Weathering influences the ice nucleation activity of microplastics

Microplastics are being increasingly detected in the atmosphere at altitudes relevant to mixed-phase cloud formation. However, the extent to which microplastics, along with their dynamic surface properties resulting from environmental weathering, could influence atmospheric microphysical processes remains largely unexplored. Here, through a series of ice nucleation experiments and droplet freezing assays, we highlight the capability of model polyethylene microplastics to induce heterogeneous ice nucleation via immersion freezing under atmospherically relevant conditions. We find that sunlight-induced weathering of the microplastic surface influences the structure of surface-bound water molecules and dictates the ice nucleation activity of the microplastics. Using polyethylene, polypropylene, polystyrene, and polyethylene terephthalate as models, we demonstrate that the ice nucleation ability of microplastics is intrinsically linked to their underlying chemistry. Our findings underscore the need to establish a connection between microplastics and atmospheric processes, as the behavior of microplastic pollutants in the atmosphere holds the potential to influence their environmental transport as well as atmospheric microphysical processes.

Development and validation of a new cloud simulation experiment for lab-based aerosol-cloud studies

The Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud expansion chamber with a volume of 84 m³ was extended for the small cloud expansion chamber AIDA mini (AIDAm) with a volume of 20 L. AIDAm is located in the cold room of AIDA and can perform automated ice-nucleation measurements over longer time periods of hours to days. AIDAm samples from the AIDA chamber, which acts as a reservoir of atmospheric aerosol types, which can slowly be modified by physical or chemical processes similar to those occurring in the atmosphere. AIDAm was validated for accurate ice-nucleation temperature control by measuring homogeneous freezing of pure water droplets at temperatures around -34 °C and for immersion freezing induced by dust aerosol particles in the temperature range between -20 and -30 °C. Further validation experiments at cirrus cloud temperatures of -45 °C revealed that AIDAm can distinguish between heterogeneous ice formation on mineral dust aerosols and homogeneous freezing of sulfuric acid solution particles. The contribution of homogeneous and heterogeneous ice formation processes to the ice-nucleation activity of coated dust particles was investigated in a 7 h long experiment, where solid dust particles were slowly coated with sulfuric acid. The continuous AIDAm measurements with a time resolution of 6 min showed a substantial suppression of the heterogeneous freezing phenomenon and an increasing role of homogeneous freezing while the coating amount was slowly increased. This experiment proved the capability of AIDAm to sensitively detect small changes in the ice-nucleation ability of aerosols, which undergo slow processing like chemical surface coating.

Methods to stabilize aqueous supercooling identified by use of an isochoric nucleation detection (INDe) device

Stable aqueous supercooling has shown significant potential as a technique for human tissue preservation, food cold storage, conservation biology, and beyond, but its stochastic nature has made its translation outside the laboratory difficult. In this work, we present an isochoric nucleation detection (INDe) platform for automated, high-throughput characterization of aqueous supercooling at >1 mL volumes, which enables statistically-powerful determination of the temperatures and time periods for which supercooling in a given aqueous system will remain stable. We employ the INDe to investigate the effects of thermodynamic, surface, and chemical parameters on aqueous supercooling, and demonstrate that various simple system modifications can significantly enhance supercooling stability, including isochoric (constant-volume) confinement, hydrophobic container walls, and the addition of even mild concentrations of solute. Finally, in order to enable informed design of stable supercooled biopreservation protocols, we apply a statistical model to estimate stable supercooling durations as a function of temperature and solution chemistry, producing proof-of-concept supercooling stability maps for four common cryoprotective solutes.

Is Ice Nucleation by Organic Crystals Nonclassical? An Assessment of the Monolayer Hypothesis of Ice Nucleation

