Computational Models for Simulating Multicomponent Aerosol Evaporation in the Upper Respiratory Airways

An effective model for predicting multicomponent aerosol evaporation in the upper respiratory system that is capable of estimating the vaporization of individual components is needed for accurate dosimetry and toxicology analyses. In this study, the performance of evaporation models for multicomponent droplets over a range of volatilities is evaluated based on comparisons to available experimental results for conditions similar to aerosols in the upper respiratory tract. Models considered include a semiempirical correlation approach as well as resolved-volume computational simulations of single and multicomponent aerosol evaporations to test the effects of variable gas-phase properties, surface blowing velocity, and internal droplet temperature gradients. Of the parameters assessed, concentration-dependent gas-phase specific heat had the largest effect on evaporation and should be taken into consideration for respiratory aerosols that contain high volatility species, such as n-heptane, at significant concentrations. For heavier droplet components or conditions below body temperatures, semiempirical estimates were shown to be appropriate for respiratory aerosol conditions. In order to reduce the number of equations and properties required for complex mixtures, a resolved-volume evaporation model was used to identify a twelve-component surrogate representation of potentially toxic JP-8 fuel based on comparisons to experimentally reported droplet evaporation data. Due to the relatively slow evaporation rate of JP-8 aerosols, results indicate that a semiempirical evaporation model in conjunction with the identified surrogate mixture provide a computationally efficient method for computing droplet evaporation that can track individual toxic markers. However, semiempirical methodologies are in need of further development to effectively compute the evaporation of other higher volatility aerosols for which variable gas-phase specific heat does play a significant role.     

MADM-A New Multicomponent Aerosol Dynamics Model

A Multicomponent Aerosol Dynamics Model (MADM) capable of solving the condensation/evaporation equation of atmospheric aerosols is presented. Condensable species may be organic and/or inorganic. For the inorganic constituents the equilibrium model ISORROPIA is used to predict the physical state of the particle, i.e., whether the aerosol is liquid or solid. The mass transfer equations for the fluxes for solid atmospheric particles are developed. MADM is able to simulate aerosol deliquescence, crystallization, solid to solid phase transitions, and acidity transitions. Aerosols of different sizes can be in different physical states (solid, liquid, or partially solid and partially liquid). Novel constraints on the electroneutrality of the species flux between the gas and aerosol phases are presented for both liquid and solid aerosols. These constraints aid in the stability of the algorithm, yet still allow changes in aerosol acidity. As an example, MADM is used to predict the dynamic response of marine aerosol entering an urban area.     

Experimental Study on the Effect of Fiber Orientation on Filter Quality

Most filtration models assume that the air stream runs perpendicularly to the orientation of the filter fibers. However, cigarette filters remove aerosol particles apparently by a different filter configuration in that the fiber orientation almost parallels the air streamlines. To focus on the effect of fiber orientation, cellulose acetate filters were used in this work to facilitate the filter performance comparison. A piece of original round cigarette filter was molded to form a cube. The same piece of filter was used for both perpendicular and parallel orientations, to avoid the variability caused by the non-uniform filter media distribution. DOP aerosol particles used in the tests were generated by either a constant output aerosol nebulizer or an ultrasonic atomizing nozzle. A Po-210 radiation source was used to neutralize the challenge aerosols to the Boltzmann charge equilibrium. A scanning mobility particle sizer (for < 0.8 μm) and an aerodynamic particle sizer (for > 0.8 μm) were used to measure aerosol number concentrations and size distributions upstream and downstream of the cigarette filters. The results showed that parallel and orthogonal filters behave similarly. However, the pressure drop across parallel filter was lower than for the perpendicular filter, indicating that the airflow is more laminar passing through the parallel filters. Possibly for the same reason, aerosol penetration through parallel filter was higher than the orthogonal filter, although the difference may not be statistically significant. When a comparison of the fiber orientation is based on filter quality, orthogonal filter performs better than parallel filter, if face velocity is lower than 60 cm/s. Parallel filter performs better only when the particles are smaller than the most penetrating size and under high face velocity.     

