Development of an Aerosol Dynamics Model for Dry Deposition Process Using the Moment Method

An aerosol dynamics model for dry deposition process is developed based on the moment method. Since it is hard to fully apply the moment method to the widely used dry deposition velocity expressions based on the resistance theory, the dry deposition velocity expression by Raupach et al. (2001) Raupach, M. R., Briggs, P. R., Ahmad, N. and Edge, V. E. 2001. Endosulfan Tansport: II. Modeling airborne Dispersal and Deposition by Spray and Vapor. J. Environ. Qual., 30: 729–740. [Crossref], [PubMed], [Web of Science ®] , [Google Scholar] is used. Detailed deposition mechanisms such as Brownian diffusion, gravitational settling, and impaction are considered in the expression. To verify the validity of the derived dynamic equation, aerosol dynamics for the dry deposition process is estimated with the expressions of Raupach et al. (2001) Raupach, M. R., Briggs, P. R., Ahmad, N. and Edge, V. E. 2001. Endosulfan Tansport: II. Modeling airborne Dispersal and Deposition by Spray and Vapor. J. Environ. Qual., 30: 729–740. [Crossref], [PubMed], [Web of Science ®] , [Google Scholar], of Wesely (1989) Wesely, M. L. 1989. Parameterizations of Surface Resistance to Gaseous Dry Deposition in Regional-Scale, Numerical Models. Atmos. Environ., 23: 1293–1304. [Crossref], [Web of Science ®] , [Google Scholar] modified for particles (Seinfeld and Pandis 1998 Seinfeld, J. H. and Pandis, S. N. 1998. Atmospheric Chemistry and Physics, 958–996. New York, , USA: Wiley.  [Google Scholar]), in CMAQ, and with constant value used in RAINS-ASIA. Those expressions give different dynamics. Generally, the result for this study is between the result of modified Wesely's expression and CMAQ. When using the modified Wesely's expression with the number of sections being equal or smaller than 10, the resultant size distribution does not give the peak shape accurately.     

A Stochastic Model for Particle Deposition and Bounceoff

A new model for particle deposition and bounceoff that combines current knowledge of turbulent bursts with the stochastic properties of turbulent fluctuations is presented. The model predictions for deposition velocities agree with experimental results in the literature for dimensionless particle relaxation time τ_p ⁺ > 2. For τ_p ⁺ > 10, most of the particles delivered to the edge of the viscous sublayer are able to deposit onto the surface due to their inertia; the deposition velocity approaches an asymptotic value because the process becomes limited by the rate of turbulent delivery to the viscous sublayer. Because of the penetration of turbulent fluctuations into the viscous sublayer, the minimum values of vertical velocities needed for particles to deposit onto the surface are smaller than those predicted by the free flight model. Most of the deposition occurs from those turbulent fluctuations at the upper tail of the distribution of the vertical component of air velocity.

In addition to the deposition velocity, the model is able to provide the distribution of particle velocities on reaching the surface which is used to compute the fraction of particle bounceoff. The model predictions for the fractions of rebound agree reasonably with the measured results from a wind tunnel experiment for τ_p ⁺ > 2. However, both the deposition velocity and the fraction of rebound are underestimated by the model for τ_p ⁺ < 2. Other mechanisms such as Brownian diffusion must be included in further revisions to this model in order to obtain satisfactory predictions for smaller values of τ_p ⁺.     

Computational Fluid Dynamic Predictions and Experimental Results for Particle Deposition in an Airway Model

An area identified as having a high priority by the National Research Council (NRC 1998) relating to health effects of exposure to urban particulate matter is the investigation of particle deposition patterns in potentially-susceptible subpopulations. A key task for risk assessment is development and refinement of mathematical models that predict local deposition patterns of inhaled particles in airways. Recently, computational fluid dynamic modeling (CFD) has provided the ability to predict local airflows and particle deposition patterns in various structures of the human respiratory tract. Although CFD results generally agree with available data from human studies, there is a need for experimental particle deposition investigations that provide more detailed comparisons with computed local patterns of particle deposition. Idealized 3-generation hollow tracheo-bronchial models based on the Weibel symmetric morphometry for airway lengths and diameters (generations 3-5) were constructed with physiologically-realistic bifurcations. Monodisperse fluorescent polystyrene latex particles (1 and 10 mu m aerodynamic diameter) were deposited in these models at a steady inspiratory flow of 7.5 L /min (equivalent to heavy exertion with a tracheal flow of 60 L /min). The models were opened and the locations of deposited particles were mapped using fluorescence microscopy. The particle deposition predictions using CFD for 10 mu m particles correlated well with those found experimentally. CFD predictions were not available for the 1 mu m diameter case, but the experimental results for such particles are presented.     

