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 reflectron 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.