Critical Size of Silver Iodide Containing Glaciogenic Cloud Seeding Particles

• Published in: Geophysical research letters Vol. 51; no. 7
• Main Authors: Chen, Jie, Rösch, Carolin, Rösch, Michael, Shilin, Aleksei, Kanji, Zamin A.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.04.2024; Wiley
• Subjects: Aerosols; Cloud formation; cloud microphysics; Cloud seeding; cloud seeding aerosol; Cloud temperatures; Clouds; Freezing; Hail; Ice formation; Ice nucleation; Impurities; Iodides; Nucleation; Parameterization; Precipitation; Silver; Silver iodide; Size effects; Wind speed
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL106680

Cloud seeding is considered a practical but unproved method to enhance precipitation or suppress hail, due to insufficient knowledge of ice formation and evolution after seeding clouds with ice nucleating particles. This study investigates the size effects on the immersion freezing of aerosol produced from commercial silver iodide (AgI) containing flares at mixed‐phase cloud temperatures from 243 to 267 K. Flare‐generated aerosol exhibited comparable ice nucleation ability (INA) to pure AgI particles in the size range of 200 and 400 nm. Non‐AgI impurities reduced the INA of flare‐generated particles ≤90 nm, which is lower than pure AgI particles ≤40 nm. The critical mass ice‐active site density of the generated aerosols (critical‐nm) was derived, indicating the minimum mass of AgI particles required for efficient ice nucleation. The new parameterization to predict critical‐nm can serve as a reference to optimize the effectiveness of cloud‐seeding materials for practical use. Plain Language Summary Ice‐forming aerosol is commonly added to clouds, expecting precipitation enhancement via promotion of ice production. In this work, silver iodide (AgI) containing aerosol was generated from commercial cloud‐seeding products under different wind speed conditions. Its ice‐forming ability was studied at mixed‐phase cloud temperatures. The lower size limit for effective ice‐forming ability of the cloud‐seeding particles (90 nm) is higher than that of pure AgI particles (40 nm). The non‐AgI components produced by cloud‐seeding products are hypothesized to decrease the ice‐forming ability of smaller particles, as the mass fraction of ice‐nucleating AgI decreases. To estimate the minimum mass of AgI in a particle required for efficient ice nucleation under cloud‐seeding relevant conditions, we derived the critical ice‐activated mass fraction of the generated aerosols. These findings provide valuable insights into the optimization of cloud‐seeding practices for enhanced precipitation. Key Points Silver iodide (AgI) containing cloud‐seeding aerosols exhibit comparable ice‐forming abilities to pure AgI at sizes of 200 and 400 nm Non‐AgI impurities produced from flare burning decrease the ice nucleation ability of particles smaller than 90 nm A new parameterization is presented to estimate the minimum mass of AgI particles required to maximize glaciogenic seeding