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 (1)PEC evolves before native EC evolves in the analysis, or (2)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  : 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/O 2 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 PEC 2.

8. Filter alone: [AEC] =1.87+/-0.03  gC/cm 2 Filter+WSOC: [AEC]=2.25+/-0.26  gC/cm 2 AEC overestimates NEC Analyzed with the same temperature program.

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 (  ) at the monitoring light wavelength. The unequal  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.