Drying and Crystallization of Evaporating Sodium Nitrate Aerosol Droplets

Spatiotemporal Plasma-Particle Characterization of Dry Aerosols Using Nanosecond, Femtosecond, and Filament Laser-Produced Plasmas

The ability to rapidly characterize dry aerosols in air using laser-induced breakdown spectroscopy (LIBS) with femtosecond laser pulses promises advancement towards real-time atmospheric sampling and standoff capabilities. Of particular interest is the ability to apply LIBS in the context of low-particle loaded environments where discrete particle interactions must be observed within the sampling volume of the laser-produced plasma (LPP). In this study, dry nanoparticles in suspension are generated from a standard solution and sampled in air using Q-switched nanosecond (ns-) pulses, short-focus (SF) femtosecond (fs-) pulses, and filaments. Short time-gated plasma images are captured to observe spatially and temporally varying discrete plasma-particle interactions, which is shown to influence early air breakdown behavior and subsequent plasma evolution. Along with images, photo-multiplier tube (PMT) measurements are conducted where strong spatiotemporal dependencies are exhibited by the collected emission signal on particle proximity and plasma expansion behavior. Finally, conditional analysis is performed on LIBS measurements to determine associated sampling probabilities and filter out spectra with poor or absent emission peaks with an adaptive threshold algorithm.

Size, Shape, and Phase of Nanoscale Uric Acid Particles

Uric acid particles are formed due to hyperuricemia, and previous works have focused on understanding the surface forces, crystallization, and growth of micron- and supermicron-sized particles. However, little to no work has furthered our understanding about uric acid nanonuclei that precipitate during the initial stages of kidney stone formation. In this work, we generate nanosized uric acid particles by evaporating saturated solution droplets of uric acid. Furthermore, we quantify the effects of drying rate on the morphology of uric acid nanonuclei. An aerosol droplet drying method generates uric acid nanoparticles in the size range of 20-200 nm from aqueous droplets (1-6 μm). Results show that uric acid nanonuclei are non-spherical with a shape factor value in the range of 1.1-1.4. The shape factor values change with drying rate and indicate that the nanoparticle morphology is greatly affected by drying kinetics. The nanonuclei are amorphous but can grow to form crystalline micron-sized particles. Indeed, a pre-crystallization phase was observed for heterogeneous nucleation of uric acid particles in the size range of a few hundred nanometers. Our findings show that the morphology of uric acid nanonuclei is significantly different from that of crystalline supermicron-sized particles. These new findings imply that the dissolution characteristics, surface properties, elimination, and medical treatment of uric acid nanonuclei formed during the initial stages of nucleation must be reconsidered.

High time resolution measurements of droplet evaporation kinetics and particle crystallisation

A refined technique for observing the complete evaporation behaviour of free-falling droplets, from droplet generation to complete solvent evaporation, with ultra-high time resolution is introduced and benchmarked. High-resolution phase-delay stroboscopic imaging is employed to simultaneously resolve the evolving droplet morphology, geometric and aerodynamic diameters, throughout the evaporative lifetime with a user-controlled < μs timescale. This allows rapid, complex morphological changes, such as crystallisation events, to be clearly observed and the corresponding mechanisms to be inferred. The dried particles are sampled for offline SEM analysis and the observed morphologies compared to the inflight imaging. Density changes can be calculated directly from the deviation between the geometric and aerodynamic diameters. The full capabilities of the new technique are demonstrated by examination of the different evaporation behaviours and crystallisation mechanisms for aqueous sodium chloride droplets evaporating under different ambient relative humidity (RH) conditions. The crystallisation window, defined as the time taken from initial to complete crystallisation, is shown to be RH dependent, extending from 0.03 s at 20% RH and 0.13 s at 40% RH. The different crystallisation mechanisms observed during the experiments are also clearly reflected in the final structure of the dry particles, with multi-crystal structures produced at low RH compared to single-crystal structures at higher RH. It is anticipated that this technique will unlock measurements which explore the evaporation behaviour and crystallisation mechanisms for rapid, complex droplet drying events, and with increasingly non-ideal solutions, relevant to industrial applications.

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.

