Kinetically Determined Hygroscopicity and Efflorescence of Sucrose-Ammonium Sulfate Aerosol Droplets under Lower Relative Humidity

Key Roles of Bulk Viscosity and Acidity on Liquid-Liquid Phase Separation of Atmospheric Organic-Inorganic Mixed Aerosols

Liquid-liquid phase separation (LLPS) and the resulting particle morphologies in atmospheric organic-inorganic mixed aerosols are key regulators of aerosol chemistry and climate forcing. However, the influence of coexisting viscous water-soluble organic compounds (WSOCs) on the LLPS behavior in complex multicomponent aerosol systems remains poorly understood. In this study, we introduced three representative WSOCs, i.e., sucrose (SUC), glycerol (GLY), and citric acid (CA), to increase the bulk viscosity of a model LLPS system composed of 1,2,6-hexanetriol (HXT) and ammonium sulfate (AS). Using microscopic imaging techniques and viscosity model predictions, we examined the impact of mass transfer limitations on LLPS. As WSOC fractions increased, both the phase separation relative humidity (SRH) and the efflorescence relative humidity (ERH) progressively decreased. For the HXT/AS/SUC and HXT/AS/CA mixed systems with molar ratios of 1:1:0.5 and 1:1:0.75, LLPS was completely suppressed, although efflorescence still occurred. In the 1:1:1 mixtures, neither LLPS nor efflorescence was observed. In contrast, the addition of GLY caused minimal changes to phase transitions due to its minor effect on the aqueous-phase viscosity. Additionally, reducing bulk acidity, along with the transformation of CA into its salts, alleviated molecular transport limitations, leading to increased SRH and ERH values for the HXT/AS/CA mixtures. These findings are critical for advancing high-resolution phase state modeling of multicomponent aerosols and assessing the atmospheric implications of particle morphologies in the presence or absence of LLPS.

Investigation of oxidative potential of fresh and O3-aging PM2.5 from various emission sources across urban and rural regions

Inhalation of atmospheric PM2.5 can induce the generation of excessive reactive oxygen species (ROS) in human alveoli, triggering local and systemic inflammation, which can directly or indirectly result in respiratory and cardiovascular diseases. In this study, we assessed the oxidative potential (OP) of fresh and O3-aged PM2.5 particles from various urban and rural emission sources using the dithiothreitol (DTT) method. Our results revealed variations in the OP of fresh PM2.5 among different emission sources, with biomass burning sources exhibiting the highest OP, followed by industrial areas, vehicular emissions, cooking emissions, and suburban areas, respectively. Water-soluble organics and transition metals might potentially exert significant influence on particle OP. O3 aging notably decreased the OP of PM2.5 particles, possibly due to the oxidation of highly DTT-active components into low redox-active small molecules. Moreover, the evolution of OP in different PM2.5 components, including methanol-soluble and insoluble fractions, exhibited distinct responses to O3 aging for source-oriented PM2.5. Additionally, differences in chemical composition between fresh and aged PM2.5 were further elucidated through measurements of component-dependent hygroscopic behaviors and phase transitions. This study systematically delineates variances in the toxic potential of fresh and O3-aged PM2.5 from various anthropogenic sources. The findings highlight the intrinsic compositional dependence of particle OP and provide essential insights for assessing the health effects of source-oriented PM2.5, as well as for formulating human health protection policies.

Evaporation Kinetics and Final Particle Morphology of Multicomponent Salt Solution Droplets

In both nature and industry, aerosol droplets contain complex mixtures of solutes, which in many cases include multiple inorganic components. Understanding the drying kinetics of these droplets and the impact on resultant particle morphology is essential for a variety of applications including improving inhalable drugs, mitigating disease transmission, and developing more accurate climate models. However, the previous literature has only focused on the relationship between drying kinetics and particle morphology for aerosol droplets containing a single nonvolatile component. Here we investigate the drying kinetics of NaCl-(NH4)2SO4, NaCl-NH4NO3, and NaCl-CaCl2 mixed salt aqueous aerosol droplets (25-35 μm radius) and the resulting morphology and composition of the dried microparticles. A comparative kinetics electrodynamic balance was used to measure evaporation profiles for each mixed salt aerosol at a range of relative humidities (RH) (0-50% RH); measurements of the evaporation kinetics are shown to be consistent with predictions from the "Single Aerosol Drying Kinetics and Trajectories" model. Populations of the mixed salt droplets were dried in a falling droplet column under different RH conditions and imaged using scanning electron microscopy to observe the impact of the drying kinetics on the morphology. Energy dispersive spectroscopy was used in tandem to obtain atomic maps and view the impact of drying kinetics on the composition of the resultant particles. It has been shown that the relationship between drying kinetics and dry particle morphology in mixed salt solution droplets is compositionally dependent and determined by the predominant salts that crystallize (i.e., (NH4)2SO4, Na2SO4, or NaCl). The degree of homogeneity in composition throughout the particle microstructure is dependent on the drying rate.

