Correlations of Ambient Temperature and Relative Humidity with Submicron Particle Number Concentration Size Distributions in On-Road Vehicle Plumes

Particle size (30 to 10,000 nm) distributions in number concentration were measured using an Electrical Low Pressure Impactor onboard a Mobile Real-time Air Monitoring Platform from June 2002 to August 2003 in Hong Kong to investigate the size distributions of on-road vehicular particles and their relationship with ambient temperature (T) and relative humidity (RH). In this article, we focus on the size distributions of particles > 30 nm in vehicle plumes where fresh vehicular particles dominated and the NOx concentration was > 400 ppb. A uni-modal size distribution with mode at either 60-108 nm or 108-170 nm was generally observed from April to October. This mode is conventionally believed to be soot particles in the literature and the different mode sizes probably depend on engine and vehicle operating conditions. From November to March, a bi-modal size distribution with a dominant mode at 30-60 nm and a minor mode at either 108-170 nm or 60-108 nm was generally observed. The existence of the 30-60 nm mode is explained in the literature by the growth of nucleated particles when vehicular exhaust cools in ambient air. Ratios of the number concentration of 30-60 nm particles to BC mass concentration correlated negatively with T, as well as with RH when RH was < 60%. The ratios appeared not to be sensitive to RH when RH was > 60%. The correlations suggested that T and RH exert significant influences on the formation of vehicular submicron particles, leading to varying size distributions of vehicular particles in Hong Kong.