Chemically Resolved\nRespiratory Deposition of Ultrafine\nParticles Characterized by Number Concentration in the Urban Atmosphere

Ultrafine particles (UFPs) dominate the atmospheric particles\nin\nnumber concentration, impacting human health and climate change. However,\nexisting studies primarily rely on mass-based approaches, leading\nto a restricted understanding of the number-based and chemically resolved\nhealth effects of atmospheric UFPs. In this study, we utilized a high-mass-resolution\nsingle-particle aerosol mass spectrometer to investigate the online\nchemical composition and number size distribution of ultrafine, fine,\nand coarse particles during the summertime in urban Shenzhen, China.\nHuman respiratory deposition dose assessments of particles with varying\nchemical compositions were further conducted by a respiratory deposition\nmodel. The results showed that during our observation, particles containing\nelemental carbon (EC) were the dominant components in UFPs (0.05–0.1\nμm). Compared to fine and coarse particles, UFPs can deposit\nmore deeply into the respiratory tract with a daily dose of ∼2.08\n± 0.67 billion particles. Among the deposited UFPs, EC-cluster\nparticles constituted ∼85.7% in number fraction, accounting\nfor a daily number dose of ∼1.78 billion particles, which poses\na greater impact on human health. Simultaneously, we found discrepancies\nin the chemically resolved particle depositions among number-, surface\narea-, and mass-based approaches, emphasizing the importance of an\nappropriate metric for particle health-risk evaluation.