Evaluating the Role of Roadside Green Belts in Mitigating Particulate Matter-Bound Polycyclic Aromatic Hydrocarbon Pollution

This study aimed to evaluate the effectiveness of roadside green infrastructure in reducing particulate matter-bound polycyclic aromatic hydrocarbons (PM-bound PAHs) in urban soils and to assess the associated human health risks in Sabzevar, Iran, a developing city characterized by limited green space and sparse environmental monitoring. We conducted soil sampling across 4 traffic-influenced zones with varying vegetation cover, including a non-vegetated control, during the summer of 2022. Sampling was performed at standardized distances along vegetated buffer strips to assess spatial distribution patterns of 15 priority PAHs. Soil extracts were analyzed via Gas Chromatography-Mass Spectrometry (GC-MS), and vegetation density was quantified using the Normalized Difference Vegetation Index (NDVI) from satellite imagery. Traffic dynamics were proxied by posted speed limits. Health risks, including lifetime cancer risk (LTCR), were probabilistically estimated using Monte Carlo simulations. Results demonstrated significant spatial variability in PAH concentrations. The highway and control sites exhibited the highest total and high-molecular-weight PAH levels, whereas the inner-city boulevard showed the lowest overall contamination but elevated phenanthrene concentrations. No statistically significant reduction in PAH concentrations was observed up to 50 m behind vegetative buffers. However, several carcinogenic PAHs (eg, benzo[a]pyrene and dibenzo[a,h]anthracene) were negatively correlated with NDVI and positively correlated with vehicle speed limits, indicating that vegetation density and traffic intensity significantly influence roadside PAH distribution. Health risk estimates remained below regulatory thresholds for all demographic groups, although relatively higher susceptibility was observed among children, the elderly, and adolescent males. In conclusion, roadside vegetation alone did not provide measurable short-distance attenuation of soil PAHs, but vegetation density and traffic characteristics were important determinants of contamination patterns. These findings suggest that effective mitigation of traffic-related PAHs in developing cities requires integrated urban green planning combined with traffic management strategies rather than reliance on vegetation buffers alone.