Particle Bounce During Filtration of Particles on Wet and Dry Filters

This paper experimentally examines the bounce and immediate re-entrainment of liquid and solid monodisperse aerosols under a stable filtration regime (precake formation) by wet and dry fibrous filters. PSL and DEHS were the solid and liquid aerosols, respectively, used in four monodisperse sizes of 0.52, 0.83, 1.50, and 3.00 w m. Three different fibrous filters were used to filter the aerosol streams, and the efficiency of the filtration process for each aerosol type under dry and wet regimes was measured. It was found that the solid particles generally exhibited a lower fractional filtration efficiency than liquid particles, although this difference decreased in the smaller size fractions. The difference between solid and liquid efficiencies was found to be greatest in the 1.5 w m size range. As particle sizes of liquid/solid aerosols and filtration parameters were similar, this difference is most likely to be due to the effect of particle bounce and or immediate re-entrainment occurring inside the filter, with the greater efficiency of filtration of the liquid particles being due to their greater capacity to plastically/elastically deform in order to absorb the impact forces. However, for the wet filtration regime (each fibre of the filter was coated by a film of water), no significant difference in filtration efficiency was detectable between solid and liquid aerosols. Therefore, the conclusion can be drawn that the either the bounce effect of the particles is inhibited by the liquid film, or the filtration conditions in the wet filter are so different that the aerosol properties are less significant with respect to capture.     

Experimental Study on Electrostatically Enhanced Granular Filters

Filtration of charged aerosols by granular bed filters enhanced by an externally applied electrostatic field was studied experimentally. The filtration efficiencies of latex aerosols by sand beds were measured for various aerosol and bed granule diameters. The results were compared with theoretical solutions. It was demonstrated that high filtration efficiencies of charged aerosols may be achieved when moderate electrostatic fields are applied to a filter.     

Comparison of Predicted and Experimentally Measured Aerosol Deposition Efficiency in BALB/C Mice in a New Nose-Only Exposure System

The use of respiratory tract dosimetry predictions to estimate the desired aerosol exposure concentration required for a specific target deposited dose in animal inhalation studies has been hindered by: (1) a lack of species/strain specific quantitative respiratory tract anatomy; and (2) verification by comparison of calculated and experimentally measured deposited doses. Using recent literature on tracheobronchial and pulmonary respiratory tract anatomy, dosimetry predictions for the Balb/c mouse were compared with deposited doses for 0.5, 1.0, and 2.0 micrometer diameter aerosols in a newly available nose-only exposure system. Spatial and temporal aerosol port to port uniformity of this nose-only exposure system was within ± 10% of the mean. Dosimetry predictions were in agreement with the measured mean deposited doses for the aerosols tested.     

Experimental and computational studies of liquid aerosol evaporation

The phenomenon of liquid aerosol evaporation was investigated both experimentally and computationally in this project. A vibrating orifice aerosol generator (VOAG) was utilised to generate a monodispersed stream of liquid droplets of a desired initial size. Three types of liquids, ethanol, hexane and water, of distinct volatilities were used. The droplets were allowed to travel through air at ambient temperature and pressure. The change in diameter of the droplets as evaporation occurs was measured dynamically using a variety of experimental techniques. Global sizing velocimetry (GSV) was applied to visualize the motion and change in diameter of droplets in real time so as to select an optimal set of operating conditions under which coalescence of droplets did not occur. A phase Doppler interferometer (PDI) was used to measure the Sauter mean diameter and axial velocity of droplets at various distances from the nozzle of the VOAG. The size and velocity profiles obtained were compared with calculations carried out using computational fluid dynamics (CFD) coupled with two different theoretical liquid evaporation models. It was observed that both liquid evaporation models produced similar droplets size profiles for ethanol and water, but a significant difference in the predicted sizes was observed for hexane. In general, a good qualitative agreement between the experimental and computational results was obtained. This represents the first report of a study involving direct comparisons of dynamic measurements of the size evolution of evaporating liquid aerosols with numerical predictions obtained from CFD calculations.     

