Short−Term Exposure to Ammonium Sulfate Modifies Ice Nucleation by Alpha-Alumina but Not Organic Monolayers or Microplastics

Accurate predictions of ice cloud formation in the atmosphere require understanding how ammonium sulfate affects the freezing behavior of ice−nucleating substances (INSs). Such information is also essential for evaluating the reliability of a freezing assay used to identify mineral dusts in atmospheric samples. Here, we used a droplet freezing technique to investigate the effects of short−term exposure (a few minutes) to ammonium sulfate on six INSs: two α-alumina samples (particle diameters < 1 µm and > 1 µm), two alcohol monolayers (C22H45OH and C30H61OH), and two microplastics (polyethylene terephthalate and low−density polyethylene). For the alcohol monolayers and microplastics, short−term exposure to ammonium sulfate did not change their ice−nucleating properties. In contrast, the submicrometer α−alumina particles exhibited enhanced ice−nucleating ability following ammonium sulfate exposure, while the supermicrometer α-alumina particles showed no change, despite having similar bulk composition. These results add corundum, the natural mineral form of α−alumina, to the growing list of minerals whose freezing properties can be modified by ammonium sulfate. The contrasting behavior of the two α−alumina samples likely reflects differences in surface structure, such as defects or trace impurities that influence nucleation activity. Overall, our findings highlight the importance of surface characteristics in determining the impact of ammonium sulfate on mineral dusts and support the use of ammonium sulfate–based freezing assays for identifying mineral components in atmospheric samples.