Heterogeneous Ice Nucleation in Model Crystalline Porous Organic Polymers: Influence of Pore Size on Immersion Freezing

Heterogeneous ice nucleation activity is affected by aerosol particle composition, crystallinity, pore size, and surface area. However, these surface properties are not well understood, regarding how they act to promote ice nucleation and growth to form ice clouds. Therefore, synthesized materials for which surface properties can be tuned were examined in immersion freezing mode in this study. To establish the relationship between particle surface properties and efficiency of ice nucleation, materials, here, covalent organic frameworks (COFs), with different pore diameters and degrees of crystallinity (ordering), were characterized. Results showed that out of all the highly crystalline COFs, the sample with a pore diameter between 2 and 3 nm exhibited the most efficient ice nucleation activity. We posit that the highly crystalline structures with ordered pores have an optimal pore diameter where the ice nucleation activity is maximized and that the not highly crystalline structures with nonordered pores have more sites for ice nucleation. The results were compared and discussed in the context of other synthesized porous particle systems. Such studies give insight into how material features impact ice nucleation activity

Isotherm-Based Thermodynamic Models for Solute Activities of Organic Acids with Consideration of Partial Dissociation

Organic acids make up a significant fraction of the organic mass in atmospheric aerosol particles. The calculation of gas–liquid–solid equilibrium partitioning of the organic acid is therefore critical for accurate determination of atmospheric aerosol physicochemical properties and processes such as new particle formation and activation to cloud condensation nuclei. Previously, an adsorption isotherm-based statistical thermodynamic model was developed for capturing solute concentration–activity relationships for multicomponent aqueous solutions over the entire concentration range (Dutcher et al. <i>J. Phys. Chem. C/A</i> <b>2011</b>, <b>2012</b>, <b>2013</b>), with model parameters for energies of adsorption successfully related to dipole–dipole electrostatic forces in solute–solvent and solvent–solvent interactions for both electrolytes and organics (Ohm et al. <i>J. Phys. Chem. A</i> <b>2015</b>). However, careful attention is needed for weakly dissociating semivolatile organic acids. Dicarboxylic acids, such as malonic acid and glutaric acid are treated here as a mixture of nondissociated organic solute (HA) and dissociated solute (H⁺ + A^–). It was found that the apparent dissociation was greater than that predicted by known dissociation constants alone, emphasizing the effect of dissociation on osmotic and activity coefficient predictions. To avoid additional parametrization from the mixture approach, an expression was used to relate the Debye–Hückel hard-core collision diameter to the adjustable solute–solvent intermolecular distance. An improved reference state treatment for electrolyte–organic aqueous mixtures, such as that observed here with partial dissociation, has also been proposed. This work results in predictive correlations for estimation of organic acid and water activities for which there is little or no activity data

Water uptake and optical properties of mixed organic-inorganic particles

Atmospheric aerosol particles are frequently mixtures of inorganic and organic species, both of which can contribute to aerosol water uptake and determine the particles’ ability to scatter and absorb light. While water uptake of purely inorganic aerosol is well represented in current regional and global chemical transport models, it is challenging to represent it for particles that are mixtures of organic and inorganic species. Here we quantified the growth factor for aerosols that consist of mixed organic-inorganic particles using an accurate lattice-based adsorption isotherm model (Ad-iso) as a benchmark. We then determined the error in the growth factor and resulting optical properties for simplifying assumptions that are commonly made in current chemical transport models. The systems studied here are representative of ambient atmospheric aerosols, consisting of model water-soluble inorganic-organic mixtures, with and without a core of absorbing black carbon, under conditions of relative humidity larger than 85%. The assumption of completely neglecting the water uptake by organic components, for particles with an organic mass fraction of 50%, led to errors of up to 7% in growth factor and up to 3.5% in single scattering albedo. Larger errors occurred for larger organic mass fractions. Approximating the organic water uptake with a constant hygroscopicity parameter, for organic mass fractions between 45 and 65%, the errors remained within 3% for the growth factor and 0.6% for the single scattering albedo. For organic mass fractions smaller than 45% or larger than 65%, the errors increased up to 6% for the single scattering albedo. The magnitudes of these errors underscore the importance of considering organic/inorganic mixtures for estimating direct aerosol radiative forcing. Copyright © 2021 American Association for Aerosol Research</p

Multistep Phase Transitions in Sea Surface Microlayer Droplets and Aerosol Mimics using Microfluidic Wells

