Evaluation of Fine Particle Number Concentrations in CMAQ

The Community Multiscale Air Quality (CMAQ) model is widely used in air quality management and scientific investigation. Numerous studies have been conducted investigating how well CMAQ simulates fine particle mass concentrations, but relatively few studies have addressed how well CMAQ simulates fine particle number distribution. Accurate simulation of particle number concentrations is important because particle number and surface area concentrations may be directly related to human health and visibility. Simulated fine particle number concentrations derived using CMAQ are compared to measurements to identify problems and to improve model performance. Evaluation is done using measured particle number concentrations in Atlanta, Georgia, from 1/1/1999 to 8/31/2000. While homogeneous binary nucleation mechanism used in CMAQ needs to be modified for better prediction of particle number concentrations, there are also other factors that affect the predicted particle level. Assumed particle size of the primary emissions in CMAQ causes number concentrations to be significantly underestimated, while particle density has a small impact. Assuming particle size distributions by three lognormal modes cannot accurately simulate particles with size less than 0.01 μ m, particularly during nucleation events. An additional mode that accounts for particles smaller than 0.01 μ m can improve the accuracy of the number concentration simulations. Though, the use of the Expectation-Maximization (EM) algorithm to estimate size distribution parameters of measured particles suggests that assumed parameters for the lognormal modes in CMAQ are generally reasonable.     

Correlation between airborne particle concentrations in seven industrial plants and estimated respiratory tract deposition by number,mass and elemental composition

The number and mass distribution of airborne particles were recorded in several industrial plants. From the data obtained, particle deposition was estimated in three regions of the respiratory tract using the ICRP grand average deposition model based on Hinds' (1999) parameterization. The median diameter was 30–70 nm (number distributions), and >4 μm (mass distributions) near most work activities, resulting in linear relationships between the deposited number/mass concentrations and the number/mass concentrations in the air. Welding and laser cutting produced particles in the 200–500-nm range; total deposition was small, not in accordance with the linear relationship observed for the other work activities. The elemental content varied between particle sizes in some workplaces, causing different elements to deposit in different respiratory regions. Iron was the most abundant element in the particles in many of the workplaces; in an iron foundry, however, Fe was most abundant only in the micron-sized particles whereas the nanoparticles mainly comprised Pb and Sb.     

Determining Size-Specific Emission Factors for Environmental Tobacco Smoke Particles

Because size is a major controlling factor for indoor airborne particle behavior, human particle exposure assessments will benefit from improved knowledge of size-specific particle emissions. We report a method of inferring size-specific mass emission factors for indoor sources that makes use of an indoor aerosol dynamics model, measured particle concentration time series data, and an optimization routine. This approach provides--in addition to estimates of the emissions size distribution and integrated emission factors--estimates of deposition rate, an enhanced understanding of particle dynamics, and information about model performance. We applied the method to size-specific environmental tobacco smoke (ETS) particle concentrations measured every minute with an 8-channel optical particle counter (PMS-LASAIR; 0.1 m 2+ w m diameters) and every 10 or 30 min with a 34-channel differential mobility particle sizer (TSI-DMPS; 0.01 m 1+ w m diameters) after a single cigarette or cigar was machine-smoked inside a low air-exchange-rate 20 m 3 chamber. The aerosol dynamics model provided good fits to observed concentrations when using optimized values of mass emission rate and deposition rate for each particle size range as input. Small discrepancies observed in the first 1-2 h after smoking are likely due to the effect of particle evaporation, a process neglected by the model. Size-specific ETS particle emission factors were fit with log-normal distributions, yielding an average mass median diameter of 0.2 w m and an average geometric standard deviation of 2.3 with no systematic differences between cigars and cigarettes. The equivalent total particle emission rate, obtained by integrating each size distribution, was 0.2-0.7 mg/min for cigars and 0.7-0.9 mg/min for cigarettes.     

