Preprints
https://doi.org/10.5194/acp-2021-245
https://doi.org/10.5194/acp-2021-245

  25 Mar 2021

25 Mar 2021

Review status: this preprint is currently under review for the journal ACP.

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

Soleil E. Worthy1, Anand Kumar1, Yu Xi1, Jingwei Yun1, Jessie Chen1, Cuishan Xu1, Victoria E. Irish1, Pierre Amato2, and Allan K. Bertram1 Soleil E. Worthy et al.
  • 1Department of Chemistry, University of British Columbia, Vancouver, BC, V6T1Z1, Canada
  • 2Institut de Chimie de Clermont-Ferrand, Université Clermont Auvergne, CNRS, Sigma-Clermont, 63000 Clermont-Ferrand, France

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 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 nine of the ten 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 ∆T_50 = -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 8 °C. 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 ammonium sulfate 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.

Soleil E. Worthy et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Review of the manuscript by Worthy et al.', Anonymous Referee #1, 26 Apr 2021
  • RC2: 'Comment on acp-2021-245', Anonymous Referee #2, 27 Apr 2021

Soleil E. Worthy et al.

Soleil E. Worthy et al.

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Short summary
We studied the effect of (NH4)2SO4 on the immersion freezing of non-mineral dust ice nucleating substances (INSs) and mineral dusts. (NH4)2SO4 had no effect on the median freezing temperature of nine of the ten tested non-mineral dust INSs, slightly decreased that of the other, and increased that of all the mineral dusts. The difference in the response of mineral dust and non-mineral dust INSs to (NH4)2SO4 suggests that they nucleate ice and/or interact with (NH4)2SO4 via different mechanisms.
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