Effect of Selected Binary and Mixed Solutions on Steam Condensation and Aerosol Behavior in Containment

In the course of severe light water reactor (LWR) core melt accidents, the formation and presence of water soluble compounds will affect the behavior of fission products in the primary system and in the containment. A liquid aerosol mixed with an insoluble component has an affinity to stick on surfaces. A relocation of the deposited aerosol may occur depending on the mole fraction of the solid component and the viscosity of the liquid component in the deposited material. In the very humid conditions expected in the containment, steam will condense on the hygroscopic particles, thereby increasing the size of the particles and settling rate. As a first step in modeling the effects of hygroscopicity, the water activity of a CsOH solution was implemented in the condensation model. The model predicts a significant contribution of CsOH hygroscopicity on the suspended mass concentration, which is in accordance with the observations from the latest large-scale containment aerosol experiments. Results of this simplified CsOH hygroscopicity model were compared with aerosol particles consisting of mixed solutions (e.g., CsOH—Cs₂CO₃—CsI—H₂O) expected in particles released during severe accidents. Water activities of binary and mixed solutions were first calculated using semiempirical methods and these results were compared with the available experimental data. Secondly, different heat and mass transfer models were compared to find a suitable method for the growth rate calculations of hygroscopic aerosol particles. We can conclude that sedimentation of hygroscopic aerosols is an effective removal mechanism for airborne fission products at a high relative humidity in the LWR containment during severe core melt accidents.     

Experimental Study of Dispersion and Deposition of Expiratory Aerosols in Aircraft Cabins and Impact on Infectious Disease Transmission

The dispersion and deposition characteristics of polydispersed expiratory aerosols were investigated in an aircraft cabin mockup to study the transmission of infectious diseases. The airflow was characterized by particle image velocimetry (PIV) measurements. Aerosol dispersion was measured by the Interferometric Mie Imaging (IMI) method combined with an aerosol spectrometer. Deposition was investigated using the fluorescent dye technique. Downward air currents were observed near the seats next to the side walls while upward airflows were observed near other seats. The downward airflow showed some effects on suppressing the dispersion of aerosols expelled by the passenger sitting in the window seat. Results show that the cough jet could bring significant amount of aerosols forward to the row of seats ahead of the cougher and the aerosols were then dispersed by the bulk air movements in the lateral direction. The aerosols expelled from a cough took 20–30 s to reach the breathing zones of the passengers seated within two rows from the cougher. Increasing the ventilation rate improved the dilution and reduced the aerosol exposure to passengers seated close to the source, but the aerosol dispersion increased, which heightened the exposure to passengers seated further away. 60–70% of expiratory aerosols in mass were deposited, with significant portions on surfaces close to the source, suggesting that disease transmission risk via indirect contact in addition to airborne risk is possible. The physical transport processes of expiratory aerosols could be used to shed insights on some epidemiological observations on in-flight transmission of certain infectious diseases.     

Authentication of Particulate Air Samples by Aerosol

A method was developed for the protection of aerosol filters against tampering that can be applied to any particulate sampler. In this method, tagged particles are dispersed into the sampled aerosol volume by liquid atomization to guarantee homogeneous distribution of the tag and deposition into the filter matrix. The tagged aerosols are collected insitu, together with the sampled dust particles. The tag can be measured in different ways. This feature can be used for self protection of the method. The authentication of the individual filters is preserved through the lifetime of the filter sample. Long-term field tests of the equipment indicate reliable performance.     

Energy-dispersive X-ray fluorescence spectrometer with capillary optics for the chemical analysis of atmospheric aerosols with high time resolution

A new method is presented for the chemical analysis with high time resolution of atmospheric aerosols deposited on a filter surface. The aerosols are collected with a special streaker sampler. This device draws the air by a 8 × 1 mm rectangular orifice through a Nuclepore polycarbonate membrane filter. The filter is mounted on a circular frame, which rotates at 1 revolution per week. The particulate matter is deposited as a 8 mm wide ring. The deposit on the filter is analysed in 1 mm steps, each corresponding to 1 h of exposure.