Potent ice nucleating organic crystals display an increase in nucleation efficiency with pressure and memory effect after pressurization that set them apart from inorganic nucleants. These characteristics were proposed to arise from an ordered water monolayer at the organic-water interface. It was interpreted that ordering of the monolayer is the limiting step for ice nucleation on organic crystals, rendering their mechanism of nucleation nonclassical. Despite the importance of organics in atmospheric ice nucleation, that explanation has never been investigated. Here we elucidate the structure of interfacial water and its role in ice nucleation at ambient pressure on phloroglucinol dihydrate, the paradigmatic example of outstanding ice nucleating organic crystal, using molecular simulations. The simulations confirm the existence of an interfacial monolayer that orders on cooling and becomes fully ordered upon ice formation. The monolayer does not resemble any ice face but seamlessly connects the distinct hydrogen-bonding orders of ice and the organic surface. Although large ordered patches develop in the monolayer before ice nucleates, we find that the critical step is the formation of the ice crystallite, indicating that the mechanism is classical. We predict that the fully ordered, crystalline monolayer nucleates ice above -2 °C and could be responsible for the exceptional ice nucleation by the organic crystal at high pressures. The lifetime of the fully ordered monolayer around 0 °C, however, is too short to account for the memory effect reported in the experiments. The latter could arise from an increase in the melting temperature of ice confined by strongly ice-binding surfaces.

Measurements of Immersion Freezing and Heterogeneous Chemistry of Atmospherically Relevant Single Particles with Micro-Raman Spectroscopy

In the atmosphere, there are several different trajectories by which particles can nucleate ice; two of the major pathways are deposition and immersion freezing. Single particle depositional freezing has been widely studied with spectroscopic methods while immersion freezing has been predominantly studied either for particles within bulk aqueous solutions or using optical imaging of single particles. Of the few existing spectroscopic methods that monitor immersion freezing, there are limited opportunities for investigating the impact of heterogeneous chemistry on freezing. Herein, we describe a method that couples a confocal Raman spectrometer with an environmental cell to investigate single particle immersion freezing along with the capability to investigate in situ the impact of heterogeneous reactions with ozone and other trace gases on ice nucleation. This system, which has been rigorously calibrated (temperature and relative humidity) across a large dynamic range, is used to investigate low temperature water uptake and heterogeneous ice nucleation of atmospherically relevant single particles deposited on a substrate. The use of Raman spectroscopy provides important insights into the phase state and chemical composition of ice nuclei and, thus, insights into cloud formation.

Ice nucleation ability of loess from the northwestern United States

The heterogeneous nucleation of ice processes involving loess particles that influences the formation of mixed-phase clouds are poorly understood. Here, the ice nucleating ability of wind-blown dust or loess accumulated from the past glaciated area was investigated at three temperatures: -26, -30, and -34 °C and at below and above saturation with respect to liquid water conditions. Total six loess samples from different regions across Columbia Basin province, WA, USA were collected, dry dispersed, size-selected at mobility diameter 200 nm, and investigated for their ice nucleation efficiency. To understand the effect of atmospheric processing during long-range transport on their ice nucleating ability, similar experiments were also performed on acid-treated loess samples. Additionally, the ice nucleating properties of Arizona Test Dust (ATD) were investigated as a surrogate for natural mineral dust particles to test the experimental approach. Results show that treated particles have lower ice nucleation efficiency compared to untreated particles at all temperature and saturation with respect to liquid water conditions. Comparison based on ice-active site density (N_s) metric indicate that loess particles at saturation with respect to liquid water conditions are marginally more efficient than the mineral and soil dust values reported in the literature, but they have lower efficiencies than the predicted N_s efficiency of K-feldspar particles at supercooled temperatures greater than -38 °C.

Water vapor exposure chamber for constant humidity and hydrogen and oxygen stable isotope composition

RATIONALE

Water vapor exposure experiments have applications for studying water physisorption and chemisorption hydration and hydroxylation reactions on a wide variety of material surfaces. The stable isotopes of hydrogen and oxygen in the water molecule are useful tracers of water exchange mechanisms and/or rates in such vapor exposure experiments.

METHODS

We designed and built a humidity chamber system that uses membrane-mediated liquid-vapor exchange of water followed by mixing with dry air to control the relative humidity of air and its δ² H and δ¹⁸ O isotopic composition. We tested the stability and precision of the humidity and its isotopic composition on hourly to 90-day timescales.

RESULTS

The humidity chamber design reported here is capable of providing relative humidity control to within ±1%, and consistent δ² H and δ¹⁸ O values of the water vapor that are similar to our cavity ringdown spectroscopy (CRDS) measurement precision (δ² H_vap  ± 0.7‰ and δ¹⁸ O_vap  ± 0.24‰). We quantify the isotopic enrichment effects of Rayleigh distillation in the system and provide information on water reservoir sizes large enough to buffer isotopic enrichment effects to within measurement precision.

