Nanoscale Curvature Controls Hygroscopic Growth and Respiratory Deposition of Atmospheric Nanoparticles

Ultrafine particles (UFPs, ≤100 nm) dominate atmospheric particle number concentrations and contribute substantially to human inhalation exposure. Accurate prediction of their respiratory deposition requires accounting for hygroscopic growth under the warm and humid conditions of the human respiratory tract. However, the hygroscopic behavior of nanometer-sized particles is strongly influenced by the Kelvin effect, which elevates deliquescence relative humidity (DRH) and suppresses water uptake, potentially introducing uncertainty into deposition estimates. In this study, hygroscopic growth factors (HGFs) and DRH of 15-100 nm sodium chloride (NaCl) particles were measured using a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA) and incorporated into the Multiple-Path Particle Dosimetry (MPPD) model to quantify the influence of size-dependent hygroscopic growth on respiratory deposition. Smaller particles (15-36 nm) exhibited elevated DRH (77-81%) and reduced hygroscopic growth compared with larger particles. At relative humidity (RH) of 80-90%, HGFs ranged from 1.95-2.25 for 15 nm particles and 1.99-2.36 for 100 nm particles. Incorporating experimentally determined hygroscopic growth significantly altered predicted deposition. Neglecting hygroscopic growth overestimated total deposition by up to 25%, with regional differences reaching 18% in the tracheobronchial region for particles ≤ 50 nm. Hygroscopic growth also redistributed deposition toward deeper lung regions for larger ultrafine particles, whereas smaller particles remained dominated by diffusion and preferentially deposited in proximal airways. These findings demonstrate that experimentally constrained hygroscopic growth measurements improve respiratory deposition estimates for hygroscopic ultrafine particles and provide benchmark data for inhalation exposure assessment and future studies of chemically complex atmospheric aerosols.