Real-time observation of the transformation of ultrafine atmospheric particle modes

An Engine Exhaust Particle Sizer was used on a mobile platform to measure ultrafine particle (5.6-560 nm) concentration profiles in streets and tunnels. Particle size distribution data along the lengths of the tunnels showed that number concentration profiles of < 30 nm particles were highly nonlinear. Conventional point measurement studies inside tunnels to determine, for instance, emission factors of ultrafine particles in this range are therefore location dependent and may not be representative. The particle concentration profiles obtained in this experiment allowed the study of the transformation of 10 nm particles to 50 nm particles. Outside of the tunnels and on heavily traveled highways, particles exhibited a dominant mode at 10 nm and a minor mode at 50 nm. Inside the tunnels, 10 nm particles gradually decreased after an abrupt increase, while 50 nm particles increased from entrance to exit. The 10 nm particles decreased at 2-4 times the rate of increase of the 50 nm particles, therefore, condensational growth of the 10 nm particles alone cannot explain the decrease. The estimated hetero-coagulation rate between 10 nm particles and particles > 30 nm inside the tunnel was substantially higher than that outside due to the increase in the concentration of particles > 30 nm. The hetero-coagulation process is proposed to play a key role in the transformation of particle modes in the tunnel. This study provides new insight on the transformation of ultrafine particle modes in vehicular plumes