Analysis of the filters is performed using an energy-dispersive X-ray fluorescence (EDXRF) spectrometer comprising an X-ray tube with Mo or Cr anode, a glass capillary with inner diameter of 1 mm, a sample holder with a rotational stage and a Si (Li) semiconductor detection system.

The detection limits in capillary optics EDXRF of streaker samples are compared with those obtainable in particle-induced X-ray emission (PIXE) and with atmospheric particulate levels observed in urban areas. The potential of the method of aerosol studies in urban and polluted environments is illustrated by the analysis of samples collected at two sites in the city of Cracow.     

Aerosol deposition in 3D models of the upper airways and trachea of rhesus macaques

Abstract

Little is known about aerosol deposition in macaques, variability in deposition between animals, or how deposition in macaques and humans compare. This is despite the use of macaques in assessments of toxic aerosols that are often translated to estimates of human exposure. We used three dimensional (3D) physical models of the upper airways and trachea (UAT) of Rhesus macaques to begin to fill in this information gap. Models of the UAT of five, living rhesus macaques were produced from CT scans, using 3D printing technology. Models were exposed to a polydisperse aerosol containing 0.54 to 9.65 micron particles, during constant flowrates of 2, 4, and 6 liters per min. Percent deposition in UAT models was quantified using an Aerodynamic Particle Sizer and was compared to in vivo upper airway deposition in ten, adult human subjects. Deposition in the UAT models increased as Stokes number increased. Deposition also varied significantly between models, but intermodel variability was reduced when plotted as a function of Stokes number. Using Stokes number, deposition in four of the five UAT models overlapped with each other and also overlapped with human upper airway deposition. These models could be used to explore the relationship between factors that affect toxic aerosol deposition in the UAT in vitro and pathology following toxic aerosol exposure in Rhesus macaques in vivo. Results from those experiments could also be applicable to humans because of deposition similarities.

Copyright © 2020 American Association for Aerosol Research     

The effect of the contact between platinum and soot particles on the catalytic oxidation of soot deposits on a diesel particle filter

Experiments were performed with two model soot aerosols brought into different forms of contact with Pt aerosol particles, to investigate the effectiveness of this contact in lowering the catalytic soot oxidation temperature. The contact was either generated between individual particles in the aerosol state (Pt-doped soot to simulate a fuel borne catalyst), or by sequential or simultaneous deposition of separately generated soot and Pt aerosols onto a sintered metal filter. (Formation of a soot cake on previously deposited Pt aerosol would simulate a catalyst coated diesel particle filter.) The catalytic activity was determined in all cases from temperature ramped oxidation in air of the filtered particles, and defined as the 50% conversion temperature.

It was found that Pt-doped soot and simultaneously filtered aerosols were both equally effective in reducing the oxidation temperature by up to 140–250 °C for the spark discharge soot (with 3–47 wt% Pt concentration in the soot cake), and by up to 140 °C for the pyrolysis soot (3 wt% Pt). Conversely, the deposition of a thin soot layer of 5–10 μm thickness onto Pt, or vice versa, produced only a slight temperature reduction on the order of about 13–42 °C. These results suggest that the distance between soot and Pt particles plays a key role in promoting an effective oxidation on the filter, which is consistent with the role of Pt particles as local generators of activated oxygen.     

湿式氨法烟气脱硫中气溶胶的形成特性研究

气溶胶的形成是湿式氨法烟气脱硫过程存在的主要问题,通过测试分析氨法脱硫前后细颗粒的浓度与粒径分布、颗粒形态及其组成的变化特性,探讨了氨法脱硫中气溶胶的形成机理,并考察了影响气溶胶颗粒形成的主要因素。结果表明:氨水挥发逸出的气态NH3与烟气中SO2发生气相反应是气溶胶形成的主要原因,气溶胶含(NH4)2SO4、(NH4)2SO3、NH4HSO3等组分,粒径集中在0.07～0.70μm范围内,氨法脱硫系统对其难以有效脱除;氨水脱硫液温度及其浓度、烟气中SO2浓度、液气比等对气溶胶形成具有重要影响,形成量随氨水脱硫液及烟气中SO2浓度升高而增多,在保持NH3:SO2化学计量比不变的情况下,随液气比增大,气溶胶颗粒形成量减少。最后,对气溶胶颗粒的控制措施提出了建议。     

