Size distributions and condensation growth of submicron particles in on-road vehicle plumes in Hong Kong

Volume concentration data of submicron particles ( 0.1 – 1.0 μ m ) in on-road vehicle plumes ( NO x > 400 ppb ) gathered by the Mobile Real-time Air Monitoring Platform (MAP) on city streets, highways and in tunnels in Hong Kong are used to study the size distributions and growth of vehicular submicron particles due to gas condensation and, particularly, its dependency on ambient temperature. Three particle volume size distributions are observed: a uni-modal distribution with an accumulation mode at 0.2 ± 0.1 μ m , and two bi-modal distributions with a minor mode at 0.2 ± 0.1 μ m and the dominant mode at either 0.5 ± 0.1 or 0.7 ± 0.1 μ m . In tunnels, the ratio of particle volume concentration to black carbon (BC) mass concentration correlates negatively with ambient temperature ( r 2 = 0.57 ) ; the dominant mode tends to be at the large particle size when the temperature is low, and when the temperature is high, the uni-mode appears at the small particle size. Thus temperature seems to exert a significant influence on the condensation growth of fresh vehicular particles. On the other hand, this ratio correlates positively with ambient particle concentrations ( r 2 = 0.35 ) . Ambient particles measured in this study are mostly > 0.3 μ m while BC in vehicle plumes is generally believed to be mainly in the < 0.3 μ m size range in the literature. Temperature-dependent gas-condensation competition between fresh BC and ambient particles is suggested to play a role in the bi-modal formation.