Molecular-resolution simulations of new particle formation: Evaluation of common assumptions made in describing nucleation in aerosol dynamics models

Aerosol dynamics models that describe the evolution of a particle distribution incorporate nucleation as a particle formation rate at a small size around a few nanometers in diameter. This rate is commonly obtained from molecular models that cover the distribution below the given formation size – although in reality the distribution of nanometer-sized particles cannot be unambiguously divided into separate sections of particle formation and growth. When incorporating nucleation, the distribution below the formation size is omitted, and the formation rate is assumed to be in a steady state. In addition, to reduce the modeled size range, the formation rate is often scaled to a larger size based on estimated growth and scavenging rates and the assumption that also the larger size is in a steady state. This work evaluates these assumptions by simulating sub-10 nm particle distributions in typical atmospheric conditions with an explicit molecular-resolution model. Particle formation is included either (1) dynamically, that is, the whole size range starting from single vapor molecules is modeled explicitly or (2) implicitly by using an input formation rate as is done in aerosol models. The results suggest that while each assumption can affect the outcome of new particle formation modeling, the most significant source of uncertainty affecting the formation rates and resulting nanoparticle concentrations is the steady-state assumption, which may lead to an overprediction of the concentrations by factors of approximately from two to even orders of magnitude. This can have implications for modeling and predicting atmospheric particle formation.

Copyright © 2017 American Association for Aerosol Research     

Using a fully-Lagrangian meshless method for the study of aerosol dispersion and deposition

Smoothed Particle Hydrodynamics was employed in the present two-dimensional simulations, thus the algorithms implemented for both continuous and dispersed phase share a common Lagrangian stencil. The results were benchmarked against those produced in earlier investigations of particle deposition resulting from the flow around a stationary square obstacle. The proposed numerical procedure also facilitates the investigation of the fundamental physics governing the transport of resuspended particles in the wake of a moving operator, e.g., inside cleanroom facilities. Aiming to illustrate this capability, the dispersion of neutrally suspended particles behind an impulsively-started plate has been numerically simulated. Two distinct particle dispersion patterns, characterized by different Reynolds numbers, have been obtained for this problem. Altogether the results have demonstrated the accuracy of the method and associated advantages for multiphase flow studies, especially in cases involving moving boundaries due to its mesh-free nature.

Copyright © 2016 American Association for Aerosol Research     

Effects of relative humidity and particle and surface properties on particle resuspension rates

Wind tunnel experiments examined the coupled effects of relative humidity (RH) and surface and particle properties on aerodynamically induced resuspension. Hydrophilic glass spheres and hydrophobic polyethylene spheres ～20 μm in diameter, with nanoscale surface features, were resuspended from hydrophilic glass, hydrophobic chemical agent resistant coating (CARC), and gold surfaces. Roughness of the glass and gold surfaces was on the nanoscale, whereas CARC surfaces had microscale roughness. Different particle–surface combinations yielded van der Waals interactions that varied by a factor of 4, but these differences had a relatively minor effect on resuspension. Wind tunnel RH was varied between 7% and 78%. Overall, RH affected the resuspension of hydrophilic particles on hydrophilic surfaces most strongly and that of hydrophobic particles on hydrophobic surfaces the least. For each particle–surface combination there was a threshold RH value below which resuspension rates were essentially constant and in good agreement with a dimensionless model of particle resuspension.

Copyright © 2016 American Association for Aerosol Research     

Numerical analysis of the dynamics of aerosol inertial collection and aggregation on raindrops