CONCLUSIONS

The humidity chamber design reported here provides a means to create constant δ² H and δ¹⁸ O values over the course of an exposure experiment. The design has applications to a wide range of studies of water sorption on material surfaces from foods and pharmaceuticals to geological materials.

Stabilization of AgI's polar surfaces by the aqueous environment, and its implications for ice formation

AgI is a potent inorganic ice nucleating particle, a feature often attributed to the lattice match between its {0001} surfaces and ice. Dissolved ions are found to be essential to the stability of these polar surfaces, and crucial to ice formation.

Performance Study and Efficiency Improvement of Ice Slurry Production by Scraped-Surface Method

In this study, the performance of ice slurry production by scraped-surface method was experimentally investigated. Temperature change characteristics, ice packing fraction (IPF) of ice slurry, power consumption of scraping system and coefficient of performance (COP) were measured by varying the concentration of sodium chloride solution, scraping speed, and solution flow rate. The effect of nanosilica on efficiency of ice slurry production was also studied. The results showed that scraping power consumption accounted for only a small proportion (about 5%) of the total power consumption of the system. An increase in the concentration of sodium chloride caused a decrease in the IPF and a decrease in the COP of the system. With the solution flow rate at 1.3 m3/h and scraping speed at 13 rpm, the maximum COP (2.43) was obtained. Furthermore, the addition of nanosilica had a significant effect on improving the system COP.

Overview of Ice Nucleating Particles

Ice particle formation in tropospheric clouds significantly changes cloud radiative and microphysical properties. Ice nucleation in the troposphere via homogeneous freezing occurs at temperatures lower than −38°C and relative humidity with respect to ice above 140%. In the absence of these conditions, ice formation can proceed via heterogeneous nucleation aided by aerosol particles known as ice nucleating particles (INPs). In this chapter, new developments in identifying the heterogeneous freezing mechanisms, atmospheric relevance, uncertainties, and unknowns about INPs are described. The change in conventional wisdom regarding the requirements of INPs as new studies discover physical and chemical properties of these particles is explained. INP sources and known reasons for their ice nucleating properties are presented. The need for more studies to systematically identify particle properties that facilitate ice nucleation is highlighted. The atmospheric relevance of long-range transport, aerosol aging, and coating studies (in the laboratory) of INPs are also presented. Possible mechanisms for processes that change the ice nucleating potential of INPs and the corresponding challenges in understanding and applying these in models are discussed. How primary ice nucleation affects total ice crystal number concentrations in clouds and the discrepancy between INP concentrations and ice crystal number concentrations are presented. Finally, limitations of parameterizing INPs and of models in representing known and unknown processes related to heterogeneous ice nucleation processes are discussed.

Direct observation of ice nucleation events on individual atmospheric particles

Nanometer scale imaging of kaolinite particles shows that ice nucleation initiates preferentially at edges of stacked planes and not on basal planes.

Immersion freezing of supermicron mineral dust particles: freezing results, testing different schemes for describing ice nucleation, and ice nucleation active site densities

Ice nucleation on mineral dust particles is known to be an important process in the atmosphere. To accurately implement ice nucleation on mineral dust particles in atmospheric simulations, a suitable theory or scheme is desirable to describe laboratory freezing data in atmospheric models. In the following, we investigated ice nucleation by supermicron mineral dust particles [kaolinite and Arizona Test Dust (ATD)] in the immersion mode. The median freezing temperature for ATD was measured to be approximately -30 °C compared with approximately -36 °C for kaolinite. The freezing results were then used to test four different schemes previously used to describe ice nucleation in atmospheric models. In terms of ability to fit the data (quantified by calculating the reduced chi-squared values), the following order was found for ATD (from best to worst): active site, pdf-α, deterministic, single-α. For kaolinite, the following order was found (from best to worst): active site, deterministic, pdf-α, single-α. The variation in the predicted median freezing temperature per decade change in the cooling rate for each of the schemes was also compared with experimental results from other studies. The deterministic model predicts the median freezing temperature to be independent of cooling rate, while experimental results show a weak dependence on cooling rate. The single-α, pdf-α, and active site schemes all agree with the experimental results within roughly a factor of 2. On the basis of our results and previous results where different schemes were tested, the active site scheme is recommended for describing the freezing of ATD and kaolinite particles. We also used our ice nucleation results to determine the ice nucleation active site (INAS) density for the supermicron dust particles tested. Using the data, we show that the INAS densities of supermicron kaolinite and ATD particles studied here are smaller than the INAS densities of submicron kaolinite and ATD particles previously reported in the literature.