Spectral Characteristics of Unsaturated and Supersaturated Inorganic Aerosols: Insights into Deliquescence Kinetics

The deliquescence phase transition of atmospheric aerosols is crucial for radiative forcing and atmospheric chemistry. However, the deliquescence kinetics of micrometer-sized aerosols, especially the formation and evolution of surface solution films, remain poorly understood. In this study, IR spectral characteristics were employed for the first time to quantify the solute concentration evolution in surface solution films. At a constant relative humidity (RH) of ∼65%, solution films on NaCl crystals exhibited a very low solute concentration (3.06 ± 0.18 mol/L), comparable to aqueous NaCl droplets above 90% RH. These films reached saturation at ∼74% RH, i.e., the deliquescence RH of NaCl, maintaining a nearly constant saturation level during deliquescence. In contrast, amorphous NaNO3 solids showed supersaturated solution films before deliquescence. Following deliquescence, the saturation level of solution phases increased due to faster solid dissolution rates than liquid water condensation. These findings address knowledge gaps in the complex nonequilibrium dissolution processes of crystalline or amorphous atmospheric aerosols.

A review of efflorescence kinetics studies on atmospherically relevant particles

The efflorescence transitions of aerosol particles have been intensively investigated due to their critical impacts on global climate and atmospheric chemistry. In the present study, we present a critical review of efflorescence kinetics focusing on three key issues: the efflorescence relative humidity (ERH) and the influence factors for aerosol ERH (e.g. particle sizes, and temperature); efflorescence processes of mixed aerosols, concerning the effect of coexisting inorganic and organic components on the efflorescence of inorganic salts; homogeneous and heterogeneous nucleation rates of pure and mixed aerosols. Among the previous studies, there are significant discrepancies for measured aerosol ERH under even the same conditions. Moreover, the interactions between organic and inorganic components remain largely unclear, causing efflorescence transition behaviours and chemical composition evolutions of certain mixed systems to be debatable. Thus, it is important to better understand efflorescence to gain insights into the physicochemical properties and characterize observed efflorescence characteristics of atmospheric particles, as well as guide further studies on aerosol hygroscopicity and reactivity.

Micro-droplet Trapping and Manipulation: Understanding Aerosol Better for a Healthier Environment

Understanding the physicochemical properties and heterogeneous processes of aerosols is key not only to elucidate the impacts of aerosols on the atmosphere and humans but also to exploit their further applications, especially for a healthier environment. Experiments that allow for spatially control of single aerosol particles and investigations on the fundamental properties and heterogeneous chemistry at the single-particle level have flourished during the last few decades, and significant breakthroughs in recent years promise better control and novel applications aimed at resolving key issues in aerosol science. Here we propose graphene oxide (GO) aerosols as prototype aerosols containing polycyclic aromatic hydrocarbons, and GO can behave as two-dimensional surfactants which could modify the interfacial properties of aerosols. We describe the techniques of trapping single particles and furthermore the current status of the optical spectroscopy and chemistry of GO. The current applications of these single-particle trapping techniques are summarized and interesting future applications of GO aerosols are discussed.

