Growth of large ionic polycyclic aromatic hydrocarbons in sooting flames

Abstract

Positive and negative ions of PAH+ have been analysed in low-pressure premixed flames of various fuels. Mass spectra were taken through a molecular beam sampling system by a linear time-of-flight mass spectrometer and reflection mass spectrometer. The main formation of PAH+ occurs in the oxidation zone of the flames where their concentrations pass through a maximum. The mass increase of PAH+ is a net result of mass growth, decomposition and oxidation. Therefore, mass growth and diminution of PAH and PAH+ is favored by an (a) increase in concentration of unsaturated small hydrocarbons; (b) increase in concentration of unsaturated small hydrocarbon radicals; (c) increase in the size of PAH or PAH+, respectively; (d) increase in temperature (for growth reactions which have an activation energy); (e) increase in temperature which favors thermal decomposition; (f) increase in concentration of oxidizing radicals such as OH; (g) increase in the concentration of O2; (h) decrease in the size (or mass) and in the degree of pericondensation, increase in the number of side chains.