Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions – Part 1: The K-feldspar microcline

• Main Authors: A. Kumar, C. Marcolli, B. Luo, T. Peter
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-05-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/18/7057/2018/acp-18-7057-2018.pdf
• ISSN: 1680-7316; 1680-7324

Potassium-containing feldspars (K-feldspars) have been considered as key mineral dusts for ice nucleation (IN) in mixed-phase clouds. To investigate the effect of solutes on their IN efficiency, we performed immersion freezing experiments with the K-feldspar microcline, which is highly IN active. Freezing of emulsified droplets with microcline suspended in aqueous solutions of NH₃, (NH₄)₂SO₄, NH₄HSO₄, NH₄NO₃, NH₄Cl, Na₂SO₄, H₂SO₄, K₂SO₄ and KCl, with solute concentrations corresponding to water activities <i>a</i>_w  =  0.9–1.0, were investigated by means of a differential scanning calorimeter (DSC). The measured heterogeneous IN onset temperatures, <i>T</i>_het(<i>a</i>_w), deviate strongly from <i>T</i>_het^{Δ<i>a</i>_w^het}(<i>a</i>_w), the values calculated from the water-activity-based approach (where <i>T</i>_het^{Δ<i>a</i>_w^het}(<i>a</i>_w) = <i>T</i>_melt(<i>a</i>_w + Δ<i>a</i>_w^het) with a constant offset Δ<i>a</i>_w^het with respect to the ice melting point curve). Surprisingly, for very dilute solutions of NH₃ and NH₄⁺ salts (molalities <i>≲</i>1 mol kg⁻¹ corresponding to <i>a</i>_w <i>≳</i> 0.96), we find IN temperatures raised by up to 4.5 K above the onset freezing temperature of microcline in pure water (<i>T</i>_het(<i>a</i>_w = 1)) and 5.5 K above <i>T</i>_het^{Δ<i>a</i>_w^het}(<i>a</i>_w), revealing NH₃ and NH₄⁺ to significantly enhance the IN of the microcline surface. Conversely, more concentrated NH₃ and NH₄⁺ solutions show a depression of the onset temperature below <i>T</i>_het^{Δ<i>a</i>_w^het}(<i>a</i>_w) by as much as 13.5 K caused by a decline in IN ability accompanied with a reduction in the volume fraction of water frozen heterogeneously. All salt solutions not containing NH₄⁺ as cation exhibit nucleation temperatures <i>T</i>_het(<i>a</i>_w) &lt; <i>T</i>_het^{Δ<i>a</i>_w^het}(<i>a</i>_w) even at very small solute concentrations. In all these cases, the heterogeneous freezing peak displays a decrease as solute concentration increases. This deviation from Δ<i>a</i>_w^het  =  const. indicates specific chemical interactions between particular solutes and the microcline surface not captured by the water-activity-based approach. One such interaction is the exchange of K⁺ available on the microcline surface with externally added cations (e.g., NH₄⁺). However, the presence of a similar increase in IN efficiency in dilute ammonia solutions indicates that the cation exchange cannot explain the increase in IN temperatures. Instead, we hypothesize that NH₃ molecules hydrogen bonded on the microcline surface form an ice-like overlayer, which provides hydrogen bonding favorable for ice to nucleate on, thus enhancing both the freezing temperatures and the heterogeneously frozen fraction in dilute NH₃ and NH₄⁺ solutions. Moreover, we show that aging of microcline in concentrated solutions over several days does not impair IN efficiency permanently in case of near-neutral solutions since most of it recovers when aged particles are resuspended in pure water. In contrast, exposure to severe acidity (pH <i>≲</i>1.2) or alkalinity (pH <i>≳</i>11.7) damages the microcline surface, hampering or even destroying the IN efficiency irreversibly. Implications for IN in airborne dust containing microcline might be multifold, ranging from a reduction of immersion freezing when exposed to dry, cold and acidic conditions to a 5 K enhancement during condensation freezing when microcline particles experience high humidity (<i>a</i>_w<i>≳</i>0.96) at warm (252–257 K) and NH₃/NH₄⁺-rich conditions.