Quantifying Olfactory and Alveolar Deposition of Ultrafine Particles Using Multiscale Modeling: Implications for...

Atmospheric particle exposure is commonly characterized using mass-based metrics that inadequately capture particle surface area, respiratory deposition, and direct brain delivery pathways. Ultrafine particles (UFPs) contribute marginally to PM_2.5 mass yet account for a disproportionate fraction of deposited surface area in both the lungs and the nasal olfactory region, a critical gateway for direct particle translocation to the brain. Using multiscale atmospheric modeling from continental to street level, evaluated against NO₂, particulate mass, chemical composition, and size-resolved particle observations, we quantify particle deposition through lung-deposited surface area (LDSA) and olfactory deposition, explicitly accounting for hygroscopic growth. UFPs account for more than one-third of alveolar surface deposition in urban environments, and particles smaller than 400 nm dominate alveolar deposition despite their marginal contribution to PM_2.5 mass. Decoupling between PM_2.5 and deposited surface area persists across cities, source contributions, and population groups, with children experiencing approximately 3-fold higher alveolar doses than adults. Residential wood heating dominates winter PM_2.5 and alveolar deposition, whereas traffic controls particle number and olfactory deposition. Critically, under the tested translocation assumptions, olfactory deposition of UFPs is estimated to exceed blood-borne translocation to the brain by several hundred-fold, supporting the relevance of neuronal pathways for brain-relevant particle delivery and consistent with recent observations of black carbon in the human olfactory bulb.