Simultaneous detection of two types of soot precursor particles using photoionization mass spectrometry

The soot precursor particles reported in the literature to date may be roughly divided into two classes. Some of their features are very different. Using photoionization mass spectrometry behind premixed atmospheric ethylene/air flames, particle mass distribution functions were measured for these precursor particles. Within a limited C/O range bimodality was found, i.e. two types of particles are formed simultaneously. Through analysis of the photoionization behaviour it was found that the ionization order (IO) is different for these two modes as is the stability of the respective particles. In accordance with earlier measurements, particles with IO = 1 are interpreted as polyaromatic hydrocarbon (PAH) stacks whereas the IO = 2 particles rather seem to be large molecules. This is consistent with the different particle classes mentioned above. Their potential role in soot formation is briefly addressed.