Mineral dust particles are one of the most abundant types of ice nucleating particles in the atmosphere. During atmospheric transport, these particles can be coated with water-soluble solutes, which can modify their ice nucleating ability. Although previous studies have shown that even low concentrations of water-soluble solutes can modify the ice nucleating properties of mineral dust particles, our understanding of this topic is far from complete. We examined the effects of a series of alkali metal nitrates at low concentrations (5 × 10 −5 M to 5 × 10 −3 M) on the surface composition and immersion freezing of potassium-rich feldspar (K-rich feldspar). Immersion freezing was investigated with the droplet freezing technique, and the surface composition was investigated with cryogenic X-ray photoelectron spectroscopy. K + increased the median freezing temperature of the droplets, while the other alkali metal cations either had no effect or decreased the median freezing temperature. The changes in the median freezing temperature of the droplets due to the presence of nitrates followed the order K + ≥ Li + ≥ Na + ≥ Rb + ≥ Cs + and, except for Cs + , were correlated to the K/Al ratio at the surface of K-rich feldspar. The K/Al ratio is possibly an indicator of the abundance of certain types of K-bearing microcline surfaces that drive the immersion freezing of K-rich feldspar, while Cs + likely influences the immersion freezing of K-rich feldspar by an additional mechanism, possibly blocking ice nucleation sites by adsorption. Our work also shows that the cation charge density (charge density over the surface area of a single cation) is not a good predictor of the effects of cations on the immersion freezing of K-rich feldspar in our experiments.

Dust is the major source of iron in atmospheric aerosols but little is known about its role in catalyzing polymerization reactions of organics in particles. Using Arizona Test Dust (AZTD) and hematite nanoparticles as laboratory standards and proxies for hematite-rich natural dust, respectively, we show that their reactions with catechol in aqueous slurries lead to the formation of black polycatechol. This observation is in contrast to oxalate and sulfate which form surface complexes promoting the dissolution of iron from the dust particles. Results from ultraviolet–visible spectroscopy and microscopy/elemental mapping show that the formation of polycatechol changed the optical properties of the dust particles and surface chemical composition. Results from ice nucleation studies using a droplet freezing technique show that polycatechol did not significantly impact ice nucleation or block ice nucleation sites on AZTD. In contrast, increasing pH decreased the ice nucleation ability of AZTD. These results highlight the complexity of iron’s role in aerosol aging processes, brown carbon formation, and ice nucleation.

Abstract. Modelling studies suggest that the climate and the hydrological cycle are sensitive to the concentrations of ice-nucleating particles (INPs). However, the concentrations, composition, and sources of INPs in the atmosphere remain uncertain. Here, we report daily concentrations of INPs in the immersion freezing mode and tracers of mineral dust (Al, Fe, Ti, and Mn), sea spray aerosol (Na+ and Cl−), and anthropogenic aerosol (Zn, Pb, NO3-, NH4+, and non-sea-salt SO42-) at Alert, Canada, during a 3-week campaign in March 2016. In total, 16 daily measurements of INPs are reported. The average INP concentrations measured in the immersion freezing mode were 0.005±0.002, 0.020±0.004, and 0.186±0.040 L−1 at −15, −20, and −25 ∘C, respectively. These concentrations are within the range of concentrations measured previously in the Arctic at ground level or sea level. Mineral dust tracers all correlated with INPs at −25 ∘C (correlation coefficient, R, ranged from 0.70 to 0.76), suggesting that mineral dust was a major contributor to the INP population at −25 ∘C. Particle dispersion modelling suggests that the source of the mineral dust may have been long-range transport from the Gobi Desert. Sea spray tracers were anti-correlated with INPs at −25 ∘C (R=-0.56). In addition, INP concentrations at −25 ∘C divided by mass concentrations of aluminum were anti-correlated with sea spray tracers (R=-0.51 and −0.55 for Na+ and Cl−, respectively), suggesting that the components of sea spray aerosol suppressed the ice-nucleating ability of mineral dust in the immersion freezing mode. Correlations between INPs and anthropogenic aerosol tracers were not statistically significant. These results will improve our understanding of INPs in the Arctic during spring.

Mineral dust particles can initiate the freezing of cloud droplets in the atmosphere. The freezing efficiency of these particles can, however, be strongly affected by solutes, such as inorganic acids, polyols, and carboxylic acids. Here, we report the effects of inorganic acids (HNO 3 and HCl), polyols, and carboxylic acids at low concentrations on the ice nucleating ability of potassium-rich feldspar (K-rich feldspar) using the droplet freezing technique. The inorganic acids and carboxylic acids decreased the median freezing temperature of droplets containing K-rich feldspar by up to 7 °C, while the polyols had no significant effect on the median freezing temperature. For the inorganic acids and carboxylic acids, the median freezing temperature was a strong function of the pH of the droplets, with the median freezing temperature decreasing as the pH decreased. By examining the surface properties of K-rich feldspar exposed to different concentrations of HCl with cryogenic X-ray photoelectron spectroscopy, we show that the decrease in the ice nucleating ability of Krich feldspar by the inorganic acids and carboxylic acids was likely caused by ion exchange (H 3 O + with parent K + in microcline) and the incongruent dissolution of Al with respect to Si at K-rich feldspar surfaces. The decrease in the ice nucleating ability of K-rich feldspar by the carboxylic acids only related to the pH of the droplets rather than the type of carboxylic acid and their expected binding mechanisms on K-rich feldspar. This study focuses on rare ice nucleating active sites (with an ice nucleating active site density of 10−600 cm −2 ) of the K-rich feldspar and short exposure times between the solutes and the K-rich feldspar. Further studies are needed to investigate more abundant ice nucleating active sites and longer exposure times, as well as K-rich feldspar samples from different sources.

<p>EGU Abstract</p><p> </p><p>A wide range of materials including mineral dust, soil dust, and bioaerosols have been shown to act as ice nuclei in the atmosphere. During atmospheric transport, these materials can become coated with inorganic and organic solutes which may impact their ability to nucleate ice. While a number of studies have investigated the impact of solutes at low concentrations on ice nucleation by mineral dusts, very few studies have examined their impact on non-mineral dust ice nuclei.</p><p>We studied the effect of dilute (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> solutions (0.05 M) on immersion freezing of a variety of non-mineral dust ice nucleating substances including bacteria, fungi, sea ice diatom exudates, sea surface microlayer, and humic substances using the droplet freezing technique. We also studied the effect of (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> on immersion freezing of mineral dust particles for comparison purposes. (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> had no effect on the median freezing temperature of 9 of the 10 tested non-mineral dust materials. There was a small but statistically significant decrease in the median freezing temperature of the bacteria <em>X. campestris</em> (change in median freezing temperature  = -0.43 ± 0.19 °C) in the presence of (NH<sub>4</sub>)<sub>2</sub>SO<sub>4 </sub>compared to pure water. Conversely, (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> increased the median freezing temperature of four different mineral dusts (potassium-rich feldspar, Arizona test dust, kaolinite, montmorillonite) by 3 °C to 9 °C and increased the ice nucleation active site density per gram of material by a factor of ~10 to ~30.</p><p>This significant difference in the response of mineral dust and non-mineral dust ice nucleating substances when exposed to (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> suggests that they nucleate ice and/or interact with (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> via different mechanisms. This difference suggests that the relative importance of mineral dust to non-mineral dust particles for ice nucleation in mixed-phase clouds could increase as these particles become coated with (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> in the atmosphere. This difference also suggests that the addition of (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> to atmospheric samples of unknown composition could be used as an indicator or assay for the presence of mineral dust ice nuclei.</p>