Size distribution and characterization of ultrafine particles in roadside atmosphere

The number concentration and number size distributions of ultrafine particles were measured with a Scanning Mobility Particle Sizer (SMPS) at a roadside in early autumn and winter, and the results are discussed with regard to the contribution of traffic activity and meteorological conditions. The number concentration of the <50 nm fraction increased in the morning under calm wind conditions, and this increase corresponded with the increase in total traffic volume and nitric oxide. The increase in ultrafine particles was influenced not only by the increase in total traffic but also by the high contribution of diesel engine vehicles. The number concentration decreased around noon as the wind speed increased, although the total traffic and the number of diesel engine vehicles were at the same level as in the morning. The number size distribution in the morning was bimodal, with a first peak diameter of around 30 nm and a second of around 90 nm in both periods. The volatility of ultrafine particles was investigated using a thermal denuder operating at 250 degrees C. The first peak consisted mainly of volatile components, whereas the second one consisted of solid materials plus some volatile components. These results were consistent with the mass size distribution of elemental and organic carbon. The number size distribution with a peak diameter of around 30 nm was also observed in the afternoon at a suburban site; however, it was produced not by vehicle emissions directly but by photochemical reactions. Although a relatively high number concentration was also observed in the morning at the suburban site due to vehicle emission, the peak diameter ranged from 40 to 90 nm, which was larger than at the roadside.