ABSTRACT
Uric acid particles contribute to kidney stones, and natural processes for the elimination of stones depend on solute-solvent interactions. The process of uric acid dissolution has previously been understood via the lens of solubility; however, for pure and mixed salt solutions, these approaches do not provide a comprehensive picture of nanoscale particle solution thermodynamics. Unlike solubility measurements, water activity measurements provide us with information about the chemical potential responsible for the migration of water molecules driving the dissolution of particles. In this work, we used in situ experimental tools to estimate water activity values for pure uric acid aqueous droplets at different stages of droplet growth. The process of cloud formation, i.e., water condensation on a solute particle resulting in aqueous droplet formation, was leveraged to compare the water affinity for nanosized uric acid particles with a well-studied inorganic salt, sodium chloride. Specifically, we investigated microscopic uric acid particles (nanoparticles <300 nm, amorphous and super micron particles >5 µm, crystalline) and the mechanism of water uptake. The growth of droplet volume (Growth Factor, GF) for uric acid particles is experimentally observed for supermicrometer crystalline particles (>1 µm) at subsaturated humidity conditions (<100% RH). In addition, the water activity of submicrometer-size uric acid particles is estimated under subsaturated and supersaturated humidity conditions. These measurements provide us with information about the volume growth of droplets as water condenses in particles exposed to different humidity conditions. Our observations under subsaturated humidity conditions show that the uric acid particles have limited volume growth (<1% change per volume and <10% change per volume for crystalline and amorphous measurement, respectively). From the experimental data, the affinity of uric acid solute with water as a solvent is quantified in terms of water activity.
ABSTRACT
Black carbon (BC) is an aerosol that is released into the atmosphere due to the incomplete burning of biomass and can affect the climate directly or indirectly. BC commonly mixes with other primary or secondary aerosols to undergo aging, thereby changing its radiative properties and cloud condensation nuclei (CCN) activity. The composition of aged BC species in the atmosphere is difficult to measure with high confidence, so their associated CCN activity can be uncertain. In this work, the CCN activity analysis of BC was performed using laboratory measurements of proxy aged BC species. Vulcan XC72R carbon black was used as the representative of BC, and three structural isomers of benzenedicarboxylic acidâphthalic acid (PTA), isophthalic acid (IPTA), and terephthalic acid (TPTA)âwere mixed with BC to generate three different proxies of aged BC species. Most studies related to CCN activity analysis of BC aerosols use the traditional Köhler theory or an adsorption theory (such as the Frenkel-Halsey-Hill adsorption theory). PTA, IPTA, and TPTA fall in the sparingly water-soluble range and therefore do not fully obey either of the aforementioned theories. Consequently, a novel hybrid activity model (HAM) was used for the CCN activity analysis of the BC mixtures studied in this work. HAM combines the features of adsorption theory via the adsorption isotherm with the features of Köhler theory by incorporating solubility partitioning. The results in this work showed that HAM improves the representation of CCN activity of pure and mixed BC aerosol species with high certainty, evident from generally better goodness of fit, R2 > 0.9. This work implies that the hygroscopicity parameterization based on HAM captures the size-dependent variability in the CCN activity of the pure and aged BC species.
ABSTRACT
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.