A water activity based model of heterogeneous ice nucleation kinetics for freezing of water and aqueous solution droplets

Immersion freezing of water and aqueous solutions by particles acting as ice nuclei (IN) is a common process of heterogeneous ice nucleation which occurs in many environments, especially in the atmosphere where it results in the glaciation of clouds. Here we experimentally show, using a variety of IN types suspended in various aqueous solutions, that immersion freezing temperatures and kinetics can be described solely by temperature, T, and solution water activity, a(w), which is the ratio of the vapour pressure of the solution and the saturation water vapour pressure under the same conditions and, in equilibrium, equivalent to relative humidity (RH). This allows the freezing point and corresponding heterogeneous ice nucleation rate coefficient, J(het), to be uniquely expressed by T and a(w), a result we term the a(w) based immersion freezing model (ABIFM). This method is independent of the nature of the solute and accounts for several varying parameters, including cooling rate and IN surface area, while providing a holistic description of immersion freezing and allowing prediction of freezing temperatures, J(het), frozen fractions, ice particle production rates and numbers. Our findings are based on experimental freezing data collected for various IN surface areas, A, and cooling rates, r, of droplets variously containing marine biogenic material, two soil humic acids, four mineral dusts, and one organic monolayer acting as IN. For all investigated IN types we demonstrate that droplet freezing temperatures increase as A increases. Similarly, droplet freezing temperatures increase as the cooling rate decreases. The log10(J(het)) values for the various IN types derived exclusively by Tand a(w), provide a complete description of the heterogeneous ice nucleation kinetics. Thus, the ABIFM can be applied over the entire range of T, RH, total particulate surface area, and cloud activation timescales typical of atmospheric conditions. Lastly, we demonstrate that ABIFM can be used to derive frozen fractions of droplets and ice particle production for atmospheric models of cirrus and mixed phase cloud conditions.

Heterogeneous nucleation of ice on carbon surfaces

Atmospheric aerosols can promote the heterogeneous nucleation of ice, impacting the radiative properties of clouds and Earth's climate. The experimental investigation of heterogeneous freezing of water droplets by carbonaceous particles reveals widespread ice freezing temperatures. It is not known which structural and chemical characteristics of soot account for the variability in ice nucleation efficiency. Here we use molecular dynamics simulations to investigate the nucleation of ice from liquid water in contact with graphitic surfaces. We find that atomically flat carbon surfaces promote heterogeneous nucleation of ice, while molecularly rough surfaces with the same hydrophobicity do not. Graphitic surfaces and other surfaces that promote ice nucleation induce layering in the interfacial water, suggesting that the order imposed by the surface on liquid water may play an important role in the heterogeneous nucleation mechanism. We investigate a large set of graphitic surfaces of various dimensions and radii of curvature and find that variations in nanostructures alone could account for the spread in the freezing temperatures of ice on soot in experiments. We conclude that a characterization of the nanostructure of soot is needed to predict its ice nucleation efficiency.

Water adsorption on clay minerals as a function of relative humidity: application of BET and Freundlich adsorption models

Water adsorption on kaolinite, illite, and montmorillonite clays was studied as a function of relative humidity (RH) at room temperature (298 K) using horizontal attenuated total reflectance (HATR) Fourier transform infrared (FTIR) spectroscopy equipped with a flow cell. The water content as a function of RH was modeled using the Brunauer, Emmett, and Teller (BET) and Freundlich adsorption isotherm models to provide complementary multilayer adsorption analysis of water uptake on the clays. A detailed analysis of model fit integrity is reported. From the BET fit to the experimental data, the water content on each of the three clays at monolayer (ML) water coverage was determined and found to agree with previously reported gravimetric data. However, BET analysis failed to adequately describe adsorption phenomena at RH values greater than 80%, 50%, and 70% RH for kaolinite, illite, and montmorillonite clays, respectively. The Freundlich adsorption model was found to fit the data well over the entire range of RH values studied and revealed two distinct water adsorption regimes. Data obtained from the Freundlich model showed that montmorillonite has the highest water adsorption strength and highest adsorption capacity at RH values greater than 19% (i.e., above ML water adsorption) relative to the kaolinite and illite clays. The difference in the observed water adsorption behavior between the three clays was attributed to different water uptake mechanisms based on a distribution of available adsorption sites. It is suggested that different properties drive water adsorption under different adsorption regimes resulting in the broad variability of water uptake mechanisms.

