Physicochemical Properties of Mineral Particles under Fog, Dust, and Clear Conditions at Surface and Elevated Levels (1330 m) at Mt. Unzen: Case Data Comparisons

Aerosol particles in near-surface and elevated air often exhibit distinct physicochemical characteristics, but their differences remain insufficiently characterized. Aerosol samples were collected under clear, fog, and dust conditions at a coastal site (5 m a.s.l.) and at the top (1330 m a.s.l.) of Mt. Unzen in southwestern Japan. Single-particle analyses using electron microscopy with energy-dispersive X-ray spectroscopy revealed bimodal size distributions under clear conditions, characterized by submicron mineral particles and supermicron sea salt particles. In contrast, unimodal size distributions were observed under fog and dust conditions, with S-rich particles dominating the submicron range and Si-rich particles prevailing in the supermicron range. The average sulfur content in mineral particles under fog conditions reached 36% by weight, significantly higher than under clear conditions (15%), although sulfur levels remained comparable between the two sites in both clear and foggy air. During dust events, sulfur accumulation was more pronounced at the coastal site (16.8%) than at the mountaintop (4.9%), likely due to slower air mass movement near the surface as indicated by backward trajectories. These results highlight the combined effects of altitude and transport dynamics on aerosol aging and the need to incorporate near-surface altitude-resolved mechanisms in atmospheric models for coastal East Asia.