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Principal Component Analysis has demonstrated promise in its ability to identify the low-dimensional chemical manifolds of turbulent reacting systems by providing a basis for the a priori parameterization of such systems based on a reduced number of parameterizing variables. Previous studies on PCA have only mentioned the importance of data pre-processing and scaling on the PCA analysis, without detailed consideration. This paper assesses the influence of data-preprocessing techniques on the size-reduction process accomplished through PCA. In particular, a methodology is proposed to identify and remove outlier observation from the datasets on which PCA is performed. Moreover, the effect of centering and scaling techniques on the PCA manifold is assessed and discussed in detail, to investigate how different scalings affect the size of the manifold and the accuracy in the reconstruction of the state-space. Finally, the sensitivity of the chemical manifold to flow characteristics is considered, to investigate the invariance of the manifold with respect to the Reynolds number. Several high-fidelity experimental datasets fro the TNF workshop database are considered in the present work, to demonstrate the effectiveness of the proposed methodologies. * Corresponding author. Phone + 32 2 650 26 80 Fax +32 2 650

a b s t r a c tWe describe a novel photoemission technique utilizing a traditional Kelvin probe as a detector of electrons/atmospheric ions ejected from metallic surfaces (Au, Ag, Cu, Fe, Ni, Ti, Zn, Al) illuminated by a deep ultra-violet (DUV) source under ambient pressure. To surmount the limitation of electron scattering in air the incident photon energy is rastered rather than applying a variable retarding electric field as is used with UPS. This arrangement can be applied in several operational modes: using the DUV source to determine the photoemission threshold (˚) with 30-50 meV resolution and also the Kelvin probe, under dark conditions, to measure contact potential difference (CPD) between the Kelvin probe tip and the metallic sample with an accuracy of 1-3 meV. We have studied the relationship between the photoelectric threshold and CPD of metal surfaces cleaned in ambient conditions. Inclusion of a second spectroscopic visible source was used to confirm a semiconducting oxide, possibly Cu 2 O, via surface photovoltage measurements with the KP. This dual detection system can be easily extended to controlled gas conditions, relative humidity control and sample heating/cooling.

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