of graphitic carbon spherules which may become more tightly packed upon aging. The spherules are composed of nanometer-sized domains of graphite crystals. The crystalline structure of the graphite itself is imperfect, with frequent substitution of carbon atoms in the hexagonal C6 molecules comprising the crystals, or intercalation of molecules between the stacked C6 rings. Such variations in structure and composition leave the complex refractive index of the material variable, as well. The theoretical analysis of soot optics is more difficult, but such analysis indicates that aggregation of the spherules tends to enhance their scattering and absorption cross sections. The greatest enhancement in absorption cross section, however, arises from encapsulation of elemental carbon by nonabsorbing host particles, such as solution droplets of ammonium sulfate.

Absorption coefficients of atmospheric aerosol are routinely inferred from reflection and/or transmission measurements of particle deposits on quartz, nuclepore, or Teflon filters. In such procedures, the filters may be regarded as optical elements which are often used in concert with integrating spheres, integrating plates, or both. The technique was originally developed by Lin et al. (1973), and a number of variations have followed (cf. Rosen et al., 1978; Sadler et al., 1981; Edwards et al., 1983; Clarke et al. 1987; Campbell et al., 1995; and Bond et al., 1999). A proper accounting of scattering in the radiative transfer is a requirement of all such measurements. Of particular relevance to filter samples, but much less understood, are the alterations of a particle's absorption (and scattering) spectrum by its close proximity to other particles and by its attachment to substrate surfaces.

Absorption by particles in a filter deposit differs from in situ absorption because of (1) multiple scattering in the deposit/substrate system, (2) alteration of absorption and scattering cross sections by electromagnetic coupling between particles, (3) electromagnetic coupling of particles to filter surfaces, (4) optical coherence between particles with separations comparable to the wavelength of the interrogating radiation, (5) induced alignment of nonspherical particles along filter surfaces, (6) shape distortion of liquid droplets, and (7) reactions among different chemical species, especially over extended sampling times.

Raman spectroscopy may provide a useful analysis of soot in that there is a pair of vibrational signatures present in the spectra of graphitic carbon that are not active in the infrared. The band strength of one relates to the quality of the crystal structure, while the other relates to the size of grain boundaries within the cryptocrystalline spherules that comprise the soot fractal (Rosen et al., 1978). Since the degree of molecular ordering in the spherules is a function of initial combustion conditions and thermal annealing, such features may be useful in distinguishing between biomass and fossil fuel soot, as well as in studying the modification of carbonaceous material during thermo-optic measurements.

If soot initially exists as an inclusion in sulfate aerosol or the amount of OC coating EC is great enough, then evaporation of those materials may increase the fractal dimension of the aggregates, thereby altering its mass-specific absorption cross section. Compositional changes during heating seem especially difficult to predict, though it seems that monitoring the infrared (in addition to the Raman) spectra of the deposits may provide some insight.

The overview by the topic leader will provide a bit more detail and some examples of the effects described above.

REFERENCES (in addition to suggested reading)

Bond, T. C., T. L. Anderson, and D. Campbell. Calibration and intercomparison of filter-based measurements of visible light absorption by aerosol particles. Aerosol Sci.
Tech., 30:582-600, 1999.

Campbell, D., S. Copeland, and T. Cahill. Measurement of aerosol absorption coefficient from teflon filters using integrating plate and integrating sphere techniques. Aerosol Sci.
Techno., 22:287-292, 1995.

Clarke, A. D., K. J. Noone, J. Heintzenberg, S. G. Warren, and D. S. Covert. Aerosol light absorption measurement techniques: Analysis and intercomparisons. Atmos. Environ., 21:1455-1465, 1987.

Edwards, J. D., J. A. Ogren, R. E. Weis, and R. J. Charlson. Particulate air pollutants:
A comparison of British “smoke” with optical absorption coefficient and elemental carbon concentration. Atmos. Environ., 17:2337-2341, 1983.

