The effect of .sub.2SO.sub.4 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 (NH.sub.4).sub.2 SO.sub.4 solutions (0.05 M) on immersion freezing of a variety of non-mineral dust ice-nucleating substances (INSs) including bacteria, fungi, sea ice diatom exudates, sea surface microlayer substances, and humic substances using the droplet-freezing technique. We also studied the effect of (NH.sub.4).sub.2 SO.sub.4 solutions (0.05 M) on the immersion freezing of several types of mineral dust particles for comparison purposes. (NH.sub.4).sub.2 SO.sub.4 had no effect on the median freezing temperature (ÎT.sub.50) of 9 of the 10 non-mineral dust materials tested. There was a small but statistically significant decrease in ÎT.sub.50 (-0.43 ± 0.19 .sup." C) for the bacteria Xanthomonas campestris in the presence of (NH.sub.4).sub.2 SO.sub.4 compared to pure water. Conversely, (NH.sub.4).sub.2 SO.sub.4 increased the median freezing temperature of four different mineral dusts (potassium-rich feldspar, Arizona Test Dust, kaolinite, montmorillonite) by 3 to 9 .sup." C and increased the ice nucleation active site density per gram of material (n.sub.m (T)) by a factor of â¼ 10 to â¼ 30. This significant difference in the response of mineral dust and non-mineral dust ice-nucleating substances when exposed to (NH.sub.4).sub.2 SO.sub.4 suggests that they nucleate ice and/or interact with (NH.sub.4).sub.2 SO.sub.4 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 potentially increase as these particles become coated with (NH.sub.4).sub.2 SO.sub.4 in the atmosphere. This difference also suggests that the addition of (NH.sub.4).sub.2 SO.sub.4 (0.05 M) to atmospheric samples of unknown composition could potentially be used as an indicator or assay for the presence of mineral dust ice nuclei, although additional studies are still needed as a function of INS concentration to confirm the same trends are observed for different INS conce