This paper reports a computational fluid dynamics simulation of airflow and fine particle (PM 2.5 ) dispersion in the street canyon in Hong Kong, using large eddy simulation. An aspect ratio (AR) of 2.7 and a Reynolds number of 5 Â 10 6 with a one main vortex, were used. This study focused mainly on the vehicle-induced momentum source and PM 2.5 concentrations were measured at 10 altitudes near the leeward wall in the street canyon, to provide high resolution measurements for model validation. The simulated PM 2.5 concentrations agreed well with the measurements, (R ¼ 0.85). The concentration was higher at the lower part near the leeward wall than the upper part.Higher concentration was found near the roof level. A near-uniform vertical dispersion of PM 2.5 near the windward wall was demonstrated; and the average concentrations were lower than found near the leeward wall. The intermittent escape of the PM 2.5 above the canyon at the roof level occurred mainly at the centre and near windward wall areas. The results demonstrated that a reduction in the AR can be conducive to pollutant dispersion in street canyon planning. The findings of this research would inform building designers to formulate effective strategies such as positioning of ventilation air intake, for the control of ingress of PM 2.5 into building environments.

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Daytime variations of PM 2.5 concentration in three street canyons in Xi'an were measured during June 11-15, 2010. Weather conditions, including the ambient wind condition and the street canyon air temperature, were also recorded. These measurements were conducted on weak-wind summer days, when the mean ambient wind velocity was 1.26 m/s. The results show that, under weak-wind conditions, there were no good correlation between PM 2.5 concentrations and vehicle flux inside the street canyons. The vehicle related PM 2.5 concentrations accumulated in the daytime inside the street canyons. The PM 2.5 concentrations at the pedestrian level were of near uniform horizontal distribution. These results indicate the absence of the typical primary air flow re-circulation inside the street canyons under weak-wind conditions, and that the dispersion and transportation of vehicle exhausts inside and/or outside the street canyons are influenced by vehicle induced air flow and thermal induced buoyancy.

A continuous photo-electric aerosol sensor (PAS) was used to characterize the particle-bound polycyclic aromatic hydrocarbons (p-PAHs) content at a heavily trafficked roadside site in Hong Kong. The measurements at the roadside station were performed for nine months from January to September 2005. For the total quantified p-PAHs, the PAS output shows good correlations to the data obtained with the offline quartz-fiber aerosol filter collection method. The average concentration of corrected p-PAHs was 5.3 ± 3.7 ng/m 3 , ranging from 0.2 to 22.0 ng/m 3 . The p-PAHs concentrations increased in the daytime in accordance with the heavy amount of road traffic, and decreased in the nighttime due to less traffic. Hourly pPAHs concentrations had high correlation coefficients with all goods vehicles (> 0.9) and large buses (~0.8), showing that diesel vehicle emissions are the primary source of p-PAHs. The day-to-day variation of p-PAHs is significant, because it was influenced by various factors (e.g., traffic levels, mixing height, and potential pollution sources).

Continuous measurements of number concentrations and size distributions of particles with diameters of 7-225 nm and 100-2,000 nm were conducted, using a TSI scanning mobility particle sizer (SMPS) and an optical particle counter (OPC), respectively, in eight weekdays of 2005 at a roadside environment in Hong Kong. PM 2.5 and PM 10 mass concentrations, black carbon (BC), particle-bound polycyclic aromatic hydrocarbons (p-PAHs), traffic counts and meteorological parameters were simultaneously monitored every hour. On average, the total particle number concentrations were 50,235 AE 27,076 cm À3 in the SMPS size range and 5,771 AE 1,793 cm À3 in the OPC size range in the winter sampling days, exceeding the summer particle number concentrations by a factor of 1.5 and 2.5, respectively. The ultrafine mode particles accounted for $90% of the particles in the SMPS size range. Daily cycle of particle numbers in the SMPS size range were obvious, with high values during daytime and low values during nigh time, which is consistent with the traffic pattern, especially the diesel-fuelled vehicles (r ¼ 0.72). Moreover, these particles were found to have good correlations with BC (r ¼ 0.84) and p-PAHs (r ¼ 0.90). The findings have suggested that diesel-fuelled vehicle exhausts are the dominant source for ultrafine particles at roadside environments of Hong Kong.

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