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Discussion papers
https://doi.org/10.5194/acp-2018-1020
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1020
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Nov 2018

Research article | 13 Nov 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions. Part 2 – Quartz and amorphous silica

Anand Kumar, Claudia Marcolli, and Thomas Peter Anand Kumar et al.
  • Institute for Atmospheric and Climate Sciences, ETH Zurich, Zurich, 8092, Switzerland

Abstract. Divergent ice nucleation (IN) efficiencies of quartz, an important component of atmospheric mineral dust, have been reported in previous studies. We show here that quartz particles obtain their IN activity from milling and that quartz aged in water loses most of its IN efficiency relative to freshly milled quartz. Also, the quartz surface – much in contrast to that of feldspars – is not prone to ammonia-induced IN enhancement. In detail we investigate the influence of solutes on the IN efficiency of various silica (SiO2) particles (crystalline and amorphous) with special focus on quartz. We performed immersion freezing experiments and relate the reported contradictory behavior to the influence of milling, and to the aging time and conditions since milling. Immersion freezing with silica particles suspended in pure water or aqueous solutions of NH3, (NH4)2SO4, NH4HSO4, Na2SO4 and NaOH, with solute concentrations corresponding to water activities aw = 0.9–1.0, were investigated in emulsified droplets by means of differential scanning calorimetry (DSC) and analyzed in terms of the onset temperature of the heterogeneous freezing signal Thet and the heterogeneously frozen water volume fraction Fhet. Quartz particles, which originate from milling coarse samples, show a strong heterogeneous freezing peak in pure water with Thet = 247–251 K. This IN activity disappears almost completely after aging for 7 months in pure water in a glass vial. During this time quartz slowly grew by incorporating silicic acid leached from the glass vial. Conversely, the synthesized amorphous silica samples show no discernable heterogeneous freezing signal unless they were milled. This implies that defects provide IN activity to silica surfaces, whereas the IN activity of a natural quartz surface is negligible, when it grew under near-equilibrium conditions. For suspensions containing milled quartz and the solutes (NH4)2SO4, NH4HSO4 or Na2SO4, Thet approximately follows Thetawhet) (aw), the heterogeneous freezing onset temperatures that obey Δawhet-criterion, i.e. Thetawhet) (aw) = Tmelt (aw + Δawhet) with Δawhet being a constant offset with respect to the ice melting point curve, similar to homogeneous IN. This water-activity-based description is expected to hold when the mineral surface is not altered by the presence of the solutes. On the other hand, we observe a slight enhancement in Fhet in the presence of these solutes, implying that the compliance with the Δawhet-criterion does not necessarily imply constant Fhet. In contrast to the sulfates, dilute solutions of NH3 or NaOH (molality ≥ 5 × 10−4 mol kg−1) reveal Thet by 3–8 K lower than Thetawhet) (aw), indicating a significant impact on the mineral surface. The lowering of Thet of quartz suspended in dilute NH3 solutions is opposite to the distinct increase in Thet that we found in emulsion freezing experiments with aluminosilicates, namely feldspars, kaolinite, gibbsite and micas. We ascribe this decrease of IN activity to the increased dissolution of quartz under alkaline conditions. The defects that constitute the active sites appear to be more susceptible to dissolution and therefore disappear first on a dissolving surface.

Anand Kumar et al.
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Status: final response (author comments only)
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Anand Kumar et al.
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Research Data supporting "Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions. Part 2 – Quartz and amorphous silica" A. Kumar, C. Marcolli, and T. Peter https://doi.org/10.3929/ethz-b-000286931

Anand Kumar et al.
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Short summary
This paper not only interests the Atmospheric Science community but has a potential to cater a broader audience. We discuss both long and short-term effects of various atmospherically relevant chemical species on a fairly abundant mineral surface. We of course discuss these chemical interactions from the perspective of fate of airborne mineral dust but the same interactions could be interesting for studies on minerals at the ground level.
This paper not only interests the Atmospheric Science community but has a potential to cater a...
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