Impact of traffic volumes on levels, patterns, and toxicity of polycyclic aromatic hydrocarbons in roadside soils

Vehicular exhaust is one of the important sources of polycyclic aromatic hydrocarbons (PAHs) in urban areas, and roadside soils can be directly contaminated with PAHs released from traffic emissions. In this study, roadside soils were collected at 10 sites in Ulsan, the largest industrial city in South Korea, to investigate the relationship between the traffic volume and the contamination characteristics of PAHs. The total concentrations of 16 US EPA priority PAHs (∑₁₆ PAHs, mean: 1079 ng g^-1) and organic-matter-normalized ∑₁₆ PAHs (mean: 224 ng g^-1 OM) were positively correlated with traffic volumes (Pearson correlation, r = 0.88 and 0.78, p < 0.01). The levels of carcinogenic PAHs were significantly higher at the high traffic sites than at the low traffic sites. High traffic sites (>25 000 vehicles per day) located at intersections showed elevated concentrations of indicator compounds (e.g., phenanthrene, fluoranthene, pyrene, and benzo[ghi]perylene) for gasoline and diesel exhaust. The diagnostic ratios also suggested a strong influence of the traffic emissions on the roadside soils, not only at urban sites but also at rural ones. Consequently, roadside soils and road dust (which are expected to be much more contaminated with PAHs than roadside soil) can act as important non-point sources of air and water pollution. The cancer risk from exposure to PAHs in the roadside soils was in an acceptable range, but continuous monitoring is required to evaluate the influence of increasing traffic on the environment and human health.