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.

Cofactors such as NAD, AMP, and Coenzyme A (CoA) are essential for a diverse set of reactions and pathways in the cell. Specific carrier proteins are required to distribute these cofactors to different cell compartments, including peroxisomes. We previously identified a peroxisomal transport protein in Arabidopsis (Arabidopsis thaliana) called the peroxisomal NAD carrier (PXN). When assayed in vitro, this carrier exhibits versatile transport functions, e.g. catalyzing the import of NAD or CoA, the exchange of NAD/NADH, and the export of CoA. These observations raise the question about the physiological function of PXN in plants. Here, we used Saccharomyces cerevisiae to address this question. First, we confirmed that PXN, when expressed in yeast, is active and targeted to yeast peroxisomes. Secondl, detailed uptake analyses revealed that the CoA transport function of PXN can be excluded under physiological conditions due to its low affinity for this substrate. Third, we expressed PXN in diverse mutant yeast strains and investigated the suppression of the mutant phenotypes. These studies provided strong evidences that PXN was not able to function as a CoA transporter or a redox shuttle by mediating a NAD/NADH exchange, but instead catalyzed the import of NAD into peroxisomes against AMP in intact yeast cells.

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.

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>Ice-nucleating particles (or INP) play an important role in controlling cloud radiative properties and lifetimes. Therefore, understanding the sources and mechanisms of ice formation in clouds is vital for understanding their impact on cloud radiative feedback. Agricultural dust contributes 25% of global dust emissions and has been shown to nucleate ice at temperatures up to -6°C. This high nucleating ability of agricultural soils suggests that they may be an essential source of INPs on regional or global scales. Many organic components, which have been shown to be important for ice nucleation in soils, have surface active properties that may enhance the ice-nucleating ability of the soil. In this work, lignin was used as a reference for investigating surfactant macromolecules as a potential component of ice nucleation. Lignin solutions showed high ice-nucleating activity in line with decreases in surface tension. We contrasted our observations of lignin with observations from soil extractions from samples taken in the field. Preliminary results suggest little correlation between surface tension measurements and the ice-nucleating activity of extracted soil samples. The presence of a correlation between ice-nucleating and surface activity in soil components such as lignin, but the absence of this correlation in complete soil samples suggests that surfactants can be important ice-nucleating macromolecules, but that highly active soil samples do not necessarily reduce the surface tension at the water-air interface.</p>