At 20...40...60 km per hour

By Priyanka Chandola
Published: Sunday 31 August 2008

-- Acceleration in vehicles increases ultrafine particles

when vehicles accelerate after stopping at a traffic light or a bus stop, the concentration of ultrafine particles increases, says a Hong Kong-based study. Ultrafine particles, less than 0.1 micron in diameter, are a fraction of particulate matter. It is considered as a significant human health concern because of its small size, large numbers and their ability to enter the respiratory tract and the circulatory system. The study aimed to evaluate the exposure of pedestrians to vehicular emissions of ultrafine particles while walking near several high volume pedestrian walkways. The findings were published in Aerosol and Air Quality Research (Vol 8, No 1).

Researchers chose three locations around a busy intersection in the city. They found the highest particle count in one of the locations and superimposed traffic patterns on the particle count versus time data for that location. Explains Hamilton Tsang, one of the lead researchers, "The effect of superimposing particle counts with traffic patterns supports correlation of traffic with heightened particle count. Traffic does cause heightened particle counts." For light-duty vehicle acceleration, many of the observed spikes of particle counts were small and lasted for around 10 to 45 seconds. It reached a maximum of 1.6 x 105 particles per cubic centimetre before descending rapidly. In case of heavy-duty vehicles, large peaks with high and prolonged particle counts were observed some 20 minutes later. When the traffic light turned green, there was a sharp rise in particle counts within three seconds due to vehicle acceleration. Six seconds later, the particle counts rose to 5.4 x 105 particles per cubic centimetre after which it slowly descended over the next 35 seconds till the next red light.

"Sudden acceleration quickly emits a large number of particles. Heavy-duty vehicles such as trucks and buses are more likely to be using diesel and have higher levels of incomplete fuel combustion which leads to more particle emissions," says Tsang. A study conducted on the Los Angeles freeway, which has the highest percentage of diesel vehicles (25 per cent) has also reported such peaks.

"Vehicle acceleration causes much higher particle emission than idling vehicles because acceleration is when vehicles burn fuel in order to move," Tsang adds. While these results are preliminary, they clearly suggest that reducing congestion would contribute greatly to improved air quality in this area thereby diminishing pedestrian exposure to toxic ultrafine particles.

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