Daytime–Nighttime Contrast of Ice-Nucleating Particles at the Summit of Mt. Fuji During Summer

Ice-nucleating particles (INPs) at cloud-forming altitudes play a crucial role in cloud microphysical processes and precipitation formation. High-altitude observatories are often used to monitor background free-tropospheric conditions; however, the extent to which local mountain-scale processes influence INP populations remains poorly constrained. Here, we conducted intensive summertime observations of immersion-mode INPs at the summit of Mt. Fuji (3776 m a.s.l.), an isolated stratovolcano in central Japan, using a custom-built droplet freezing assay system (SNICS). Aerosol samples were collected separately during daytime (10:00–18:00) and nighttime (20:00–04:00) periods between July and August 2021. INP concentrations exhibited a clear diurnal contrast, with systematically higher values during daytime than nighttime across the mixed-phase temperature range (− 15 to − 30 °C), and the separation between daytime and nighttime distributions was particularly evident around − 20 to − 25 °C. Heat treatment experiments (95 °C for 1 h) substantially reduced INPs active at warmer temperatures, whereas INPs active at colder temperatures largely persisted. The post-heating spectra showed a temperature dependence broadly similar to that of suspensions of local volcanic rock samples. These results suggest that daytime aerosol populations include a mixture of heat-labile INPs, likely of biological origin, and heat-resistant INPs. The observed diurnal variability highlights the importance of mountain-scale transport processes in modulating INP populations at high-altitude sites and suggests that isolated mountain peaks can episodically inject surface-derived INPs into the free troposphere.