Active sites for ice nucleation differ depending on nucleation mode

Area of Science:

• Atmospheric Chemistry
• Climate Science
• Mineral Physics

Background:

• Ice nucleation in clouds is crucial for Earth's climate but remains poorly understood.
• Nucleation occurs at active sites on airborne particles, with distinct pathways under varying conditions.
• Two key pathways are immersion freezing (in supercooled droplets) and deposition (in supersaturated vapor).

Purpose of the Study:

• To directly compare active sites for immersion freezing and deposition nucleation.
• To determine if the same active sites are responsible for nucleation in both modes.
• To elucidate the differing properties governing site activity in each nucleation pathway.

Main Methods:

• Performed simultaneous immersion freezing and deposition experiments on identical mineral thin sections (feldspar and quartz).
• Directly compared the specific active sites identified in each experimental mode.
• Utilized pore condensation and freezing concepts to analyze site characteristics.

Main Results:

• Nucleation is dominated by a limited number of active sites on both feldspar and quartz.
• A significant lack of correlation was found between sites active in immersion freezing versus deposition.
• Only 6 out of 73 immersion-active sites were also active for deposition nucleation across both minerals.

Conclusions:

• Different properties dictate active site efficacy for immersion freezing and deposition nucleation.
• Deposition nucleation sites likely require specific size and/or geometry, unlike immersion freezing sites.
• Nucleation pathway dependence necessitates explicit consideration of nucleation mode in cloud modeling.