Lin, C.-I., M. Baker, and R. J. Charlson. Absorption coefficient of atmospheric aerosol:
A method for measurement. Appl. Opt., 12:1356-1363, 1973.

Rosen, H., A. D. A. Hansen, L. Gundel, and T. Novakov. Identification of the optically
absorbing component in urban aerosol. Appl. Opt., 17:3859-3861, 1978.

Sadler, M., R. J. Charlson, H. Rosen, and T. Novakov. An intercomparison of the integrating plate and the laser transmission methods for determination of aerosol absorption coefficients. Atmos. Environ., 15:1265-1268, 1981.


TOPIC # 7 . . .
How might current analysis methods be enhanced or combined to obtain more information about the nature of OC, EC, and other carbon fractions in filter samples? What can be done with existing analysis methods and samples? What might be provided by collocated measurements? What hardware and software changes would permit more of the commonly applied protocols to be applied with the same analytical instruments?

Hans Hansson, Air Pollution Laboratory, Institute of Applied Environmental Research and Department of Meteorology, Stockholm University, Sweden

Interest in carbon compounds in the atmospheric aerosol is not mainly from the point of view of basic science, but rather for their potentially large influence on climate and health. The climate influence is especially due to the absorbing properties of what is commonly called soot, Elemental Carbon or Black Carbon. This has been noted in the recently published results of the large field experiment INDOEX. The air pollution plume originating over India and flowing south over the India Ocean, has shown indications of increasing climate forcing by 10 W/m2 in the air above ground, due to the high content of light absorbing material.

Cloud drop formation depends on the size and chemical composition of the particles on which the droplets are formed. The amount of inorganic salts has been considered to totally determine whether or not a particle will nucleate (activate) as a cloud droplet, as described through the original Köhler equations. However recent theoretical investigations, where the Köhler equations had to be further developed, indicate that organic components have a crucial influence on the activation properties and possibly the resulting cloud droplet size distribution, as well. The global climate forcing due to this phenomenon is not known.

A major source of particulate carbon to the atmosphere is the combustion engine. The emission factor by mass varies greatly and depends on the type of engine, its age and the kind of fuel it uses. New developments have strongly reduced the carbon mass emitted, but this reduction has been achieved by reducing the size of the particles, not the number. The new particle traps for diesel engines decrease particle number and mass by about 99%, but measurements show that, due to nucleation after the particle trap, the number of particles, in the nanometer range, increase strongly, and that the total number of particles is comparable to what is found without the particle trap.

Globally, the total number of particles emitted to the atmosphere from combustion in general, and specifically from engines, is very large. A simple box model calculation for Germany gives an estimate of several thousand particles per cc at the border of the emission area. Such concentrations can prevail over several thousand kilometers assuming a lifetime of 4-8 days for OC/EC. Even if the particles are small, some tenths of nm, they might have a profound influence on particle formation and resulting size distribution far away from the source area.

The recent WHO review on the health effects of ambient particles, states that there is strong evidence that fine particles (< 2.5 mm, PM2.5) are more hazardous than larger ones (coarse particles) in terms of mortality and cardiovascular and respiratory endpoints in panel studies. This does not imply that the coarse fraction of PM10 is innocuous. Amongst the characteristics found to contribute to toxicity in epidemiological and controlled exposure studies, are metal content, presence of PAHs and other organic components, endotoxin content as well as small (< 2.5 mm) and extremely small size (< 100 nm). Further, it states that a number of source types are associated with health effects, in particular motor vehicle emissions and coal combustion. These sources produce primary as well as secondary particles, both of which have been associated with adverse health effects.

In summary, several important characteristics of particles, other than mass, such as number size distribution and physical and chemical properties, are necessary to estimate the effects of OC/EC. But still TC/EC/OC has to be measured. However efforts have to be made to minimize artifacts, like adsorption of VOC’s and losses of semi-volatile particulate matter, in the sampling procedure.