Oceanic sea spray is one of the largest contributors of atmospheric aerosol particles worldwide. The phase of aerosol particles is known to impact radiative forcing and cloud nucleation. However, as chemically complex aqueous systems that include mixtures of biological, organic, and salt constituents, it is a challenge to predict the phase of sea spray aerosol especially as they age in the atmosphere. In this study, phase behavior (liquid–liquid phase separation, LLPS, and crystallization) of sea surface microlayer (SSML) sample and chemical mimics are investigated using a microfluidic pervaporation approach. Internally mixed aqueous droplets with varying compositions and concentrations are trapped in microfluidic wells, and phase transitions of the droplets are optically determined in a slow dehydration process. A system containing SSML sample combined with an organic acid 3-methyl glutaric acid (3-MGA) undergoes multiple phase changes, including two crystallization and two LLPS events. The added organic acid increases the sample’s organic-to-inorganic ratio and moves the system into the range typical of aged SSA. To better understand the contributing constituents to the observed phase changes, control experiments with inorganic salt components NaCl, MgCl2, and Na2SO4 are performed with and without 3-MGA, at varying organic to inorganic ratios. 3-MGA leads to LLPS, and the presence of Mg2+ more readily facilitates LLPS than Na+. With the systems studied, LLPS is more prevalent for the chemical mixtures in an intermediate OIR range. This study provides new insight into sea spray aerosol phase as a function of composition and relative humidity and demonstrates multistep phase transitions for these complex systems

Direct Measurement of pH in Individual Particles via Raman Microspectroscopy and Variation in Acidity with Relative Humidity

Atmospheric aerosol acidity is an important characteristic of aqueous particles, which has been linked to the formation of secondary organic aerosol by catalyzing reactions of oxidized organic compounds that have partitioned to the particle phase. However, aerosol acidity is difficult to measure and traditionally estimated using indirect methods or assumptions based on composition. Ongoing disagreements between experiments and thermodynamic models of particle acidity necessitate improved fundamental understanding of pH and ion behavior in high ionic strength atmospheric particles. Herein, Raman microspectroscopy was used to determine the pH of individual particles (H₂SO₄+MgSO₄) based on sulfate and bisulfate concentrations determined from ν_s(SO₄^2–) and ν_s(HSO₄^–), the acid dissociation constant, and activity coefficients from extended Debye–Hückel calculations. Shifts in pH and peak positions of ν_s(SO₄^2–) and ν_s(HSO₄^–) were observed as a function of relative humidity. These results indicate the potential for direct spectroscopic determination of pH in individual particles and the need to improve fundamental understanding of ion behavior in atmospheric particles

Liquid–Liquid Phase Separation Can Drive Aerosol Droplet Growth in Supersaturated Regimes

It is well known that atmospheric aerosol size and composition impact air quality, climate, and health. The aerosol composition is typically a mixture and consists of a wide range of organic and inorganic particles that interact with each other. Furthermore, water vapor is ubiquitous in the atmosphere, in indoor air, and within the human body’s respiratory system, and the presence of water can alter the aerosol morphology and propensity to form droplets. Specifically, aerosol mixtures can undergo liquid–liquid phase separation (LLPS) in the presence of water vapor. However, the experimental conditions for which LLPS impacts water uptake and the subsequent prediction of aerosol mixtures are poorly understood. To improve our understanding of aerosol mixtures and droplets, this study explores two ternary systems that undergo LLPS, namely, the 2MGA system (sucrose + ammonium sulfate + 2-methylglutaric acid) and the PEG1000 system (sucrose + ammonium sulfate + polyethylene glycol 1000). In this study, the ratio of species and the O:C ratios are systematically changed, and the hygroscopic properties of the resultant aerosol were investigated. Here, we show that the droplet activation above 100% RH of the 2MGA system was influenced by LLPS, while the droplet activation of the PEG1000 system was observed to be linearly additive regardless of chemical composition, O:C ratio, and LLPS. A theoretical model that accounts for LLPS with O:C ratios was developed and predicts the water uptake of internally mixed systems of different compositions and phase states. Hence, this study provides a computationally efficient algorithm to account for the LLPS and solubility parameterized by the O:C ratio for droplet activation at supersaturated relative humidity conditions and may thus be extended to mixed inorganic–organic aerosol populations with unspeciated organic composition found in the ambient environment