Polycyclic aromatic hydrocarbons (PAHs) represent harmful pollutants widely present in the atmosphere. PAHs are formed from the incomplete combustion of fuels at high-temperature, high-pressure conditions typical of modern engines and turbines. Once in the atmosphere, the PAHs undergo chemical and photochemical degradation through gas-phase or heterogenous reactions. The modeling of these processes is still inaccurate compared to other pollutants, mainly due to large uncertainties in the chemical kinetic predictions of the PAH emissions. In particular, the development of detailed kinetic models requires speciation measurements as benchmark for their validation, but this is particularly challenging in the case of large PAHs due to: i) condensation/adsorption; ii) lack of isomer selectivity from laboratory-based diagnostics. The present work presents an innovative technique for PAH identification based on miniature, high-repetition-rate shock tube (HRRST) technologies coupled by molecular beam sampling to synchrotron-based double-imaging photoelectron/photoion coincidence (i<sup>2</sup>PEPICO) spectroscopy. To generate PAHs, toluene (0.1% dilute in argon) was pyrolyzed at temperatures and pressures around 1488 K and 7.6 bar. Toluene is a key species in PAH formation. The experiments yielded mass spectra and concentration/time profiles for fuel and products. PAHs up to around m/z = 280 were observed and each mass peak is composed of several isomers. A key aspect of the i<sup>2</sup>PEPICO is that the photoelectron spectrum (PES) for each mass is also obtained. By fitting the PES, the contributions of different isomers to the experimental PES are obtained. For many PAHs literature PESs are not available, and a large number of PESs were calculated by ab-initio methods. The figure contains an example of curve fitting for m/z = 166. Results for different m/z ratios will be presented together with their kinetic implications.

Polycyclic aromatic hydrocarbon identification by miniature shock tube and double-imaging photoelectron/photoion coincidence techniques

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