The effect of (NH4)2SO4 on the freezing properties of non-mineral dust ice nucleating substances of atmospheric relevance

A wide range of materials including mineral dust, soil dust, and bioaerosols have been shown to act as ice nuclei in the atmosphere. During atmospheric transport, these materials can become coated with inorganic and organic solutes which may impact their ability to nucleate ice. While a number of studies have investigated the impact of solutes at low concentrations on ice nucleation by mineral dusts, very few studies have examined their impact on non-mineral dust ice nuclei. We studied the effect of dilute (NH4)2SO4 solutions (0.05 M) on immersion freezing of a variety of non-mineral dust ice nucleating substances including bacteria, fungi, sea ice diatom exudates, sea surface microlayer, and humic substances using the droplet freezing technique. We also studied the effect of (NH4)2SO4 on immersion freezing of mineral dust particles for comparison purposes. (NH4)2SO4 had no effect on the median freezing temperature of 9 of the 10 tested non-mineral dust materials. There was a small but statistically significant decrease in the median freezing temperature of the bacteria X. campestris (change in median freezing temperature = -0.43 ± 0.19 °C) in the presence of (NH4)2SO4 compared to pure water. Conversely, (NH4)2SO4 increased the median freezing temperature of four different mineral dusts (potassium-rich feldspar, Arizona test dust, kaolinite, montmorillonite) by 3 °C to 9 °C and increased the ice nucleation active site density per gram of material by a factor of ~10 to ~30. This significant difference in the response of mineral dust and non-mineral dust ice nucleating substances when exposed to (NH4)2SO4 suggests that they nucleate ice and/or interact with (NH4)2SO4 via different mechanisms. This difference suggests that the relative importance of mineral dust to non-mineral dust particles for ice nucleation in mixed-phase clouds could increase as these particles become coated with (NH4)2SO4 in the atmosphere. This difference also suggests that the addition of (NH4)2SO4 to atmospheric samples of unknown composition could be used as an indicator or assay for the presence of mineral dust ice nuclei.