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

Anand Kumar, Claudia Marcolli, Beiping Luo, Thomas Peter
2018 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> 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 NH<sub>3</sub>, (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>,
more » ... NH<sub>4</sub>NO<sub>3</sub>, NH<sub>4</sub>Cl, Na<sub>2</sub>SO<sub>4</sub>, H<sub>2</sub>SO<sub>4</sub>, K<sub>2</sub>SO<sub>4</sub> and KCl, with solute concentrations corresponding to water activities <i>a</i><sub>w</sub><span class="thinspace"></span> = <span class="thinspace"></span>0.9–1.0, were investigated by means of a differential scanning calorimeter (DSC). The measured heterogeneous IN onset temperatures, <i>T</i><sub>het</sub>(<i>a</i><sub>w</sub>), deviate strongly from <i>T</i><sub>het</sub><sup>Δ<i>a</i><sub>w</sub><sup>het</sup></sup>(<i>a</i><sub>w</sub>), the values calculated from the water-activity-based approach (where <i>T</i><sub>het</sub><sup>Δ<i>a</i><sub>w</sub><sup>het</sup></sup>(<i>a</i><sub>w</sub>) = <i>T</i><sub>melt</sub>(<i>a</i><sub>w</sub> + Δ<i>a</i><sub>w</sub><sup>het</sup>) with a constant offset Δ<i>a</i><sub>w</sub><sup>het</sup> with respect to the ice melting point curve). Surprisingly, for very dilute solutions of NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup> salts (molalities <i>≲</i>1<span class="thinspace"></span>mol<span class="thinspace"></span>kg<sup>−1</sup> corresponding to <i>a</i><sub>w</sub><span class="thinspace"></span><i>≳</i><span class="thinspace"></span>0.96), we find IN temperatures raised by up to 4.5<span class="thinspace"></span>K above the onset freezing temperature of microcline in pure water (<i>T</i><sub>het</sub>(<i>a</i><sub>w</sub> = 1)) and 5.5<span class="thinspace"></span>K above <i>T</i><sub>het</sub><sup>Δ<i>a</i><sub>w</sub><sup>het</sup></sup>(<i>a</i><sub>w</sub>), revealing NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup> to significantly enhance the IN of the microcline surface. Conversely, more concentrated NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup> solutions show a depression of the onset temperature below <i>T</i><sub>het</sub><sup>Δ<i>a</i><sub>w</sub><sup>het</sup></sup>(<i>a</i><sub>w</sub>) by as much as 13.5<span class="thinspace"></span>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 NH<sub>4</sub><sup>+</sup> as cation exhibit nucleation temperatures <i>T</i><sub>het</sub>(<i>a</i><sub>w</sub>) &amp;lt; <i>T</i><sub>het</sub><sup>Δ<i>a</i><sub>w</sub><sup>het</sup></sup>(<i>a</i><sub>w</sub>) 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><sub>w</sub><sup>het</sup><span class="thinspace"></span> = <span class="thinspace"></span>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<sup>+</sup> available on the microcline surface with externally added cations (e.g., NH<sub>4</sub><sup>+</sup>). 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 NH<sub>3</sub> 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 NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup> 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<span class="thinspace"></span>K enhancement during condensation freezing when microcline particles experience high humidity (<i>a</i><sub>w</sub><i>≳</i>0.96) at warm (252–257<span class="thinspace"></span>K) and NH<sub>3</sub>/NH<sub>4</sub><sup>+</sup>-rich conditions.</p>
doi:10.5194/acp-18-7057-2018 fatcat:oruprywulvcthlb3666z36nigq