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  1. Free, publicly-accessible full text available August 3, 2024
  2. Heterogeneous ice nucleation in the atmosphere impacts climate, but the magnitude of the effect of ice clouds on radiative forcing is uncertain. Surfaces that promote ice nucleation are varied. Because O, Si, and Al are the most abundant elements in the Earth's crust, understanding how the Si : Al ratio impacts the ice nucleation activity of aluminosilicates through exploration of synthetic ZSM-5 samples provides a good model system. This paper investigates the immersion freezing of ZSM-5 samples with varying Si : Al ratios. Ice nucleation temperature increases with increasing surface Al content. Additionally, when ammonium, a common cation in aerosol particles, is adsorbed to the zeolite surface, initial freezing temperatures are reduced by up to 6 °C in comparison to proton-terminated zeolite surfaces. This large decrease in ice nucleation activity in the presence of ammonium suggests that the cation can interact with the surface to block or modify active sites. Our results on synthetic samples in which the surface composition is tunable gives insight into the role of surfaces in heterogeneous ice nucleation processes in the atmosphere. We emphasize the importance of examining surface chemical heterogeneities in ice nucleating particles that could result from a variety of aging pathways for a deeper understanding of the freezing mechanism. 
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  3. The pH of aerosol particles remains challenging to measure because of their small size, complex composition, and high acidity. Acidity in aqueous aerosol particles, which are found abundantly in the atmosphere, impacts many chemical processes from reaction rates to cloud formation. Only one technique – pH paper – currently exists for directly determining the pH of aerosol particles, and this is restricted to measuring average acidity for entire particle populations. Other methods for evaluating aerosol pH include filter samples, particle-into-liquid sampling, Raman spectroscopy, organic dyes, and thermodynamic models, but these either operate in a higher pH range or are unable to assess certain chemical species or complexity. Here, we present a new method for determining acidity of individual particles and particle phases using carbon quantum dots as a novel in situ fluorophore. Carbon quantum dots are easily synthesized, shelf stable, and sensitive to pH in the highly acidic regime from pH 0 to pH 3 relevant to ambient aerosol particles. To establish the method, a calibration curve was formed from the ratiometric fluorescence intensity of aerosolized standard solutions with a correlation coefficient ( R 2 ) of 0.99. Additionally, the pH of aerosol particles containing a complex organic mixture (COM) representative of environmental aerosols was also determined, proving the efficacy of using carbon quantum dots as pH-sensitive fluorophores for complex systems. The ability to directly measure aerosol particle and phase acidity in the correct pH range can help parametrize atmospheric models and improve projections for other aerosol properties and their influence on health and climate. 
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  4. null (Ed.)
    Heterogeneous ice nucleation is a common process in the atmosphere, but relatively little is known about the role of different surface characteristics on the promotion of ice nucleation. We have used a series of iron oxides as a model system to study the role of lattice mismatch and defects induced by milling on ice nucleation activity. The iron oxides include wüstite (FeO), hematite (Fe 2 O 3 ), magnetite (Fe 3 O 4 ), and goethite (FeOOH). The iron oxides were characterized by X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) surface area measurements. The immersion freezing experiments were performed using an environmental chamber. Wüstite (FeO) had the highest ice nucleation activity, which we attribute to its low lattice mismatch with hexagonal ice and the exposure of Fe–OH after milling. A comparison study of MnO and wüstite (FeO) with milled and sieved samples for each suggests that physical defects alone result in only a slight increase in ice nucleation activity. Despite differences in the molecular formula and surface groups, hematite (Fe 2 O 3 ), magnetite (Fe 3 O 4 ), and goethite (FeOOH) had similar ice nucleation activities, which may be attributed to their high lattice mismatch to hexagonal ice. This study provides further insight into the characteristics of a good heterogeneous ice nucleus and, more generally, helps to elucidate the interactions between aerosol particles and ice particles in clouds. 
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