Evaluation of spatial distribution of cancer risk induced by traffic-related ultrafine particles within an urban microenvironment in New Taipei City, Taiwan.
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Ultrafine particles (UFPs) pose substantial health risks due to their deep lung penetration and high surface area, yet their spatial distribution and associated cancer risks in urban microenvironments remain poorly quantified. This study applied microscale computational fluid dynamics (CFD) simulations (1.2 × 1.2 km2 domain with about 5 m horizontal resolution near the ground) coupled with a size-resolved convection-diffusion model for 20-100 nm particles to quantify UFP dispersion and excess lifetime cancer risk (ELCR) under seasonal wind regimes. Emissions from four traffic line-source categories (urban roads, elevated expressways, interchanges, and freeways) were simulated under seasonal wind regimes and peak/off-peak traffic to produce spatial maps of lung-deposited surface area (SAUFP) and ELCR. Ground-level urban roads create narrow near-road bands with SAUFP maxima of about 40 μm2/cm3, while elevated corridors generate broader plumes with lower near-ground intensity that still form distinct downwind hotspots. Domain-averaged SAUFP during peak hours ranges from roughly 15 μm2/cm3 in winter to 31 μm2/cm3 in summer. Peak adult ELCR reaches about 2.2 × 10-4 in winter and 3.0 × 10-4 in summer, and 9-year-old children experience the highest risks, up to about 6.5 × 10-4, due to higher inhalation doses and lung deposition efficiencies. Many near-road and intersection locations harbor a high risk of exposure. The source-resolved, size-aware CFD-ELCR framework reveals strong spatial heterogeneity and age-dependent vulnerability that cannot be captured by sparse fixed-site monitoring alone, and it provides a transferable tool to identify traffic-related UFP risk hotspots and support targeted mitigation in dense urban microenvironments.