Hygroscopicity of an Ice‐Nucleating Protein Below Its Immersion Freezing Temperature

Ice nucleation is a key process in cloud formation, affecting precipitation and atmospheric radiative balance. Biological materials, such as proteins from Pseudomonas syringae , act as highly efficient ice‐nucleating particles (INPs), inducing freezing at relatively high temperature. Snomax, a commercial product of the ice‐nucleating protein, is widely used in laboratory studies. However, its hygroscopicity at ice‐relevant temperature has not been fully characterized. Here, we investigated the hygroscopicity of Snomax particles within 252–296 K using the low‐temperature hygroscopicity tandem differential mobility analyzer (low‐ T HTDMA), also with measurements of cloud condensation nuclei (CCN) activity and immersion freezing temperature. For comparison, glycine, serine, and aspartic acid were also studied to assess whether their water interactions related to Snomax's ice‐nucleating efficiency. Snomax induced droplet freezing between 266 and 270 K, consistent with literature data. Snomax particles exhibited hygroscopic growth across the whole investigated temperature range, following the Flory‐Huggins model. Snomax was also highly CCN active ( κ CCN = 0.22 ± 0.03), suggesting a potential role on liquid cloud formation. The investigated amino acids were CCN active ( κ CCN = 0.23–0.42), but showed negligible influence on ice nucleation. Their hygroscopicity was dependent on phase state and temperature. These results indicate that Snomax's ice‐nucleating efficiency cannot be explained by the properties of individual amino acids.