A Model of Deposition of Hygroscopic Particles in the Human Lung

Many aerosols in the environment are hygroscopic and grow in size once inhaled into the humid respiratory tract. The deposited amount and the distribution of the deposited particles among airways differ from insoluble particles of the same initial diameter. As particles grow in size, diffusive behavior tends to diminish while impaction and sedimentation effects increase. A multiple-path model for deposition of hygroscopic particles in the respiratory tract was developed for symmetric and asymmetric lung geometries by implementing particle size change in a model of insoluble particle deposition in lungs. Particle growth by molecular diffusion of water vapor to the particle surface was formulated. The growth model included temperature depression, solute, Kelvin, and Fuchs effects. Particle growth during travel time in each lung airway was computed. Average loss efficiency per airway was calculated by incorporating contributions from particles of various sizes acquired in that airway. A mass balance on the number of particles that entered, exited, deposited, or remained suspended was performed per airway to obtain regional and local deposition fractions of particles in the lung. The deposition fractions calculated for salt particles showed a drop for submicrometer particles in the tracheobronchial region and a significant increase in deposition for micrometer particles or larger. Consequently, very few fine and coarse salt particles reached the alveolar region to be available for deposition. Overall, lung deposition of ultrafine particles decreased for salt particles. Deposition for fine and coarse salt particles in the lung was larger than that of insoluble particles of the same initial particle size.     

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     

A Model for Removal of Compact,Rough, Irregularly Shaped Particles from Surfaces in Turbulent Flows

A model for removal of compact, rough, irregularly shaped particles from surfaces in turbulent flow was developed. Following the approach of our previous bumpy particle model, irregularly shaped particles were modeled as spherical particles with a number of bumps on them. To improve the model, the effect of surface roughness was added to the bumps. Each bump was modeled with large number of asperities and the Johnson-Kendall-Roberts (JKR) adhesion theory was used to model the adhesion and detachment of each bump and asperity in contact with the surface. The total adhesion force for each bump was obtained as the summation of each asperity force in contact with the substrate. To account for the variability observed in the removal of particles, the number of bumps and roughness values of particles are assumed to be random, respectively, with Poisson and log-normal distributions. For particle separation from the surface, the theory of critical moment was used, and the orientation of bumps on the surface was considered when determining the range of shear velocity needed for removal of the irregularly, shaped particles. The effects of particle size, turbulent flow, particle irregularity, and particle surface roughness on detachment and resuspension were studied for different particles and surfaces. Model prediction for removal of rough, irregularly shaped graphite particles from steel substrate was compared with the available experimental data and earlier numerical models, and good agreement was obtained. This study may find application in adhesion and detachment of irregular particles from flooring in indoor and outdoor environments.     

Theoretical and Experimental Evaluation of a Portable Electrostatic TEM Sampler

In this study, we present the design, calibration, and performance evaluation of a portable TEM sampler to collect ultrafine particles on TEM grids. The sampler is based on electrostatic deposition. We performed calibration experiments with monodisperse model particles of different size, morphology, and chemistry to evaluate the efficiency of the device. Additionally, we modeled the performance of the TEM sampler using finite element calculations to calculate the sampling efficiency as function of particle size. We find a good agreement between the model calculations and calibration experiments, but with a systematic deviation of about 30%. We present further experiments with polydisperse particles and derive particle size distributions and absolute number concentrations from TEM images. The shape of the size distribution is very similar to the corresponding SMPS measurements, while the total number concentration has a larger uncertainty. Thanks to the compact design of the sampler, it can be used to collect ultrafine particles even in remote or hostile environments, where no electric power is available.     

Particle Deposition in a Cast of Human Tracheobronchial Airways

Abstract

Regional particle deposition efficiency and deposition patterns were studied experimentally in a human airway replica made from an adult cadaver. The replica includes the oral cavity, pharynx, larynx, trachea, and four generations of bronchi. This study reports deposition results in the tracheobronchial (TB) region. Nine different sizes of monodispersed, polystyrene latex fluorescent particles in the size range of 0.93–30 μm were delivered into the lung cast with the flow rates of 15, 30, and 60 l min^{? 1}. Deposition in the TB region appeared to increase with the increasing flow rate and particle size. Comparison of deposition data obtained from physical casts showed agreement with results obtained from realistic airway replicas that included the larynx. Deposition data obtained from idealized airway models or replicas showed lower deposition efficiency. We also compared experimental data with theoretical models based on a simplified bend and bifurcation model. A deposition equation derived from these models was used in a lung dosimetry model for inhaled particles, and we demonstrated that there was general agreement with theoretical models. However, the agreement was not consistent over the large range of Stokes number. The deposition efficiency was found as a function of the Stokes number, bifurcation angle, and the diameters of parent and daughter tubes. An empirical model was developed for the particle deposition efficiency in the TB region based on the experimental data. This model, combined with the oral deposition model developed previously, can be used to predict the particle deposition for inertial effects with improved accuracy.     

