Publications

Soot precursor formation and limitations of the stabilomer grid

Johansson, Karl O.; Lai, J.Y.W.; Skeen, Scott A.; Popolan-Vaida, D.M.; Wilson, K.R.; Hansen, Nils H.; Violi, A.; Michelsen, Hope A.

We have combined experimental and theoretical approaches to gain new insight into the mechanisms of PAH growth and soot formation. The experimental approach involves aerosol-mass spectrometry in conjunction with vacuum-ultraviolet photoionization of volatile species vaporizing from particles sampled from an Ar-diluted C2H2/O2 counter-flow diffusion flame at nearly atmospheric pressure (700 Torr). We recorded aerosol mass spectra at different distances from the fuel outlet for fixed ionization energies and in a fixed position while tuning the photoionization energy. The mass spectra contain a large distribution of peaks, highlighting the importance of small building blocks and showing a variety of chemical species that extends beyond the traditional classification of PAHs based on thermodynamic stability. In addition, we performed stochastic simulations of PAH growth in the flame in order to provide better insight into the chemical composition of species associated with peaks in the measured mass spectra. These simulations were conducted using a stochastic nanoparticle simulator (SNAPS). Synthesis of experimental and simulated results showed that peaks in the observed mass spectra generally consisted of a mixture of PAH isomers. At m/z =154 and 202, for example, experiments and simulations suggested that additional isomers than biphenyl and pyrene are important. Furthermore, the results highlight the importance of odd-carbon numbered species and complex growth paths. The experimental results suggest that species of higher masses can build up concentration ahead of species of lower masses. Our experimental results show, for example, that the peak at m/z = 278 appears closer to the burner outlet than the peak at m/z = 202, i.e., suggesting that a single monotonic growth mechanism is not enough.