Charring Minimization in Thermal Analysis of Aerosol Carbon

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Charring Minimization in Thermal Analysis of
Aerosol Carbon

• Jian Zhen Yu and Qianfeng Li
• Department of Chemistry
• Hong Kong University of Science Technology

EC/OC workshop, Durango, CO March 2003
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Thermal conditions that maximize OC removal /
minimize charring of OC

• Low-temperature oxidation (Cadle et al., 1983
  Cachier et al., 1989)
• Flash heating (Tanner et al., 1982)

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Determination of the temperature upper-limit for
low-temperature oxidation

• Criterion EC remains intact.

We need EC materials that are free of OC and
representative of aerosol EC .
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Thermal Creation of EC-Only Test Samples from
Atmospheric Aerosols
Thermal conditions for creation of EC-only
(mixture of PEC and native aerosol EC) samples
Carrier gas Temperature (oC) Time (sec)
He He-1 He-2 50 600 800 Purging online 60 300

• Successful creation of EC-only samples is
  evidenced by
• In He atmosphere at temperatures as high as 800oC
• FID signal remains at baseline.
• No discernable increase in filter laser
  transmittance is observed.

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Thermal evolution of an EC-only sample in 2
O2/He
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Percentage of EC Evolved as a Function of
Oxidation Temperature
350 oC
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• Earlier Work Using a Low-temperature Oxidation
  Step in thermal analysis
• Optimal temperature for OC/EC 300-350 oC, (Dod
  et al., 1978 Ellis et al., 1984 Ohta and Okita,
  1984).
• 2-step method for OC/EC (Cachier et. al., Tellus,
  1989, 41B, 379).
• 340 oC, pure O2 , precombustion 2 hours. Pure
  graphite was intact.

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Flash Heating

• Kinetics Competition
• Vaporization versus Decomposition
• Ref Buehler R.J., et.al. J.Am. Chem.Soc.
  Proton Transfer Mass Spectrometry of Peptides. A
  Rapid Heating Technique for Underivatized
  Peptides Containing Arginine, 1974, 96, 3990

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Thermal Methods Used for Comparison
IMPROVE IMPROVE NIOSH Optimized-1 Optimized-2
He Purging online (10s) Purging online (10s) Purging online (10s) Purging online (10s) Purging online (10s)
O2/He 200s, 350 oC 200s, 350 oC
He-1 180 s, 120 oC 60 s, 250 oC 60 s, 250 oC 10s, purging 10s, purging
He-2 180 s, 250 oC 60 s , 500 oC 60 s , 500 oC 60 s, 500 oC
He-3 180 s, 450 oC 60 s , 650 oC 60 s , 650 oC 60 s, 650 oC
He-4 180 s, 550 oC 90 s, 850 oC 90 s, 850 oC 90 s, 850 oC 200 s, 850 oC
He-5 Purging, 60 s Purging, 60 s Purging, 60 s Purging, 60 s
O2/He-1 240 s, 550 oC 30 s, 650 oC 30 s, 650 oC 30 s, 650 oC 30 s, 650 oC
O2/He-2 210 s, 700 oC 30 s, 750 oC 30 s, 750 oC 30 s, 750 oC 30 s, 750 oC
O2/He-3 210 s, 800 oC 60 s, 850 oC 60 s, 850 oC 60 s, 850 oC 60 s, 850 oC
O2/He-4 120 s, 940 oC 120 s, 940 oC 120 s, 940 oC 120 s, 940 oC
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Combination of low-temperature oxidation and
flash heating forms least charring from
water-soluble aerosol OC
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Transmittance Increase before the OCEC split in
the He/O2 step
Smaller transmittance increase is expected with
lower amount of charring.
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Deviation of EC measurements by non-optimal
methods from the optimized method