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

Abstract

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.

The financial support by the Hong Kong Jockey Club Charities Trust in the Mobile Real-time Air Monitoring Platform (MAP) project is gratefully acknowledged. We thank the MAP team, Messer. K.L. To, Win Tsang, Alex Tam, and Alta Choi, who are responsible for the real-time mobile air quality data used in this article.

Notes

*BM30/60 represents bi-modal size distributions, where the concentration of 108–170 nm particles was smaller than that of 60–108 nm particles; BM30/108 represents bi-modal size distributions, where the concentration of 108–170 nm particles was larger than that of 60–108 nm particles.

**Zero “0” is assigned to size distribution occurring at a frequency lower than the dominant size distributions in vehicle plumes in each sampling. For example, in June, the frequency of BM30/60 occurring was either less than that of UM60 or UM108 in vehicle plumes in each sampling period.

***There were two days when UM60 were the dominant size distribution in vehicle plumes. The other size distributions may or may not be detected in vehicle plumes. The frequency of occurrence of UM60 was higher than that of the other distributions in the sampling periods of these two days.

*BC_a and BC_v are black carbon ambient air and in vehicle plumes, respectively, concentrations in μ g m^{− 3};

**dN_a/dlog Dp is the particle number concentration in ambient air, in ×10⁴ cm^{− 3};

***dN_v/dlog Dp is the particle number concentration in vehicle plumes, in × 10⁴ cm^{− 3};

**** PN/BC = (dN_v/dlogDp-dN_a/dlogDp)/([BC_v]-[BC_a]) in 10⁴× cm^{− 3}/ μ g m^{− 3};

^#BM30/60 represents bi-modal size distributions, where the concentration of 108–170 nm particles was smaller than that of 60–108 nm particles; BM30/108 represents bi-modal size distributions, where the concentration of 108–170 nm particles was larger than that of 60–108 nm particles;

^##Average and standard deviation.