Particle Deposition in a Multiple-Path Model of the Human Lung

Predicting the amount of particle deposition in the human lung following exposure to airborne particulate matter is the first step toward evaluating risks associated with exposure to airborne pollutants. Realistic deposition models are needed for accurate predictions of deposition in the lung, but a major limitation is the degree to which the lung geometry can be accurately reconstructed. Morphometric data for the entire airway tree of the human lung are not available. So far, idealistic lung structures have been used for deposition calculations. In this study, 10 statistical lung structures based on morphometric measurements of Raabe et al. (1976) were generated for the conducting airways of the human lung. A symmetric, dichotomous branching alveolar airway structure was attached to the end of the conducting airway tree of each lung structure. The total volume of the alveolar region was the same among the lung geometries. Using a mathematical scheme developed previously (Anjilvel and Asgharian 1995), regional, lobar, and per-generation depositions of particles were calculated in these geometries. The results were compared to deposition predictions using typical-path and five-lobe symmetric lung geometry models. All three lung models showed very similar regional and generation-by-generation deposition results. Lobar deposition was found to strongly depend on the detailed morphometry of the lung structure that was used.     

Solid Phase Extraction for Determination of Polycyclic Aromatic Hydrocarbons from Atmospheric Wet and Dry Deposition Samples

Solid phase extraction with C-18 sorbent tubes was employed for extraction and preconcentration of trace level (ng/l) of polycyclic aromatic hydrocarbons (PAHs) from water samples obtained by collecting wet and dry atmospheric deposition. Recoveries of spiked PAHs from 0.2 to 10.0 l water samples ranged from 60% to 90%. Five deuterated PAHs (naphthalene- d 8 , acenaphthene- d 10 , anthracene- d 10 , chrysene- d 12 , and perylene- d 12 ) were added to the samples as surrogates. The recoveries of surrogates were in the same range as the spiked PAHs. The recoveries of surrogates were used to estimate the recoveries of ambient PAHs with a similar ring structure. Factors that contributed to the relatively low recoveries include breakthrough of the sorbates, adsorption on the container surface, and degradation in water during storage.     

Particle Deposition on Semiconductor Wafers

The deposition of aerosol particles on semiconductor wafers in the typical manufacturing environment of the clean room has been calculated using the equations of convective diffusion and sedimentation. The result shows that the deposition velocity decreases with increasing particle size in the diffusion regime and increases with increasing particle size in the sedimentation regime, with a minimum deposition velocity occurring in the vicinity of 0.2 μm. The minimum deposition velocity varies from approximately 0.2 × 10^?3 to 0.7 × 10^?3 cm/s, depending on the size of the wafer, the airflow velocity, and whether the wafer is freestanding or placed on top of a workbench.     

大气干湿沉降物中高氯酸盐对洁净水体的污染研究

使用被动采样方法,将盛放一定体积去离子水的采样缸在露天状况下连续放置一年。通过连续观测发现,洁净水体在受到大气干(自然降尘、沙尘暴降尘)、湿(雨、雪、雹)沉降物中污染物的影响后,水体中高氯酸盐浓度的年变化逐步上升。春节期间由于受到除夕夜大量燃放烟花爆竹的影响,采样缸水体中的高氯酸盐浓度较非燃放期平均上升337.7%。在受到干沉降影响的14天内,平均每天上升24.1%。在之后的湿沉降影响下,高氯酸盐浓度再次被提升了158.1%。在受湿沉降影响的4天中,平均每天上升39.5%。     

Selective Particle Deposition in Crossflow Filtration

Abstract

To study the mechanism of particle deposition in crossflow filtration, hydrodynamic forces exerted on a spherical particle touching the surface of filter medium are analyzed to derive the critical selective cut-diameter of the deposited particles under various crossflow velocities and filtration rates in a crossflow filtration system. Experimental data of turbulent crossflow filtration of dilute light calcium carbonate suspension agree with the prediction of this theory within 30% error under the crossflow velocity of from 0.57 to 1.14 m/s. Equations to estimate the characteristics of crossflow filtration, such as steady-state filtration rate and average specific resistance of cake, are also presented.     

