Minimizing Charring in OC EC Analysis by Thermal Method

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  • 1.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
  • 2.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)
  • 3.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 .
  • 4.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 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.
  • 5.Thermal evolution of an EC-only sample in 2% O2/He
  • 6.Percentage of EC Evolved as a Function of Oxidation Temperature 350 oC
  • 7.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.
  • 8.Flash Heating Kinetics Competition: Vaporization versus Decomposition Ref: Buehler R.J., J.Am. Chem.Soc. “Proton Transfer Mass Spectrometry of Peptides. A Rapid Heating Technique for Underivatized Peptides Containing Arginine”, 1974, 96, 3990
  • 9.Thermal Methods Used for Comparison
  • 10.Combination of low-temperature oxidation and flash heating forms least charring from water-soluble aerosol OC
  • 11.low high C loading
  • 12.low high C loading
  • 13.Transmittance Increase before the OCEC split in the He/O2 step Smaller transmittance increase is expected with lower amount of charring.
  • 14.Deviation of EC measurements by non-optimal methods from the optimized method