The starting point for the discussion will be to question if one standard for measuring EC/OC will be useful for addressing all different effects. Different approaches will be suggested depending on the effect to be estimated.

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TOPIC # 8 . . .
What new and innovative sampling, analytical, and interpretive techniques are needed to determine the properties and sources of carbonaceous aerosol in the atmosphere?
Topic Leader: Hans Moosmüller, Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512

The total carbon content (TC) of carbonaceous aerosol can be determined accurately with thermal analysis methods. In addition, thermal analysis methods utilize the refractory nature of elemental carbon (EC) and the volatility of organic carbon (OC) to quantify these two components of TC. However, while heating filter samples in an inert atmosphere to volatilize OC, part of the OC pyrolyzes into EC. Despite procedures correcting for this charring, the resulting EC/OC split depends sensitively on the heating process and the charring correction. Therefore, EC and OC determined by thermal-optical analysis are method-dependent operational definitions, poorly suited as primary standards. However, this situation may change if the charring process and compounds removed and created through charring can be better quantified.

Alternative techniques for determining the EC/OC split are needed for source apportionment, radiative transfer (including visibility), and health effect applications. Of particular interest are real time techniques with high (1 s) time resolution, which can be used to characterize both ambient aerosol and emissions.

OC is a conglomerate of hundreds or thousands of individual chemical compounds and it is difficult to define common properties which can be used for OC quantification. On the other hand, EC consists of graphitic micro crystals, which can be quantified using several unique properties including 1) strong light absorption, 2) graphitic Raman spectrum, 3) insoluble in polar and non-polar solvents, and 4) thermally refractory.

1) Measurement of light absorption directly yields the primary parameter needed for radiative transfer applications and also quantifies EC or rather BC (black carbon). Both filter-based and photoacoustic techniques can be used for this measurement, yielding some real time instruments with excellent time resolution.

2) Measurement of the graphitic Raman spectrum is currently undergoing a small resurgence. It quantifies graphitic carbon (GC) thought to be responsible for carbonaceous light absorption and potentially identical with BC. In addition, Raman spectroscopy may yield information on the size of graphitic micro crystals.

3) Dissolving the OC and quantifying the remaining carbon yields non-extractable carbon (NEC), which is closely related to EC. Dissolving part of the OC may also be useful as pretreatment for thermal-optical analysis to reduce charring. However, extraction methods are not well suited for continuous or real time analysis.

4) Measurement of EC through its thermally refractory properties has been discussed above.

Topic 8 Follow Up AbstractSource Apportionment Using Semi-Continuous Measurement of OC, EC, and Other Markers of Combustion Emissions
Delbert J. Eatough, Brigham Young University, Provo, Utah

There has been a significant increase in the number of semi-continuous instruments available for the determination of PM2.5 mass and composition. Commercial instruments are now available for the semi-continuous measurement of nonvolatile mass, total mass, EC, OC, sulfate, nitrate and other fine particulate components. Data are often provided as 1-h average measurements. The availability of data on a time period comparable to or shorter than meteorological and emission changes in an urban atmosphere significantly improves the ability of programs such as UNMIX to do source apportionment in an urban air shed. EC, in particular, is a good marker for diesel emissions. However, the separation of diesel, gasoline and wood combustion emissions is of vital importance and difficult to de using EC and OC data alone. The addition of other 1-h average gaseous data for potential markers of primary emissions (e.g. NOx, CO, SO2) and secondary formation processes (e.g. NO2, O3, H2O2) significantly improves the use of semi-continuous EC data in the apportionment of EC and OC from various combustion emissions and secondary formation processes. The potential use of such data will be illustrated with data obtained at the Pittsburgh Supersite and in the Wasatch Front EMPACT study. The development of other 1-h average carbon fraction data sets would be expected to further improve the power of this approach.

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