A Universal Calibration Curve for the TSI Aerodynamic Particle Sizer

Interest in mordenite as an inhalation hazard arose when it was discovered that the mineral exists in the subsurface of Yucca Mountain, NV, the site of a federally proposed nuclear waste repository. During preliminary geologic investigations at Yucca Mountain, workers performing air coring (dry-drilling) operations were potentially exposed to aerosols of mordenite. Mordenite is also increasingly used in industrial applications, such as cation exchange, molecular absorbency, and reversible dehydration. Concern that the fibrous nature of mordenite may present an inhalation hazard is supported by the ''Stanton Hypothesis," which states that the carcinogenicity of any fiber type depends upon dimension and durability rather than physicochemical properties. To date, little scientific literature is available on the inhalation health hazards of mordenite. This study initiates research in this area. Mordenite specimens collected from different geologic localities were analyzed macroscopically and microscopically. Mineral verification was performed using energy dispersive x-ray and x-ray diffraction analysis. Fibrous aerosols were generated to simulate aerosols created during air coring operations. Anderson cascade impactors were used to obtain aerosol mass median aerodynamic diameters. Electron microscopy of nucleopore filters allowed for individual aerosol fibers to be morphologically sized and applied to the Stanton Hypothesis for mesothelioma induction. Physical fiber dimensions were used to calculate aerodynamic diameters and to estimate pulmonary deposition. Results obtained from this study indicate that under similar conditions of aerosolization, using similar mordenite materials, inhalation of mordenite fibers could produce substantial deep-lung deposition.     

Aerosol filter characterization by electron microscopical methods

Scanning (SEM) as well as transmission electron microscopy (TEM) have been proved to be suitable methods for the characterization of different aerosol filters.

An equipment and a standardized procedure are described for a qualitative and quantitative evaluation of different characteristics for high efficiency aerosol filters. Solid aerosols, incl. radiolabeled aerosols, are used for this evaluation. The test aerosol is sampled in the up- and downstream on Nuclepore filters. By means of SEM- and TEM-methods aerosol concentrations, particle size distributions, particle forms and chemistry or radioactivity are measured on both Nuclepore filters. The total and the selective penetration and collection efficiencies are then estimated for particles of different sizes, forms, chemical composition and radioactivities. The SEM-procedure can also supply very useful information about deposition mechanisms of particles on the surface and inside the tested filter, about the clogging mechanism, about filter corrosion or destruction, etc.     

Development and Validation of a Simple Numerical Model for Estimating Workplace Aerosol Size Distribution Evolution Through Coagulation,Settling, and Diffusion

Recent research has indicated that the toxicity of inhaled ultrafine particles may be associated with the size of discrete particles deposited in the lungs. However, it has been speculated that in some occupational settings rapid coagulation will lead to relatively low exposures to discrete ultrafine particles. Investigation of likely occupational exposures to ultrafine particles following the generation of aerosols with complex size distributions is most appropriately addressed using validated numerical models. A numerical model has been developed to estimate the size-distribution time-evolution of compact and fractal-like aerosols within workplaces resulting from coagulation, diffusional deposition, and gravitational settling. Good agreement has been shown with an analytical solution to lognormal aerosol evolution, indicating good compatibility with previously published models. Validation using experimental data shows reasonable agreement when assuming spherical particles and coalescence on coagulation. Assuming the formation of fractal-like particles within a range of diameters led to good agreement between modeled and experimental data. The model appears well suited to estimating the relationship between the size distribution of emitted well-mixed ultrafine aerosols, and the aerosol that is ultimately inhaled where diffusion loses are small.     

