The effect of (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> on the freezing properties of non-mineral dust ice-nucleating substances of atmospheric relevance

Abstract

Abstract. 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 (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 (NH4)2SO4 solutions (0.05 M) on the immersion freezing of several types of mineral dust particles for comparison purposes. (NH4)2SO4 had no effect on the median freezing temperature (ΔT50) of 9 of the 10 non-mineral dust materials tested. There was a small but statistically significant decrease in ΔT50 (−0.43 ± 0.19 ∘C) for the bacteria Xanthomonas campestris 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 to 9 ∘C and increased the ice nucleation active site density per gram of material (nm(T)) 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 potentially increase as these particles become coated with (NH4)2SO4 in the atmosphere. This difference also suggests that the addition of (NH4)2SO4 (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 concentrations than those used here. A comparison with results in the literature does suggest that our results may be applicable to a range of mineral dust and non-mineral dust INS concentrations.

Ice nucleating particles in the Canadian High Arctic during the fall of 2018

“…The reason why ammonium sulfate improves the ice nucleating ability of many types of mineral dust likely involves ion exchange of NH 4 + with parent cations at the mineral dust surface or adsorption of NH 4 + at the mineral dust surface. 46,48,49 Both processes could affect the orientation of water molecules at the mineral dust surface and, hence, ice nucleation.…”

“…An ammonium sulfate assay was used as an indicator for the presence of mineral dust INPs in the samples, as suggested previously. 46,47 This assay, which has not been used previously for field studies, is based on laboratory studies of mineral dust and non-mineral dust INPs in the presence of dilute ammonium sulfate solutions. Several previous studies have shown that the ice nucleation ability of many types of mineral dusts (feldspars, kaolinite, montmorillonite, micas, and quartz) but not all (amorphous silica and gibbsite) increases in the presence of dilute ammonium salt solutions (< 0.1 M) after correction for freezing point depression by the salts.…”

“…The reason why ammonium sulfate improves the ice nucleating ability of many types of mineral dust likely involves ion exchange of with parent NH + 4 cations at the mineral dust surface or adsorption of at the mineral dust surface. 46,48,49 Both NH + 4 processes could affect the orientation of water molecules at the mineral dust surface and, hence, ice nucleation. On the other hand, previous studies have shown that the ice nucleation ability of many nonmineral dust aerosols typically found in the atmosphere (bacteria, fungi, sea-ice diatom exudates, sea-surface microlayer samples, humic substances, and leaf derived INPs) either decrease or are not affected by the presence of dilute ammonium sulfate solutions (< 0.1 M), after correction for freezing point depression by the salt.…”

“…On the other hand, previous studies have shown that the ice nucleation ability of many nonmineral dust aerosols typically found in the atmosphere (bacteria, fungi, sea-ice diatom exudates, sea-surface microlayer samples, humic substances, and leaf derived INPs) either decrease or are not affected by the presence of dilute ammonium sulfate solutions (< 0.1 M), after correction for freezing point depression by the salt. 46,47,52 See Table S2 for a summary of previous work with non-mineral aerosol particles. Consistent with these previous studies, we assumed that an increase in the ice nucleation ability of a sample after the addition of ammonium sulfate at a concentration of 0.05 M and after correction for freezing point depression by the salt indicates the presence of mineral dust INPs.…”

“…Several previous studies have shown that the ice nucleation ability of many types of mineral dusts (feldspars, kaolinite, montmorillonite, micas, and quartz) but not all (amorphous silica and gibbsite) increases in the presence of dilute ammonium sulfate solutions (<0.1 M) aer correction for freezing point depression by the salts. [46][47][48][49][50][51] See Table S1 † for a summary of these previous studies. The reason why ammonium sulfate improves the ice nucleating ability of many types of mineral dust likely involves ion exchange of NH 4 + with parent cations at the mineral dust surface or adsorption of NH 4 + at the mineral dust surface.…”