Modeling Particle Deposition in Extrathoracic Airways

The extrathoracic (ET) airways filter, warm, and humidify the inspired air and provide olfactory function. These multiple functions are reflected in its complex anatomy and physiology. The ET airways form the first line of defense against inhaled pollutants, both gaseous as well as particulate. To accurately assess the risk posed by inhaled particulate matter to the lung, it is essential to understand the filtering efficiency of the ET airways. In this paper computational fluid dynamics is used to simulate the airflow patterns and the thermodynamics of the ET airways. We detail the procedure to develop a computer reconstruction of the ET airways and the computer model to simulate the flow variable. Using this information we compute the particle trajectories, for both hygroscopic and nonhygroscopic aerosols, and use this data to evaluate the particle deposition pattern in the ET airways. The model predicts high relative humidity conditions in the ET airways. The model also shows that the high relative humidity conditions are conducive for rapid growth of hygroscopic particles and dramatically alter the deposition characteristics of ambient (hygroscopic) aerosol.     

Wind Tunnel Modeling of Small Particle Deposition

An inexpensive microsphere dispersion, deposition, and sampling system is described. This system was used to examine the transport and deposition of small particles (? 1 μm diameter) across the aerodynamic boundary layer which developed over a prototype deposition surface (smooth, flat, acrylic plate). Unit density, polystyrene latex microspheres (0.8 and 1.1 μm diameter) were deposited onto both oil-coated and dry, upper and lower surfaces. Observed deposition velocities were significantly larger than those reported in the literature, possibly because low relative humidities increased electrostatic charge on the experimental plate. The experimental results were greater than those predicted by a turbulent flow particle deposition model.     

Simulation of Particle Deposition in an Idealized Mouth with Different Small Diameter Inlets

The deposition of monodisperse particles (1.0-12.5 w m diameter) in an idealized mouth geometry has been studied numerically for three different inlet diameters (3.0, 5.0, and 8.0 mm). The continuous phase flow is solved using a RANS (Reynolds Averaged Navier-Stokes) k m y turbulence model at an inhalation flow rate of 16.3, 21.7, and 32.2 L/min. The particulate phase is simulated using a random-walk/Lagrangian stochastic eddy-interaction model (EIM). When optimized near-wall corrections are included in the EIM, the particle deposition results in the idealized mouth geometry are in relatively good agreement with measured data obtained in separate experiments. Without the near-wall corrections in the EIM, poor agreement with experiment is seen.     

Deposition of Dry Powder Generated by Solovent in Sophia Anatomical Infant Nose-Throat (SAINT) Model

Purpose: To quantify deposition of ^99mtechnetium-labeled powder in the Sophia Anatomical Infant Nose-Throat (SAINT) model of a 9-month old. Methods: Powder was generated by the Solovent (BD Technologies), an active dry powder inhaler with spacer, during 30 seconds of tidal volume (TV) breathing. Activity that passed through the model was captured on a filter and represented powder that was available for deposition in the lungs. Deposition in the nasal cavity, on the filter, and in the spacer was expressed as a percentage of the injected dose into the spacer. Results: Mean (± SD) injected dose averaged 89.5 ± 0.09%, 90.3 ± 0.11%, and 91.3 ± 0.05% at 50, 100, and 200 mL TV, respectively. Mean nasal deposition increased significantly from 50 mL to 100 mL and 200 mL TV with 0.60 ± 0.002%, 1.72 ± 0.007%, and 6.75 ± 07.21%, respectively (all p ≤ 0.05). Similarly, mean filter deposition increased significantly from 50 mL to 100 mL to 200 mL with 0.28 ± 0.00%, 1.14 ± 0.00%, and 3.87 ± 0.01%, respectively (all p < 0.05). Mean retention in the spacer was similar at 50 mL (93.38 ± 0.02%) and at 100 mL TV (89.97 ± 0.04%), but decreased significantly to 71.47 ± 0.05% at 200 mL TV (all p < 0.05). Conclusions: These data suggest for the first time the feasibility of delivering a dry powder formulation to infants and toddlers by actively introducing the powder into a spacer. Lung deposition and nasal deposition, as a percent of injected dose, were dependent on tidal volume with deposition increasing with increasing TV. Nevertheless, deposition, as a percent of injected dose, was low in both regions. This was likely due to significant retention in the spacer at all 3 tidal volumes.

