The Impact of (bio-)organic substances on the ice nucleation activity of the K-feldspar microcline in aqueous solutions

Abstract

Abstract. Potassium-feldspars (K-feldspars), such as microcline, are considered key dust minerals inciting ice nucleation in mixed phase clouds. Besides the high ice nucleation activity of microcline, recent studies also revealed a high sensi-tivity of microcline towards interaction with solutes on its surface. Here, we investigate the effect of organic and bio-organic substances on the ice nucleation activity of microcline, with the aim to better understand the underlying sur-face interactions. We performed immersion freezing experiments with microcline in solutions of three carboxylic acids, five amino acids and two polyols to represent these compound classes. By means of a differential scanning calorimeter we investigated the freezing of emulsified droplets of microcline suspended in various solutions. Depend-ing on the type of solute, different effects were observed. In the case of carboxylic acids (acetic, oxalic and citric acid), the measured heterogeneous onset temperatures, Thet, showed no significant deviation from the behavior pre-dicted by the water activity criterion, Thet(aw) = Tmelt(aw+Δaw), which relates Thet with the melting point temperature Tmelt via a constant water activity offset Δaw. While this behavior could be interpreted as a lack of interaction of the solute molecules with the surface, the carboxylic acids caused the fraction of heterogeneously frozen water, Fhet(aw), to decrease by up to 40 % with increasing solute concentrations. In combination, unaltered Thet(aw) and reduced Fhet(aw) suggest that active sites were largely deactivated by the acid molecules, but amongst those remaining active are also the best sites with the highest Thet. A deviation from this behavior is citric acid, which showed not only a de-crease in Fhet, but also a decrease in Thet of up to 4 K for water activities below 0.99, pointing to a depletion of the best active sites by interactions with the citrate ions. When neutralized solutions of the acids were used instead, the de-crease in Fhet became even more pronounced. The slope of Thet(aw) was different for each of the neutralized acid solu-tions. In the case of amino acid solutions, we found a decrease in Thet (up to 10 K), significantly below the Δaw-criterion, as well as a reduction in Fhet (up to 60 %). Finally, in case of the investigated polyols, no significant devia-tion of Thet from the Δaw-criterion was observed, and no significant deviation of Fhet in comparison to a pure water suspension was found. Furthermore, we measured the effects of aging on the ice nucleation activity in experiments with microcline suspended in solutions for up to seven days, and tested the reversibility of the interaction with the solutes after aging for 10 days. For citric acid, an ongoing irreversible degradation of the ice nucleation activity was observed, whereas the amino acids showed completely reversible effects. In summary, our experiments demonstrate a remarkable sensitivity of microcline ice nucleation activity to surface interactions with various solutes, underscoring the importance of the history of such particles from source to frozen cloud droplet in the atmosphere.

Measurement report: The Urmia playa as a source of airborne dust and ice-nucleating particles – Part 1: Correlation between soils and airborne samples

“…Feldspars, and especially K-feldspars proved to nucleate ice at the highest temperatures among the most common mineral dust species and have therefore been considered the most important minerals for cloud glaciation (Atkinson et al, 2013). However, its ice nucleation (IN) activity proved to be highly sensitive to the presence of other aerosol constituents and to decline in the presence of ions or acids (Kumar et al, 2018;Whale et al, 2018: Yun, et al, 2020Klumpp et al, 2022). Freezing experiments with quartz particles also yielded mixed results.…”

The Urmia playa as a source of airborne dust and ice-nucleating particles – Part 2: Unraveling the relationship between soil dust composition and ice nucleation activity

“…Figure 9 shows that the melting points of the salt solutions decrease to slightly below 271 K for the highest investigated salt concentration of 0.4 M. Table 6 compares the measured melting point depression T melt (DSC) with the ones based on water activities calculated with AIOMFAC, T melt (a w ) (Zuend et al, 2008(Zuend et al, , 2011. From these data, the freezing point depression T het (a w ) that is expected in the absence of specific interactions between the IN-active surface and the solute (Klumpp et al, 2022), which are given in Fig. 8 as blue dashed lines, can be derived.…”

“…For DSC measurements, 5 to 10 mg of emulsion were placed in an aluminum pan, and the pan was hermetically sealed. Thermograms were registered at a rate of 1 K min −1 in the temperature ranges of freezing and melting and evaluated in terms of the onset temperatures of the heterogeneous (T het ) and homogeneous freezing peaks (T hom ), the heterogeneously frozen fraction (F het ), and the melting temperature T melt , as has been explained in more detail in Kumar et al (2018) and Klumpp et al (2022). The stability of the emulsions was tested for some samples by subjecting them to three freezing cycles, with the first and third cycles performed with a cooling rate of 10 K min −1 as control cycles, following the procedure introduced by Marcolli et al (2007).…”

The Microfluidic Ice Nuclei Counter Zürich (MINCZ): a platform for homogeneous and heterogeneous ice nucleation

“…A range of techniques has been developed to study homogeneous and heterogeneous nucleation in atmospherically relevant systems (Diehl et al, 2014;Kaufmann et al, 2016;Miller et al, 2021;Rogers, 1988;Stetzer et al, 2008), and each technique can be associated with a particular drawback. For example, single-particle levitation devices (Diehl et al, 2014;Krämer et al, 1996) are time-consuming for investigating a large number of droplets sufficient for statistical analysis, whereas differential scanning calorimetry measurements of water-in-oil emulsions typically give only qualitative insight into nucleation behaviour due to the polydispersity in droplet size (Kaufmann et al, 2016;Klumpp et al, 2022;Kumar et al, 2018). To overcome such shortcomings, microfluidic techniques can be used to generate a stable, monodisperse population of water droplets at high throughput that is suitable for quantifying nucleation rates.…”

“…We report data for the homogeneous freezing of pure water and for the heterogeneous freezing of microcline suspensions in water. Microcline, a K-feldspar, is selected as an example, since it is commonly found in collected mineral dust samples and it is a highly active INP (Harrison et al, 2016;Kanji et al, 2017;Klumpp et al, 2022;Welti et al, 2019).…”