Spatial distributions of ultrafine particles (UFPs; 6 < D p < 560 nm) and related gaseous and particulate pollutants were estimated from on-road measurements undertaken on busy roadways of Seoul, Korea, using a mobile laboratory (ML). The objectives of the study were to determine the spatial variations in UFP size distributions and concentrations of associated gaseous and particulate pollutants and to observe the relationships of UFP number concentrations with other pollutants on roadways in an urban area in Korea. The pollutants associated with diesel vehicles such as black carbon (BC) and particlebound polycyclic aromatic hydrocarbons (PM-PAHs) exhibited a high determination coefficient (r 2 = 0.65), indicating the influence of diesel vehicles on emissions in the study area. Further supporting evidence for the influence of diesel vehicles on emissions was given by the higher determination coefficients of PM-PAHs and BC concentrations with larger sizeclassified particles, ranging from 60 < D p < 220 nm, than with total UFP number concentrations or smaller particles in the 6 < D p < 60 nm size range. Peak concentrations of measured pollutants were observed mostly at intersections, reflecting the relationships of transient driving modes (i.e., deceleration and acceleration) with emissions of UFPs, associated pollutants, and concentrated traffic volumes at such locations.

An investigation of the carbonaceous components of PM 2.5 (D p < 2.5 µm) and PM 0.1 (D p < 0.1 µm) in conjunction with online measurements of gaseous and particulate pollutants, which are dominated by motor vehicle emissions, was undertaken over 15 consecutive days in a field sampling campaign at a roadside and reference site for daytime (06:00 to 17:30) and nighttime (18:00 to 05:30) conditions. The results indicated that traffic-related air pollutants strongly influenced pollution levels in the sampling area, especially during the nighttime, although the concentrations were lower than during the daytime. Water-soluble organic carbon/organic carbon (WSOC/OC) ratios in PM 0.1 were 0.60 ± 0.11 and 0.63 ± 0.16 at the roadside and reference sites, respectively, while the corresponding ratios in PM 2.5 were found to be 0.60 ± 0.11 and 0.63 ± 0.14. These results demonstrated the relatively high contribution of WSOC in the study area compared to in previous reports. It was observed that elemental carbon (EC) in PM 0.1 are emitted approximately by a factor of 0.1 in comparison with those observed for EC in PM 2.5 when considering the slopes of the linear relationship with particle-bound PAHs and NO x , while OC displayed no such relationship. Our measurements indicate that a significant portion of the OC in PM 0.1 is not directly co-emitted with EC from motor vehicles but is readily adsorbed or condensed onto the existing EC fractions through condensation. The relationship between the EC2 fraction and the particle number concentration indicates that the EC2 fraction is the primary chemical component of traffic related ultrafine particles in terms of particle number concentration.