Copyright 2012 American Association for Aerosol Research     

Particle Deposition in Cyclic Bifurcating Tube Flow

Deposition efficiency (DE) of aerosol particles in a bifurcating tube was determined experimentally with cyclic flow conditions at a frequency of 16, 30, and 50 cycles/min. Two bifurcation tube models with identical dimensions but different branching angles (θ = 30° and 45°) were used. Monodispersed oleic acid droplets (3, 5, and 7 μm in diameter) tagged with uranine were used as test aerosols and sampled through the model tube at a mean Reynolds number (Re) of 679–5548. Stokes number (Stk) ranged between 0.028 and 0.25. The model tube was washed section by section with deionized water and the fluorescence content in the washed solution was used to determine deposition efficiency. DE increased monotonically with increasing Stk at Re < ～ 2000. However, at higher values of Re, Stk failed to unify DE. There was no significant difference in DE between θ = 30° and 45° and among different cyclic frequencies used. However, DE with cyclic flows was 80–200% higher than those obtained with constant flows. The implications of the results were also discussed.     

Aerosol Particle Deposition in a Recirculation Region

Digital simulation results concerning aerosol particle transport and deposition in a recirculation region are presented. It is assumed that the particles are shed from sources near the back face of a block in a turbulent duct flow. The results show that a large number of particles may be captured by the block and the upper wall of the channel due to impaction and interception. The capture efficiencies increase as the source distance from the wall decreases. The gravitational effects on the particle deposition rate are also studied.     

Measurement of the Effect of Cartilaginous Rings on Particle Deposition in a Proximal Lung Bifurcation Model

Although cartilaginous rings are present in the trachea and main bronchi of actual human conducting airways, and despite previous authors' theoretical predictions that these effects are significant, little systematic experimental study has been conducted to quantify the effects of such localized morphological features on particle deposition. In the present study, the possible effects of cartilaginous rings upon particle deposition in an idealized airway model are investigated experimentally. The airway model includes the oral cavity, pharynx, larynx, trachea, and first three generations of bronchi. Gravimetry is used to determine the deposition of monodisperse aerosol particles with mass median diameters ranging between 2.9–6.3 µm for steady inhalation flow rates of 30 and 60 l/min. Particle deposition efficiency obtained from a model with cartilaginous rings present in the trachea is compared with that from a smooth-walled tracheo-bronchial model. Significantly enhanced deposition fraction in the trachea with cartilaginous rings present in the trachea is observed for all inhalation rates and particle sizes. The data also indicates that the disturbance of the airflow within the trachea by the presence of cartilaginous rings promotes deposition of particles through the entire trachea, but this influence does not propagate to bifurcations further downstream. The present work indicates that cartilaginous rings may be a critical element to be integrated into future modelling of airways due to their significant effect on inhaled aerosol deposition.     

Experimental Study of Particle Deposition on Semiconductor Wafers

A sensitive method for detecting particle deposition on semiconductor wafers has been developed. The method consisted of generating a monodisperse fluorescent aerosol, depositing the known-size monodisperse aerosol on a wafer in a laminar flow chamber, and analyzing the deposited particles using a fluorometric technique. For aerosol particles in the size range of 0.1–1.0 μm, the mobility classification-inertial impaction technique developed by Romay-Novas and Pui (1988) was used to generate the monodisperse test aerosols. Above a particle diameter of 1.0 μm, monodisperse uranine-tagged oleic acid aerosols were generated by a vibrating-orifice generator. The test wafer was a 3.8-cm diameter silicon wafer placed horizontally in a vertical laminar flow chamber which was maintained at a free stream velocity of 20 cm/s. A condensation nucleus counter and an optical particle counter were used to obtain the particle concentration profile in the test cross section and to monitor the stability of aerosol concentration during the experiment. The results show that the measured particle deposition velocities on the wafers agree well with the theory of Liu and Ahn (1987) in the particle size range between 0.15 and 8.0 μm. The deposition velocity shows a minimum around 0.25 μm in particle diameter and increases with both smaller and larger particle size owing to diffusional deposition and gravitational settling, respectively.     

