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Uncertainties in Optical Charring Correction Schemes

Uncertainties in Optical Charring Correction Schemes. Jian Zhen Yu and Hong Yang Department of Chemistry Hong Kong University of Science & Technology. EC/OC workshop, Durango, CO March 2003. Two assumptions underlying charring correction scheme.

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Uncertainties in Optical Charring Correction Schemes

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  1. Uncertainties in Optical Charring Correction Schemes Jian Zhen Yu and Hong Yang Department of Chemistry Hong Kong University of Science & Technology EC/OC workshop, Durango, CO March 2003

  2. Two assumptions underlying charring correction scheme • PEC evolves before native EC evolves in the analysis, or • PEC and native EC have the same absorption coefficients at the monitoring light wavelength. The EC/OC split is set at where filter transmittance returns to its initial value.

  3. Definition of terms • PEC: pyrolytically-generated EC • NEC: native EC • AEC: apparent EC, as determined by the instrument using optical charring correction scheme • s: filter light absorption coefficient at the monitoring wavelength • A: filter light absorbance at the monitoring wavelength

  4. Hypothetic thermograms of pure PEC, pure NEC, and a mixture of the two in a He/O2 atmosphere

  5. How much does AEC deviate from NEC?

  6. Unburned EC on the filter in the He/O2 stage

  7. Implication of an invalid assumption 2 If NEC < PEC2 , AEC underestimates NEC If NEC > PEC2 , AEC overestimates NEC The magnitude of deviation of AEC from NEC is proportional to: (1) the amount of NEC that is oxidized before the split line (2) the ratio between the  values of NEC and PEC2.

  8. AEC overestimates NEC Analyzed with the same temperature program. Filter alone: [AEC] =1.87+/-0.03 mgC/cm2 Filter+WSOC: [AEC]=2.25+/-0.26 mgC/cm2

  9. Conclusions • The two assumptions underlying the charring correction scheme by optical methods are proved not to hold. • Pyrolytically generated EC is indistinguishable from native EC. • The two types of EC have different light absorption coefficients (s) at the monitoring light wavelength. • The unequal s values of PEC and NEC could lead to either a negative or a positive bias in the EC measurement using the charring correction scheme. • The deviation from the underlying assumptions highlights the importance of minimizing PEC formation in order to minimize uncertainties in OC and EC determination.

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