Measurements of ultrafine particle concentration and size distribution in the urban atmosphere

Particle size distributions were measured at three adjacent sites in Birmingham: a busy roadside A38.; 30 m away from the road and a nearby urban background site. Two scanning mobility particle sizers SMPS., an electrical low pressure impactor ELPI., a condensation particle counter and a thermophoretic precipitator were employed to measure and collect particles. Excellent agreement on the number weighted size distribution was found between the SMPS and ELPI, as well as with sizes measured by transmission electron microscopy. The average number concentration at roadside measured on four separate days was between 1.6 and 1.9=105 cmy3 with similar size distributions and more than half of measured particles smaller than 30 nm. Traffic was the main source of ultrafine particles at the roadside. A 24-h average of 2.68=104"1.29=104 cmy3 was measured, which is close to that in Hughes et al. Physical and chemical characterization of atmospheric ultrafine particles in the Los Angeles area. Environ Sci Technol 1998; 32:1153]1161. in Pasadena, CA, USA. Total particle number concentration declined downwind of the traffic, faster than the mass concentration. Model calculations show that dilution with background air is the main mechanism for the rapid drop in particle number concentration and change in particle size distribution when moving away from traffic.