Performance of Electrically Augmented Fibrous Filters,Measured with Monodisperse Aerosols

A study of the performance of filters in an external electric field has been carried out in controlled conditions using monodisperse aerosols with a charge distribution close to a Boltzmann. The effects of electric field strength, particle diameter, fiber diameter, and filtration velocity have been measured. The results are in basic agreement with existing theory for the capture of particles by polarization forces and indicate that this process is dominant, although other filtration processes are also significant, particularly when the electric field is low. Reasonable correlation between single fiber efficiency and the dimensionless parameter for capture by polarization forces is observed. The agreement is improved by the inclusion of the hydrodynamic factor into the calculation, but this does not remove all discrepancy. If filters are exposed to an electric field for a protracted period, a small persistent residual polarization of the fibers is observed. Loading the filter with aerosol in the laboratory has little effect on its performance, though eventually the aerosol captured in the vicinity of the live electrode causes shorting out.     

Filtration Efficiency of Non-Uniform Fibrous Filters

Although theoretical models of the filtration efficiency of fibrous filters are typically based on a single type of fiber in an ordered array, many actual fibrous filters comprises fibers that are inherently randomly distributed. It is desirable to be able to estimate the filtration efficiency of such non-uniform fibrous filters from their composition arrangement and the filtration property of individual fibers. Toward that end, we approximate the filter system as a series of cells comprising individual fibers with random distribution, deviating from the homogeneity assumption in the classical models. With better characterization of the filter structure based on the Voronoi diagram, we theoretically revisit filtration efficiency by the top-down (TD) approach and the bottom-up approach. The proposed models are compared with the existing experimental and numerical results under different fiber volume fractions, indicating the high accuracy of the TD model for the filtration of submicron aerosol particles. The influence of the degree of randomness of fiber distribution on filtration efficiency is also quantified.

Copyright 2015 American Association for Aerosol Research     

Electronic cigarette-generated aldehydes: The contribution of e-liquid components to their formation and the use of urinary aldehyde metabolites as biomarkers of exposure

Electronic cigarettes (e-cigarette) have emerged as a popular electronic nicotine delivery system (ENDS) in the last decade. Despite the absence of combustion products and toxins such as carbon monoxide (CO) and tobacco-specific nitrosamines (TSNA), carbonyls including short-chain, toxic aldehydes have been detected in e-cigarette-derived aerosols up to levels found in tobacco smoke. Given the health concerns regarding exposures to toxic aldehydes, understanding both aldehyde generation in e-cigarette and e-cigarette exposure is critical. Thus, we measured aldehydes generated in aerosols derived from propylene glycol (PG): vegetable glycerin (VG) mixtures and from commercial e-liquids with flavorants using a state-of-the-art carbonyl trap and mass spectrometry. To track e-cigarette exposure in mice, we measured urinary metabolites of 4 aldehydes using ULPC-MS/MS or GC-MS. Aldehyde levels, regardless of abundance (saturated: formaldehyde, acetaldehyde???unsaturated: acrolein, crotonaldehyde), were dependent on the PG:VG ratio and the presence of flavorants. The metabolites of 3 aldehydes – formate, acetate, and 3-hydroxypropyl mercapturic acid (3-HPMA; acrolein metabolite) – were increased in urine after e-cigarette aerosol and mainstream cigarette smoke (MCS) exposures, but the crotonaldehyde metabolite (3-hydroxy-1-methylpropylmercapturic acid, HPMMA) was increased only after MCS exposure. Interestingly, exposure to menthol-flavored e-cigarette aerosol increased the levels of urinary 3-HPMA and sum of nicotine exposure (nicotine, cotinine, trans-3′-hydroxycotinine) relative to exposure to a Classic Tobacco-flavored e-cigarette aerosol. Comparing these findings with aerosols of other ENDS and by measuring aldehyde-derived metabolites in human urine following exposure to e-cigarette aerosols will further our understanding of the relationship between ENDS use, aldehyde exposure, and health risk.

© 2018 American Association for Aerosol Research