Nitrate Photolysis in Mixed Sucrose-Nitrate-Sulfate Particles at Different Relative Humidities

Atmospheric particles can be viscous. The limitation in diffusion impedes the mass transfer of oxidants from the gas phase to the particle phase and hinders multiphase oxidation processes. On the other hand, nitrate photolysis has been found to be effective in producing oxidants such as OH radicals within the particles. Whether nitrate photolysis can effectively proceed in viscous particles and how it may affect the physicochemical properties of the particle have not been much explored. In this study, we investigated particulate nitrate photolysis in mixed sucrose-nitrate-sulfate particles as surrogates of atmospheric viscous particles containing organic and inorganic components as a function of relative humidity (RH) and the molar fraction of sucrose to the total solute (F_SU) with an in situ micro-Raman system. Sucrose suppressed nitrate crystallization, and high photolysis rate constants (∼10^-5 s^-1) were found, irrespective of the RH. For F_SU = 0.5 and 0.33 particles under irradiation at 30% RH, we observed morphological changes from droplets to the formation of inclusions and then likely "hollow" semisolid particles, which did not show Raman signal at central locations. Together with the phase states of inorganics indicated by the full width at half-maxima (FWHM), images with bulged surfaces, and size increase of the particles in optical microscopic imaging, we inferred that the hindered diffusion of gaseous products (i.e., NO_x, NO_y) from nitrate photolysis is a likely reason for the morphological changes. Atmospheric implications of these results are also presented.

Effect of waiting time on the water transport kinetics of magnesium sulfate aerosol at gel-forming relative humidity using optical tweezers

With the loss of water, the amorphous gel states in aqueous magnesium sulfate (MgSO₄) aerosol forms and results in nonequilibrium dynamics, owing to the extended time scales for diffusive mixing. The mass transfer resistance in MgSO₄ aerosol droplets during evaporation or condensation is investigated using aerosol optical tweezers (AOTs) coupled with Raman spectroscopy. In addition, the kinetics of water transport during hydration and dehydration after different waiting time is studied. With the cyclic change of the relative humidity (RH) below gel-forming, the waiting time is varied to examine the effect of the duration of drying and humidifying on water transport kinetics during subsequent hydration and dehydration process. Apparent diffusion coefficients (D_ap) of water molecules in the gel state after different waiting time are obtained. The results indicate that the duration of drying will affect water transport kinetics for subsequent humidifying process due to the different structure and composition in MgSO₄ aerosol droplet at different ambient humidities. However, the duration of humidifying has little effect on water transport kinetics for subsequent drying process below gel-forming RH.

Evaporation of mixed citric acid/(NH4)2SO4/H2O particles: Volatility of organic aerosol by using optical tweezers

The condensation and evaporation processes of semi-volatile organic compounds (SVOCs) in atmospheric aerosols can induce significant evolutions of their chemical and physical properties. Hence, for interpreting and predicting composition changes of atmospheric aerosols, it is indispensable to provide insight into the partitioning behaviors of SVOCs between condensed and gas phases. In this research, optical tweezers coupled with cavity-enhanced Raman spectroscopy were employed to observe the volatility of internally mixed citric acid (CA)/(NH₄)₂SO₄ (AS) particles, and the effect of AS on the gas/particle partitioning behaviors of atmospheric organic acids was investigated. The radii and refractive indexes of the levitated droplets were determined in real time from the wavelength positions of simulated Raman spectra and the effective vapor pressures of CA at different relative humidities (RHs) were obtained according to Maxwell equation. For the CA/AS particle with organic to inorganic mole ratio (OIR) of 1:1, the effective vapor pressure of CA decreased with the decreasing of RH. When the RH decreased from 67% to 8.2%, the effective vapor pressure of CA decreased from (1.35±0.508)×10^-4Pa to (3.0±1.0)×10^-6Pa. Meanwhile, the CA/AS particles with OIR of 3:1, 1:3 were also studied, and the results show the same phenomenon compared to the particles with OIR of 1:1. When under constant RHs, the effective vapor pressures of CA decreased with the increasing of AS contents, suggesting that the presence of AS suppressed the partitioning of CA to aqueous particles. In addition, the mass transfer processes of water in CA and CA/AS/H₂O systems were further studied. The characteristic time ratio between the droplet radius and RH was used to describe the water mass transfer difference dependent on RH. Compared to the characteristic time ratio of pure CA, the characteristic time ratio of CA/AS particles apparently increased. For CA/AS particles under the same RH steps, the characteristic time ratio increased with the AS content increase. According to the differential isotherm, the diffusion coefficients of citric acid and citric acid/ammonium sulfate at low RHs (RH ≈7%-1%, RH≈1%-7%) were calculated respectively. Generally, the key aspect of the current work was to deeply explore the relationship between the evaporation rates of SVOCs and water transport process.