Laboratory Investigation of Renoxification from the Photolysis of Inorganic Particulate Nitrate

Nitrogen oxides (NO_x) play a key role in regulating the oxidizing capacity of the atmosphere through controlling the abundance of O₃, OH, and other important gas and particle species. Some recent studies have suggested that particulate nitrate, which is conventionally considered as the ultimate oxidation product of NO_x, can undergo "renoxification" via photolysis, recycling NO_x and HONO back to the gas phase. However, there are large discrepancies in estimates of the importance of this channel, with reported renoxification rate constants spanning three orders of magnitude. In addition, previous laboratory studies derived the rate constant using bulk particle samples collected on substrates instead of suspended particles. In this work, we study renoxification of suspended submicron particulate sodium and ammonium nitrate through controlled laboratory photolysis experiments using an environmental chamber. We find that, under atmospherically relevant wavelengths and relative humidities, particulate inorganic nitrate releases NO_x and HONO less than 10 times as rapidly as gaseous nitric acid, putting our measurements on the low end of recently reported renoxification rate constants. To the extent that our laboratory conditions are representative of the real atmosphere, renoxification from the photolysis of inorganic particulate nitrate appears to play a limited role in contributing to the NO_x and OH budgets in remote environments. These results are based on simplified model systems; future studies should investigate renoxification of more complex aerosol mixtures that represent a broader spectrum of aerosol properties to better constrain the photolysis of ambient aerosols.

Optical Interrogation of Single Levitated Droplets in a Linear Quadrupole Trap by Cavity Ring-Down Spectroscopy

Optical trapping is a well-established technique to manipulate and levitate micro- and nanoscale particles and droplets. However, optical traps for single aerosol studies are most often limited to trapping spherical nonabsorbing droplets, and a universal optical trap for the stable confinement of particles regardless of their absorption strength and morphology is not established. Instead, new opportunities arise from levitating droplets using electrodynamic traps. Here, using a combined electrodynamic linear quadrupole trap and a cavity ring-down spectrometer, we demonstrate that it is possible to trap single droplets and simultaneously measure their extinction cross sections and elastic scattering phase functions over extended periods of time. To test the novel setup, we evaluated the evaporation of 1,2,6-hexanetriol under low-humidity conditions, and the evolution of aqueous (NH₄)₂SO₄ and NaCl droplets experiencing changing environmental conditions. Our studies extended beyond spherical droplets and we measured particle extinction cross sections after the efflorescence (crystallization) of the inorganic salt particles. Comparison of measured cross sections for crystallized particles with light scattering model predictions (using Mie theory or the T-matrix/extended boundary-condition method (EBCM) implementations for random orientation, with either the spheroid or superellipsoid parameterizations) enables information on particle shape to be inferred. Specifically, we find that cross sections for dry (NH₄)₂SO₄ particles are accounted for by Mie theory and, thus, particle shape is represented well by a sphere. Conversely, the cross sections for dry NaCl particles are only reconciled with light scattering models pertaining to nonspherical shapes. These results will have implications for accurate remote sensing retrievals of dry salt optical properties and for parameterizations implemented in radiative forcing calculations with changing humidity. Moreover, our new platform for precise and accurate measurement of optical properties of micron-scale and sub-micron particles has potential applications in a range of areas of atmospheric science, such as precise light scattering measurements for ice crystals and mineral dust. It represents a promising step toward accurate characterizations of optical properties for nonspherical and light-absorbing aerosols.

On the particle formation of leucine in spray drying of inhalable microparticles

The particle formation of L-leucine, a dispersibility-enhancing amino acid used in the spray drying of inhalable pharmaceutical aerosols, was extensively studied using three experimental methods, and the results were interpreted with the aid of theory. A comparative-kinetics electrodynamic balance was used to study the shell formation behavior in single evaporating microdroplets containing leucine and trehalose. Different concentration thresholds of solidification and shell formation were determined for trehalose and leucine, which were then used in the particle formation model to predict the properties of spray-dried particles. Furthermore, a droplet chain instrument was used to study the particle morphologies and particle densities that were not accessible in the single particle experiments. Lab-scale spray drying was also used to produce powders typical for actual pharmaceutical applications. Raman spectroscopy confirmed that a glass former, such as trehalose, can inhibit the crystallization of leucine. The surface compositions of these spray-dried powders were analyzed via time-of-flight secondary ion mass spectrometry. The leucine surface coverage in a polydisperse powder was determined to be a function of the particle size or the initial droplet diameter of each respective particle. This observation confirms the important role of leucine crystallization kinetics in its shell-forming capabilities. A critical supersaturation ratio of 3.5 was also calculated for leucine, at which it is assumed to instantaneously nucleate out of solution. This ratio was used as the threshold for the initiation of crystallization. Crystallinity predictions for the leucine-trehalose particles based on this supersaturation ratio were in good agreement with the solid-state characterizations obtained by Raman spectroscopy. This study improves the fundamental understanding of the particle formation process of leucine-containing formulations, which can apply to other crystallizing systems and potentially facilitate the rational design of such formulations with reduced experimental effort.