MEASUREMENT OF SIZE DISTRIBUTION AND PHOTOELECTRIC ACTIVITY OF PARTICLES IN A GAS-DIFFUSION FLAME

The aerodynamic diameter D of the particles in a gas diffusion flame and the concentration of particle-bound polyaromatic hydrocarbons (PAH) are determined by in situ aerosol measurement techniques. Therefore a small gas volume is extracted at different heights from the combustion zone through a thin quartz capillary. By rapid cooling on expansion and approximately 600-fold dilution with air at ambient temperature the physical and chemical processes affecting the particles are quenched. We find 5 less-than-or-equal-to D less-than-or-equal-to 10 nm, and D increases with increasing height above the burner. The number concentration of the particles has, however, a maximum in the middle of the flame, indicating particle formation and growth by condensation and agglomeration up to the middle and annihilation by burning in the upper part of the flame. Photoelectric charging of the particles indicates that PAHs are present in or on the particles everywhere, yet PAH concentration is higher at lower heights and also with smaller particles. Although the total mass of particles in the diffusion mode of the flame is much higher compared to the premixed mode, the size distribution is very similar in both cases. This indicates that the size of the particles and total particulate mass are not directly related.