Freezing of water and aqueous NaCl droplets coated by organic monolayers as a function of surfactant properties and water activity

This study presents heterogeneous ice nucleation from water and aqueous NaCl droplets coated by 1-nonadecanol and 1-nonadecanoic acid monolayers as a function of water activity (a(w)) from 0.8 to 1 accompanied by measurements of the corresponding pressure-area isotherms and equilibrium spreading pressures. For water and aqueous NaCl solutions of ~0-20 wt % in concentration, 1-nonadecanol exhibits a condensed phase, whereas the phase of 1-nonadecanoic acid changes from an expanded to a condensed state with increasing NaCl content of the aqueous subphase. 1-Nonadecanol-coated aqueous droplets exhibit the highest median freezing temperatures that can be described by a shift in a(w) of the ice melting curve by 0.098 according to the a(w)-based ice nucleation approach. This freezing curve represents a heterogeneous ice nucleation rate coefficient (J(het)) of 0.85 ± 0.30 cm(-2) s(-1). The median freezing temperatures of 1-nonadecanoic acid-coated aqueous droplets decrease less with increasing NaCl content compared to the homogeneous freezing temperatures. This trend in freezing temperature is best described by a linear function in a(w) and not by the a(w)-based ice nucleation approach most likely due to an increased ice nucleation efficiency of 1-nonadecanoic acid governed by the monolayer state. This freezing curve represents J(het) = 0.46 ± 0.16 cm(-2) s(-1). Contact angles (α) for 1-nonadecanol- and 1-nonadecanoic acid-coated aqueous droplets increase as temperature decreases for each droplet composition, but absolute values depend on employed water diffusivity and the interfacial energies of the ice embryo. A parametrization of log[J(het)(Δa(w))] is presented which allows prediction of freezing temperatures and heterogeneous ice nucleation rate coefficients for water and aqueous NaCl droplets coated by 1-nonadecanol without knowledge of the droplet's composition and α.

Why Do Sulfuric Acid Coatings Influence the Ice Nucleation Properties of Mineral Dust Particles in the Atmosphere?

Laboratory studies with supermicrometer particles have shown that mineral particles coated with sulfuric acid are relatively poor ice nuclei. We investigated this phenomenon, which is of atmospheric relevance, by probing the structure of water at the mineral-aqueous acid interface as a function of the sulfuric acid concentration using sum frequency generation vibrational spectroscopy. We found that ordered water structures at water/mica interfaces drastically diminished at molarities of sulfuric acid equal to 0.5 M and totally disappeared when the molarities reached 5 M. The decrease in ordered water structures at the interface was caused by a combined effect of the decreased mica surface potential at low pH, the adsorption of sulfates on mica, and the lack of free water molecules in high concentrations of acidic solution. The good ice nucleation ability above liquid water saturation is correlated with the presence of structured water, suggesting that structured water at the interface may be needed for efficient heterogeneous ice nucleation.

10th Anniversary review: applications of analytical techniques in laboratory studies of the chemical and climatic impacts of mineral dust aerosol in the Earth's atmosphere

It is clear that mineral dust particles can impact a number of global processes including the Earth's climate through direct and indirect climate forcing, the chemical composition of the atmosphere through heterogeneous reactions, and the biogeochemistry of the oceans through dust deposition. Thus, mineral dust aerosol links land, air, and oceans in unique ways unlike any other type of atmospheric aerosol. Quantitative knowledge of how mineral dust aerosol impacts the Earth's climate, the chemical balance of the atmosphere, and the biogeochemistry of the oceans will provide a better understanding of these links and connections and the overall impact on the Earth system. Advances in the applications of analytical laboratory techniques have been critical for providing valuable information regarding these global processes. In this mini review article, we discuss examples of current and emerging techniques used in laboratory studies of mineral dust chemistry and climate and potential future directions.