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.
Highlights
Ice can form homogenously in the atmosphere at temperatures below −35 ◦C (Koop and Murray, 2016) or heterogeneously at warmer temperatures when an ice-nucleating substance (INS) is present to initiate freezing (Murray et al, 2012; Kanji et al, 2017; Hoose and Möhler, 2012)
INSs studied include bacteria, fungi, exudates from sea ice diatoms collected in Antarctica, INSs from the sea surface microlayer collected in the Arctic, and humic substances
For the bacteria and fungi grown in the laboratory, blanks correspond to the fraction frozen curves for the growth medium at the same dilution factor as in the INS suspensions
Summary
Ice can form homogenously in the atmosphere at temperatures below −35 ◦C (Koop and Murray, 2016) or heterogeneously at warmer temperatures when an ice-nucleating substance (INS) is present to initiate freezing (Murray et al, 2012; Kanji et al, 2017; Hoose and Möhler, 2012). Several studies have investigated the effects of solutes at low concentrations on the freezing properties of mineral dusts in the immersion mode. Other inorganic salts, including NaOH and NaCl, decrease the freezing temperatures of some types of mineral dust (Kumar et al, 2019a, b, 2018; Reischel and Vali, 1975; Whale et al, 2015). Reischel and Vali (1975) studied the effects of a range of inorganic salts on the freezing properties of leaf-derived nuclei and found only small changes (less than 1.5 ◦C) in the freezing temperatures of this INS in the presence of each of the tested solutes. Because (NH4)2SO4 causes an increase in the ice nucleation ability of most mineral dust particles even at low concentrations, we investigated whether it would have a similar effect on nonmineral dust INSs in the immersion mode. Spheric samples in response to the addition of low concentrations of (NH4)2SO4 could potentially be used to identify the presence of mineral dust INSs in atmospheric samples
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