A three-dimensional stochastic model is developed for predicting atmospheric aerosol collection and aggregation on the surface of a falling raindrop at its terminal velocity. Potential flow and viscous flow are assumed as the flow fields in the vicinity of the large and the small raindrops, respectively. The results show that hydrophobic coarse mode aerosols collected by either small raindrops (d_c < 100 μm) or large drops (d_c > 100 μm) form aggregations on the surfaces of drops, and accumulation mode aerosols tend to be captured by the aggregations or hydrophobic coarse particles which have been collected by the drops, and this may significantly enhance the capability of the raindrop for fine aerosol collection. When the aggregation effect is considered in the calculation, fine aerosol efficiency can be promoted by one to two orders of magnitude. Therefore, fine particle collision efficiency by raindrops is underestimated by employing the classical dynamic theory which neglects the particle aggregation effect. However, the collection efficiency of coarse particles remains almost constant with the increase in the amount of particles collected by large drops, while there is only a slight increase in efficiency by small raindrops upon increasing in particle concentration. This implies that the traditional limiting trajectory method can still be used for the calculation of coarse particle collection efficiencies by either small or large raindrops.

Copyright © 2018 American Association for Aerosol Research     

Modeling for particle size prediction and mechanism of silicon nitride nanoparticle synthesis by chemical vapor deposition

Particle size is a vital characterization for silicon nitride nanoparticle as its scale determines its application area. Particle size prediction for synthesis of silicon nitride nanoparticle by chemical vapor deposition (CVD) is much needed. In this study, a model is proposed for particle growth during silicon nitride nanoparticle synthesis by CVD in order to predict particle size. Comparison between modeling and experimental results validated the model. The modeling results showed that lower pressure in the condensation room would be an effective way of obtaining silicon nitride nanoparticles with smaller particle size. An expression is established to reveal the relation between the mean particle diameter of silicon nitride nanoparticle and pressure in the condensation room based on the modeling. The modeling method is capable of predicting the mean particle size of ultrafine silicon nitride powder to within 3.6% accuracy. Corresponding manufacturing thermal parameters are recommended for silicon nitride nanoparticle production with different mean particle sizes. Modeling and analysis in this article may provide theoretical guidance for production of silicon nitride nanoparticle by CVD.

© 2017 American Association for Aerosol Research     

Emissions and radiative impacts of sub-10 nm particles from biofuel and fossil fuel cookstoves

Abstract

Combustion sources have been shown to directly emit particles smaller than 10?nm. The emission of 1-3?nm particles from biofuel or fossil fuel cookstoves has not been studied previously, nor have the radiative impacts of these emissions been investigated. In this work, emissions (number of particles) were measured during a water boiling test performed on five different cookstoves (three-stone fire, rocket elbow, gasifier, charcoal, and liquified petroleum gas [LPG]) for particle diameters between ～1 and ～1000?nm. We found significant emissions of particles smaller than 10?nm for all cookstoves (>5?×?10¹⁵ # kg-fuel^?1). Furthermore, cleaner (e.g., LPG) cookstoves emitted a larger fraction of sub-10?nm particles (relative to the total particle counts) than traditional cookstoves (e.g., three-stone fire). Simulations performed with the global chemical transport model GEOS-Chem-TOMAS that were informed by emissions data from this work suggested that sub-10?nm particles were unlikely to significantly influence number concentrations of particles with diameters larger than 80?nm that can serve as cloud condensation nuclei (CCN) (<0.3%, globally averaged) or alter the cloud-albedo indirect effect (absolute value <0.005?W m^?2, globally averaged). The largest, but still relatively minor, localized changes in CCN-relevant concentrations (<10%) and the cloud-albedo indirect effect (absolute value <0.5?W m^?2) were found in large biofuel combustion source regions (e.g., Brazil, Tanzania, Southeast Asia) and in the Southern Ocean. Enhanced coagulation-related losses of these sub-10?nm particles at sub-grid scales will tend to further reduce their impact on particle number concentrations and the aerosol indirect effect, although they might still be of relevance for human health.

