Particle Counts and Size Distributions in the Roadside Environment

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 PM₁₀ 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 ± 27,076 cm⁻³ in the SMPS size range and 5,771 ± 1,793 cm⁻³ 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 = 0.72). Moreover, these particles were found to have good correlations with BC (r = 0.84) and p-PAHs (r = 0.90). The findings have suggested that diesel-fuelled vehicle exhausts are the dominant source for ultrafine particles at roadside environments of Hong Kong.