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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. [ABSTRACT FROM AUTHOR]

Volume concentration data of submicron particles in on-road vehicle plumes 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 , and two bi-modal distributions with a minor mode at and the dominant mode at either or . In tunnels, the ratio of particle volume concentration to black carbon (BC) mass concentration correlates negatively with ambient temperature ; 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 . Ambient particles measured in this study are mostly while BC in vehicle plumes is generally believed to be mainly in the 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. [Copyright &y& Elsevier]

Abstract: Some studies have reported that small submicron atmospheric particles are more acidic than large submicron particles; other studies demonstrated a reversed trend. In this study, the size dependence of in situ pH in submicron particles in Hong Kong was investigated. The equivalent ratios of [NH4 +]measured to [SO4 2−]measured in submicron particles were found to be generally less than unity and size dependent, suggesting the possibility of incomplete gas–aerosol equilibrium. The Aerosol Inorganic Model-II (AIM-II) model using measured ionic compositions with the gas–aerosol partitioning disabled was used to estimate the in situ pH in different sized particles. The estimated in situ pH of different sized submicron atmospheric particles was between −2.5 and 1.5 and it generally decreases with increasing submicron particle size. At such low in situ pH, the estimated HSO4 − equivalent concentrations were 3.2 times (on average) of those of H+ in different sized particles. The trends of the size dependence of the [NH4 +]measured to [SO4 2−]measured ratio and pH under different regimes of relative humidity are discussed. [Copyright &y& Elsevier]

Dicarboxylic acids in atmospheric aerosols have received much attention because of their potential roles in affecting the global climate. The composition and the sources of dicarboxylic acids in PM2.5 were studied at one remote and two urban sites in Hong Kong in the winter of 2000 and in the summer of 2001. Oxalate was the dominant dicarboxylic acid in all samples. The winter oxalate concentrations were high and spatially uniform, with an average value of 0.36 μg m−3, but the summer oxalate concentrations were low and had a large spatial variation. The influence of meteorological factors on the concentrations of dicarboxylic acids was also studied. The ratio of malonate to succinate was used to distinguish primary sources from secondary sources of these acids. This ratio at all three sites was close to that from direct vehicular exhaust in the winter, but it was close to that of secondary reactions in the summer. Hence, the acids were attributed to vehicular emissions in the winter and secondary sources in the summer. This hypothesis is also supported by a good correlation of oxalate with sulfate in the summer but a poor one in the winter. The correlations of oxalate with malonate, succinate, sulfate and K+ were also studied in terms of the routes of secondary formation of these dicarboxylic acids. [Copyright &y& Elsevier]