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 sensitivity of microcline to 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 surface 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. Depending 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 predicted 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 decrease 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 decrease in Fhet became even more pronounced. The slope of Thet(aw) was different for each of the neutralized acid solutions. 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 the case of the investigated polyols, no significant deviation 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 7 d, and tested the reversibility of the interaction with the solutes after aging for 10 d. 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 the source to frozen cloud droplets in the atmosphere.

The Urmia Playa as source of airborne dust and ice nucleating particles – Part 1: Correlation between soils and airborne samples

“…Yet, quartz and microcline are anti-correlated with Fhet and Thet, although the freezing onset temperatures of the soil and dust samples fall in the typical range of these two minerals. The lack of correlation between K-feldspars could be due to their high sensitivity to the presence of ions and organic substances (Kumar et al, 2018;Whale et al, 2018: Yun, et al, 2020Klumpp et al, 2022), which constitute a relevant fraction of the investigated samples. The correlation of soil and dust physicochemical properties with IN activity (Table 8) shows a significantly positive correlation of Thet and Fhet with total clay fraction in accordance with their positive correlation with clay minerals and phyllosilicates (Table 7), which are the major constituents of the clay size fraction.…”

Comparing the ice nucleation properties of the kaolin minerals kaolinite and halloysite

“…The general setup of the immersion freezing experiments used here has been described in Klumpp et al (2022) and will be briefly repeated in this section. Immersion freezing experiments were conducted with a Differential Scanning Calorimeter (DSC Q10 from TA instruments).…”

“…Experiments were run at least twice, each time with a freshly prepared suspension. Our key parameters to analyse the DSC thermograms are the homogeneous (Thom) and the heterogeneous (Thet) freezing onset temperatures, and the heterogeneously frozen fraction (Fhet), which was evaluated as the integral of the heat release due to heterogeneous freezing divided by the total heat release due to homogeneous and heterogeneous freezing as described in Klumpp et al (2022). Occasionally occurring spikes in the thermograms stemming from larger droplets in the droplet size distribution are excluded from the analysis.…”

Ice nucleation by smectites: The role of the edges

“…CC BY 4.0 License. Kiselev et al, 2016) with freezing onsets around 251-252 K in emulsion freezing experiments (Kumar et al, 2018;Klumpp et al, 2022a). Yet, they also proved to be highly sensitive to the presence of solutes, as the IN activity of feldspar suspensions in aqueous salt solutions was found to decline for solute concentrations >0.02 M (Whale et al, 2018;Kumar et al, 2018;Kumar et al, 2019b;Yun et al, 2020;.…”

“…Yet, they also proved to be highly sensitive to the presence of solutes, as the IN activity of feldspar suspensions in aqueous salt solutions was found to decline for solute concentrations >0.02 M (Whale et al, 2018;Kumar et al, 2018;Kumar et al, 2019b;Yun et al, 2020;. Moreover, they irreversibly lose IN activity in the presence of acidic solutes (Kumar et al, 2018;Klumpp et al, 2022a). Conversely, the presence of ammonia or ammonium containing salts boosts their IN activity above the pure water case (Kumar et al, 2018;2019b;Whale et al, 2018;Worthy et al, 2021).…”