Copyright © 2020 American Association for Aerosol Research     

Assessment and validation of a hygroscopic growth model with different water activity estimation methods

Abstract

Hygroscopic growth models are currently of interest as aids for targeting the deposition of inhaled drug particles in preferred areas of the lung that will maximize their pharmaceutical effect. Mathematical models derived to estimate hygroscopic growth over time have been previously developed but have not been thoroughly validated. For this study, model validation involved a comparison of modeled values to measured values when the growing droplet had reached equilibrium. A second validation process utilized a novel system to measure the growth of a droplet on a microscope coverslip relative to modeled values when the droplet is undergoing the initial rapid growth phase. Various methods currently used to estimate the water activity of the growing droplet, which influences the droplet growth rate, were also compared. Results indicated that a form of the hygroscopic growth model that utilizes coupled-differential equations to estimate droplet diameter and temperature over time was valid throughout droplet growth until it reached its equilibrium size. Accuracy was enhanced with the use of a polynomial expression to estimate water activity relative to the use of a simplified estimate of water activity based on Raoult’s Law. Model accuracy was also improved when constraining the film of salt solution surrounding the dissolving salt core at saturation.

Copyright © 2020 American Association for Aerosol Research     

Evaporation from a nonspherical aerosol particle situated in an absorbing gas

The problem of an aerosol particle evaporating in an infinite expanse of an absorbing gas is considered. The relevant Helmholtz equation (resulting from the steady-state diffusion equation with an absorption term included) with density jump boundary conditions is converted into a boundary integral equation via the use of the Green’s function. The resulting integral equation is valid for particles of arbitrary shape. Explicit numerical results for the local and average evaporation rates are reported for several axisymmetric particles for a range of values of the dimensionless absorption parameter (λ²), where λ is the ratio of the radius of the particle (a) to the diffusion length (l). Here, the diffusion length is defined as l=[D/(vΣ_a)]^1/2, in which v (cm s^-1) is the average thermal speed of the vapor molecules, Σ_a (cm^-1) is the cross-section for absorption of the vapor by the gas, and D (cm² s^-1) is the diffusion coefficient of the vapor in the gas. Our numerical results for the local and average evaporation rates for a sphere exhibit excellent agreement with the corresponding analytical values (maximum deviation <0.40%). We find that the evaporation rate increases with increasing absorption and that this increase depends on the degree of departure of the particle from a spherical shape. The jump distance has a large impact in that it significantly lowers the evaporation rates as it increases in magnitude. It should be remarked that the results of this paper are also directly applicable to the problem of either neutrons or photons undergoing diffusion from a source situated in an absorbing medium.     

Evolution of atmospheric aerosol particle size distributions via Brownian coagulation numerical simulation

Current atmospheric observations tend to support the view that continental tropospheric aerosols, particularly urban aerosols, show multimodal mass distributions. One of the obvious mechanisms causing the multimodality is the mixing of different primary sources. Other modes involve dissimilar aerosol formation processes in the atmosphere. Fine aerosol particles are generated from secondary processes such as nucleation, condensation and chemical reaction, whereas coarse particles usually consist of dust, fly ash and mechanically generated aerosols. With the use of a newly developed computer code GROWTH in our laboratory, we report here the simulated results of Brownian coagulation dynamics involving multimodal mass density functions for long periods of time. In our model calculations we assume that the aerosol particles are well mixed in an atmospheric volume so that spatial variation in the distribution is negligible. Our accurate numerical simulation of the Brownian coagulation dynamics indicates that once formed, an atmospheric multimodal aerosol distribution in the range 0.1 to 100 μm will maintain its identity for a very long period of time (at least hours) unless “atmospheric perturbations” such as meteorological instabilities, rain-washout and gravitational settling occur. It is our belief that understanding the complex domain of atmospheric aerosols requires systematic investigation of each process. This paper is a continuation of such an investigation.     

Modelling of heavy and buoyant particle dispersion in a two-dimensional turbulent mixing layer

