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TOPIC #8What New and Innovative Sampling, Analytical,

and Interpretive Techniques are Needed to Determine the Properties and Sources of

Carbonaceous Aerosol in the Atmosphere?

Hans Moosmüller

Desert Research Institute, Reno, NV

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Duct Tape is like “The Force”.

It has a dark side and a light side and it holds the

universe together

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Carbonaceous Aerosol is like “The Force”.

It has a dark side (EC) and a light side (OC) and it seems to be difficult to tell them apart?

But does it hold the universe together? Needs more research!

Don’t forget to mention the possibility in your next proposal!

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Why can’t we tell OC/EC apart?Charring/Pyrolysis causes trouble!

Do we fully understand the charring process?Not until Episode 3&4 of the OC/EC Workshop!

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Why Do We Care About EC/OC?

1. Radiative Transfer and Visibility (Scattering and Absorption and Indirect Effects)

2. Source Apportionment

3. Health Effects (small, toxic particles)

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Appropriate Measurements: EC/OC?1. Radiative Transfer and Visibility:

Absorption, Scattering, CCN

2. Source Apportionment:Any conservative properties

3. Health Effects:Mass for soluble part, surface area or number for insoluble part, toxic compounds.

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All I want for Christmas…1. First Principle Definition and Measurement

of EC/OC or more appropriate parameters.

2. Real Time Measurement with large dynamic range and with 1 hour time resolution for ambient and 1 s for sources measurements

3. Absorption and Scattering Coefficients

4. Understand Health Effects of EC/OC

5. Remote Sensing of EC/OC

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What Did I Get?1. First principle definition and measurement of TC.

2. Real time measurement with 1 hour time resolution of operationally defined EC/OC.

3. Wide range of optical absorption and scattering coefficients.

4. Little understanding of EC/OC health effects.

5. No remote sensing of EC/OC.

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What’s the Problem with Santa?• Thermal optical analysis (TOA) is commonly used

for EC/OC speciation.• TOA is based on first principles, the refractory

properties of EC and the volatility of OC.• Charring transforms OC into EC and TOA from a

first principle method into an operational definition.• TOA is unlikely to reach a time resolution of 1s,

needed for mobile and other highly variable sources.

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Possible Approaches• As we can measure TC based on first principles (but

not with 1 s time resolution), we only need a good method for either EC or OC.

• OC is tough to measure because it is a conglomerate of hundreds or thousands of individual chemical compounds and it is difficult to identify common properties, to be used for OC quantification.

• EC consists of graphitic micro crystals. EC can be characterized using four(?) unique properties.

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Four Unique EC Properties1. Strong light absorption (Visibility & RT)

2. Graphitic Raman spectrum (Structure)

3. Insoluble in polar and non-polar solvents (Health)

4. Thermally refractory (Many fractions for source apportionment. Are they conservative measures)

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Strong Light Absorption (EC or BC)1. Relevant parameter for radiative transfer and

visibility.

2. Much larger absorption efficiency than other atmospheric aerosols.

3. Instruments for fast (1s) absorption measurements exist.

4. Either in situ (photoacoustic) or filter measurement.

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Light Absorption (EC or BC)• For Visibility and Radiative Transfer

applications, light absorption is the primary, EC related parameter needed.

• Why don’t we directly measure light absorption instead of using an inaccurate EC measurement together with inaccurate absorption efficiencies to calculate light absorption (Bond et al., 1998)?

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Why is Absorption a Good Measure of BC Mass?

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0.01 0.1 1 10

Particle Diameter (µm)

Extinction E

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White Smoke

Black Smoke

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Filter Based Measurement of BCMeasure attenuation through a filter

1. Batch Processing:Integrating Sphere or Plate Techniques

2. Real Time Measurements:Aethalometer, PSAP, CARUSO

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Filter Based Measurement of BC

Problems:

1. Interference between absorption by particles and scattering by particles and filter medium.

2. Limited dynamic range.

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Filter Based Measurement of BCPartial Solutions:1. Interference: Either correct for through

additional measurements and radiative transfer approach (CARUSO, real time, Petzold et al., 2002) or eliminate scattering with index matching (batch, Ballach et al., 2001).

2. Limited dynamic range: Variable filter change interval .

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In Situ Measurement of BCPhotoacoustic Method (Arnott et al., 1999)

1. Based on and calibrated with first principles.

2. High time resolution (≈ 1s).

3. Large dynamic range (from ambient to sources).

4. Good sensitivity (≈ 0.5 Mm-1).

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Graphitic Carbon (EC or GC)

1. Initial work by Rosen, Novakov (1977).

2. Recent resurgence of interest:a) Sloan’s group at the Univ. of Waterloo (Sze et al., 2001) b) Institute for Tropospheric Research (several preprints and abstracts).

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Graphitic Carbon (GC)1. G peak @1585 cm-1: Main mode of bulk graphite

proportional to crystal volume, i.e., mass of GC.

2. D (disorder) peak @ 1360 cm-1: due to symmetry breaking at the crystal edges. D/G indicates edge over volume ratio.

3. D’ peak @1620 cm-1: Analogue of the G peak for graphite layers located at the crystal boundaries, sensitive to the material around the crystal. Frequency gives information about the electronic properties of the surroundings, D’/G gives information on surface to volume ratio.

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Graphitic Carbon (GC)

N. Siddique, L. Neil, D. Fleming, J. J. Sloan(2001)

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Graphitic Carbon (GC)

• Problems:1. Poor time resolution, no real time capability.

2. Must reduce fluorescence from filter substrate.

3. Must avoid sample heating.

4. Quantitative results require modest filter loading.

• Potential:1. Measurement of GC mass.

2. Measurement of GC crystal size

3. Measurement of GC environment.

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Are GC and BC Identical?Theory: The bulk graphitic structure gives rise to the

strong light absorption.

Experiment: There has been some work on showing correlation between PSAP measurement of BC and GC (unpublished: NOAA CMDL and Institute for Tropospheric Research (Germany).

Needed: Correlation of GC and photoacoustic BC. Characterization of pyrolyzed OC.

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Non-Extractable Carbon (EC or NEC)

1. Takes time (≈1day) for extraction, therefore no potential for high time resolution?

2. Other non-extractable compounds?

3. Insensitive (≈ 5 µg/m3).

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Items for Discussion

1. We should use appropriate measurements for each application, not necessarily EC/OC (e.g., light absorption useful for visibility & radiative transfer).

2. Compare GC with light absorption.

3. Can we better understand pyrolitic carbon from its Raman spectrum?

4. Can we measure TC in real time with good time resolution?