[Particle number size distribution near a major road with different traffic conditions].

The profiles of number concentration of ambient particles at a roadside site in Beijing were studied with different traffic conditions. A Scanning Mobility Particle Sizer (SMPS) was utilized to measure the number concentrations of fine and ultrafine (10-100 nm) particles in August 2008 and August 2009, which represented the normal and Olympic traffic conditions, respectively. Size distributions of particle number concentration were identified and their temporal variations were also discussed. Results indicated that with normal traffic pattern, the total number concentration of ultrafine and 10-478 nm particles were (1.15 +/- 0.49) x 10(4) cm(-3) and (1.61 +/- 0.57) x 10(4) cm(-3), respectively. While the concentrations were decreased to (0.55 +/- 0.14) x 10(4) cm(-3) and (1.21 +/- 0.24) x 10(4) cm(-3), respectively, with special traffic condition during the Beijing Olympic Games. Largest reduction of 52.2% was observed for ultrafine particles among all size ranges. With normal traffic condition, bimodal distribution was found with two peak values in number concentrations around 22.5 nm and 113.0 nm. During the Olympic period, nucleation mode particles were significantly reduced due to a series of temporal control measures on motor vehicles such as the removal of yellow-labeled vehicles from the roads and travel restrictions based on odd-even license plate numbers. As a result, the peak in particle number concentration at 22.5 nm disappeared. The temporal variation indicated that significantly higher ultrafine particle number concentrations occurred around 00 : 00-04 : 00, 11 : 00-13 : 00 and 17 : 00-20 : 00 with normal traffic situation, which primarily attributed to the contributions of diesel exhaust at night, secondary formation at noon and traffic jam in the evening, respectively. However during the Olympic period, the temporal variation pattern changed significantly. The gap in the number concentrations of ultrafine particle between these three time periods and other time periods were narrowed, which should be attributed to the reduction of traffic flow as well as the increase of fleet speed during the Olympics.