Real-time measurements of PM_2.5, PM_10-2.5, and BC in an urban street canyon

A continuous dichotomous beta gauge monitor was used to characterize the hourly content of PM_2.5, PM_10-2.5, and Black Carbon (BC) over a 12-month period in an urban street canyon of Hong Kong. Hourly vehicle counts for nine vehicle classes and meteorological data were also recorded. The average weekly cycles of PM_2.5, PM_10-2.5, and BC suggested that all species are related to traffic, with high concentrations on workdays and low concentrations over the weekends. PM_2.5 exhibited two comparable concentrations at 10:00-11:00 (63.4 μg/m³) and 17:00-18:00 (65.0 μg/m³) local time (LT) during workdays, corresponding to the hours when the numbers of diesel-fueled and gasoline-fueled vehicles were at their maximum levels: 3179 and 2907 h^-1, respectively. BC is emitted mainly by diesel-fueled vehicles and this showed the highest concentration (31.2 μg/m³) during the midday period (10:00-11:00 LT) on workdays. A poor correlation was found between PM_2.5 concentration and wind speed (R = 0.51, P-value > 0.001). In contrast, the concentration of PM_10-2.5 was found to depend upon wind speed and it increased with obvious statistical significance as wind speed increased (R = 0.98, P-value < 0.0001).