大气有害颗粒物室内自然沉降实验研究

大气有害颗粒物(PM)对人体有极大的危害,紧闭门窗可使颗粒物自沉降,从而实现空气洁净。为研究室内有害颗粒物自然沉降的净化效果,该研究采用激光离散式颗粒物传感器与数据采集系统,对自然条件下室内大气有害颗粒物自然沉降进行实验研究。通过实验发现:室内大气有害颗粒物在重度污染(粒径小于2.5μm的PM150μg/m~3)浓度下,有较明显的自然沉降速率,PM粒径2.5μm～10μm、1.0μm～2.5μm、0.3μm～1.0μm自然沉降速率分别为:1.76μg/(m~3·min)、0.67μg/(m~3·min)和0.51μg/(m~3·min),当颗粒物为中度与轻度污染浓度时(150μg/m~3),自然沉降速率随浓度降低逐渐放缓;当密闭室内存在人员时,会加快室内大气有害颗粒物下降速率;此外,不同粒径的大气有害颗粒在空气中的分布均匀状态不同,粒径越大在空气中分布越不均匀,粒径越小越均匀,其在空气中自然沉降的速率越低。     

Modeling Dry Deposition of Aerosol Particles onto Rough Surfaces

Dry deposition is a primary mechanism by which suspended particles are transported from gas onto surfaces. Prediction of this transport rate is needed in a vast range of applications, including environmental, industrial, and engineering, and in studying the impacts of aerosols. Besides air flow characteristics and properties of aerosol particles, the dry deposition velocity depends greatly on surface properties. However, existing models describe rough surfaces with only one parameter, the surface roughness height, and are therefore of limited accuracy. Here, we introduce a new, and yet simple, physical approach to account for the influence of surface roughness on the dry deposition velocity. The approach relies on a hybrid parameter that combines the surface roughness height and the peak-to-peak distance between roughness elements. Our new approach is able to predict the deposition velocity accurately, being superior to many of the earlier models, which overpredict deposition velocities by a factor as high as 25. In addition, our approach is more general and covers a wide size range of aerosol particle diameter (0.001–100 μm).

Copyright 2012 American Association for Aerosol Research     

Ultrafine Particle Deposition in a Human Tracheobronchial Cast

The deposition of 0.20, 0.15, and 0.04 μm diameter particles was measured in a human central airway cast using a variable larynx with cyclic inspiratory flow. Data were compared with theoretical predictions for deposition from laminar flow for the first seven airway generations. With the exception of tracheal deposition, which on average exceeded predictions by a factor of 9, the measured deposition was about twice that predicted. The enhanced deposition is attributable to secondary swirling flows. Less enhancement is observed at higher inspiratory flow rates as turbulence increases. The surface density of particles deposited at bifurcations was approximately 20% greater than along the airway lengths. This increased deposition at bifurcations should be considered when calculating tissue dose for particles which act before the initial deposit is removed by clearance processes.     

Particle Deposition in a Cast of Human Oral Airways

Oral and nasal airways are entryways to the respiratory tract. Most people breathe through the nasal airway during rest or light exercise, then switch to oral/nasal breathing during heavy exercise or work. Resistance through the oral airways is much lower than through the nasal airways, so fewer aerosol particles are deposited in the oral airways. Aerosol drugs are usually delivered by inhalation to the lung via the oral route for that reason. Oral deposition data from humans are limited, and those available show great intersubject variability. The purpose of this study was to investigate the effects of particle size and breathing rate on the deposition pattern in a human oral airway cast with a defined geometry. The airway replica included the oral cavity, pharynx, larynx, trachea, and 3 generations of bronchi. The oral portion of the cast was molded from a dental impression of the oral cavity in a human volunteer, while the other airway portions of the cast were made from a cadaver. Nine different sizes of polystyrene latex fluorescent particles in the size range of 0.93-30 mu m were used in the study. Regional deposition was measured at a constant inspiratory flow rate of 15, 30, and 60 L min^-1. Deposition in the oral airway appeared to increase with an increasing flow rate and particle diameter. Deposition at the highest flow rate of 60 L min^-1 was close to 90% for particles >20 mu m. Particles> about 10 mu m deposited mainly in the oral cavity. Deposition efficiency has been found to be a unique function of the Stokes number, suggesting that impaction is the dominant deposition mecha nism. Oral deposition can be approximated by a theoretical deposition model of inertial impaction in a 180 degrees curved tube, assuming perfect mixing in a turbulent flow. Our model suggests that the minimum dimension near the larynx and the average cross-sectional area are important parameters for oral airway deposition; however, additional data from the oral airway replica are needed to ascertain whether these are indeed the critical dimensions. Information from the present study will add to our knowledge of the deposition mechanism, the correlation of particle deposition with airway geometry, and eventually the best way to deliver aerosol drugs.