Heavy and buoyant particle dispersion in the turbulent mixing layer was investigated numerically using a two-phase flow discrete vortex modelling. It was revealed from the modelling that inclusion of two-way momentum coupling is essential for properly modelling heavy particle dispersive transport in turbulent free shear flows. For heavy particles with small Stokes numbers, the dispersion is predominated by the large-scale vortex structures and they exert small influence on the carrier fluid flow. Heavy particles with large St directionally align along the braid region between the neighbouring vortices. However, the lateral dispersion of particles of large St is smaller than that of particles of small St.For buoyant particles with the density being slightly greater than that of the carrier fluid, numerical simulation revealed that the buoyant particles scatter over the whole vortex core rather than collect along the fringes of the vortex. The Lagrangian statistics calculation of buoyant particle dispersion showed that both the inertial and crossing-trajectory effects affect the particle dispersion behaviour and particle eddy diffusivity. The dispersion behaviour of buoyant particles is highly associated with the particle Stokes number. Large St buoyant particles exhibit a larger dispersion. It was also indicated from the numerical simulation that buoyant particles might disperse larger than the fluid tracers. The correlation between the buoyant particle and fluid tracer velocities was affected by including the coupling effect.     

Analysis of particle cloud height dynamics in a stirred tank

Local and temporal variations of the particle cloud formed in a cylindrical mixing vessel were investigated experimentally. Different particle sizes (0.5, 1, and 2 mm) and volumetric concentration up to 20 vol % were evaluated at different impeller speeds. The time‐averaged cloud height was linear with impeller frequency and with volume concentration. Suspensions with larger particles had a lower average cloud height, while the standard deviation for the temporal cloud height variation was larger. Two strong periodic phenomena were identified to be dominating the particle cloud height variations. The frequencies were linear with impeller speed, resulting in dimensionless frequencies of S₁=0.02–0.03 and S₂=0.05–0.06. The frequencies were affected by neither the particle size nor the volumetric concentration. The amplitude showed no dependency on the particle size, but the S₂ amplitude significantly decreases and S₁ increases with increasing solid concentration. The results were compared to LES/discrete element method simulations and showed a fair agreement. © 2015 American Institute of Chemical Engineers AIChE J, 62: 338–348, 2016     

Investigating particle emissions and aerosol dynamics from a consumer fused deposition modeling 3D printer with a lognormal moment aerosol model

ABSTRACT

Particle emissions from consumer-fused deposition modeling 3D printers have been reported previously; however, the complex processes leading to observed aerosols have not been investigated. We measured particle concentrations and size distributions between 7 nm and 25 μm emitted from a 3D printer under different conditions in an emission test chamber. The experimental data was combined with a moment lognormal aerosol dynamic model to better understand particle formation and subsequent evolution mechanisms. The model was based on particles being formed from nucleation of unknown semivolatile compounds emitted from the heated filament during printing, which evolve due to condensation of emitted vapors and coagulation, all within a small volume near the printer extruder nozzle. The model captured observed steady state particle number size distribution parameters (total number, geometric mean diameter and geometric standard deviation) with errors nominally within 20%. Model solutions provided a range of vapor generation rates, saturation vapor pressures and vapor condensation factors consistent with measured steady state particle concentrations and size distributions. Vapor generation rate was a crucial factor that was linked to printer extruder temperature and largely accounted for differences between filament material and brands. For the unknown condensing vapor species, saturation vapor pressures were in the range of 10^?3 to 10^?1 Pa. The model suggests particles could be removed by design of collection surfaces near the extruder tip.

Copyright © 2018 American Association for Aerosol Research     

Inertial migration and multiple equilibrium positions of a neutrally buoyant spherical particle in Poiseuille flow

The radial migration of a single neutrally buoyant particle in Poiseuille flow is numerically investigated by direct numerical simulations. The simulation results show that the Segré and Silberberg equilibrium position moves towards the wall as the Reynolds number increases and as the particle size decreases. At high Reynolds numbers, inner equilibrium positions are found at positions closer to the centerline and move towards the centerline as the Reynolds number increases. At higher Reynolds numbers, the Segré and Silberberg equilibrium position disappears and only the inner equilibrium position exists. We prove that the inner annuluses in the measurements of Matas, Morris & Guazzelli (J. Fluid Mech. 515, 171–195, 2004) are not transient radial positions, but are real equilibrium positions. The results on the inner equilibrium positions and unstable equilibrium positions are new and convince us of the existence of multiple equilibrium radial positions for neutrally buoyant particles.