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 a nd 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 combustio n zone through a thin quartz capillary. By rapid cooling on expansion and approximately 600-fold dilution with air at ambient temperature th e 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 concent ration of the particles has, however, a maximum in the middle of the f lame, indicating particle formation and growth by condensation and agg lomeration up to the middle and annihilation by burning in the upper p art of the flame. Photoelectric charging of the particles indicates th at PAHs are present in or on the particles everywhere, yet PAH concent ration is higher at lower heights and also with smaller particles. Alt hough the total mass of particles in the diffusion mode of the flame i s much higher compared to the premixed mode, the size distribution is very similar in both cases. This indicates that the size of the partic les and total particulate mass are not directly related.