CHARACTERIZATION OF ATMOSPHERIC DRY DEPOSITED PARTICLES AT URBAN AND NON-URBAN LOCATIONS

Microscopic techniques were used to size particles collected on a deposition surface and to generate mass size distributions of deposited particles sampled from urban and non-urban locations. The volume shape factor was defined as a conversion factor between the projected area diameter and equivalent volume diameter of a particle and was used as an indication of the irregularity of the particle shape. The average volume shape factor of deposited particles at the urban location (1.61±0.21) was higher than the average at the non-urban locations (1.16±0.10). This suggests that particles are more irregular in the urban areas. Since non-urban areas have less larger particles in ambient air, depositional mass-size distributions at the urban location had a larger average peak (58 μm) and average mass median diameter (49 μm) of coarse particle mode of the distributions than they did at non-urban locations (averaged 33 and 27 μm). Evaluation of correlation coefficients between parameters (wind speed, deposition flux, peak diamater, mass median diameter) indicates that there are more airborne coarse particles at urban locations than at non-urban locations and this distribution plays an important role in dry deposition. By directly observing the deposited particles, it was found that particles larger than 10 μm diameter contributed to more than 90% (in mass) of the atmospheric dry deposition even when ambient coarse particle concentration is low at non-urban locations.     

Identification of thermal zones and population balance modelling of fluidized bed spray granulation

Air temperature measurements in a fluidized bed of glass beads top sprayed with water showed that conditions for particles growth were fulfilled only in the cold wetting zone under the nozzle which size and shape depended on operating conditions (liquid spray rate, nozzle air pressure, air temperature, and particles load). Evolution of the particle size distribution during agglomeration was modelled using population balance and representing the fluidized bed as two perfectly mixed reactors exchanging particles with particle growth only in the one corresponding to the wetting zone. The model was applied to the agglomeration of non-soluble glass beads and soluble maltodextrin particles spraying respectively an acacia gum solution (binder) and water. Among the three adjustable parameters, identified from experimental particle size distributions evolution during glass beads agglomeration, only one describing the kinetics of the size distribution evolution depended on process variables. The model allowed satisfying simulation of the evolution of the particle size distribution during maltodextrin agglomeration.     

Loss processes affecting submicrometer particles in a house heavily affected by road traffic emissions

The fraction of outdoor aerosol that penetrates into indoor environments plays an important role in determining the contribution of outdoor particles to the total lung dose of particles in human exposure. The objective of this study was to investigate the physical processes affecting migration of outdoor traffic particles into indoor environments. Particle number size distributions were measured by a fast mobility particle sizer system in both indoor and outdoor environments of a house located in close proximity to a busy street in Bologna (Italy) in the period February–April 2012. Indoor to outdoor (I/O) ratios for submicron particle number concentrations showed strong dependence on particle size and meteorological conditions. The loss rates of particles due to deposition, coagulation, and evaporation were determined using dynamic mass balance and coagulation models. Higher loss rates were found for small particles (nucleation and Aitken mode) indoors than for larger particles (accumulation mode). The coagulation and evaporation processes made a significant contribution to the loss of traffic nanoparticles indoors, especially during the day time. Application of positive matrix factorization to the indoor and outdoor particle size distributions showed a substantial loss of traffic-generated nucleation mode particles in the indoor environment, with evaporation playing a major role.

Copyright © 2017 American Association for Aerosol Research     

Simulation of the effect of local obstructions and blockage on airflow and aerosol deposition in central human airways

Investigation of the effect of sidewall and carinal tumours, airway constrictions and airway blockage on the inspiratory airflow and particle deposition in the large central human airways was the primary objective of this study. A computational fluid and particle dynamics model was implemented, validated and applied in order to simulate the air and particle transport and to quantify the aerosol deposition in double airway bifurcation models. Our investigations revealed that surface abnormalities and tubular constrictions can significantly alter the airstreams and the related local aerosol deposition distributions. Sidewall tumours have lead to an enhanced deposition of large particles and caused lower deposition efficiency values of nano-particles compared to the deposition efficiency in healthy airways. Central tumours multiplied the deposition efficiency of large particles but hardly affected the deposition efficiency of nano-particles. Airway blockage caused a significant redistribution of particle deposition sites. The deposition efficiency of the inhaled aerosols in constricted airways was much higher than the same deposition efficiency in healthy airways. Current results might help in the understanding of the adverse health effects of the inhaled air-pollutants in patients with lung disease and might be integrated into future aerosol therapy protocols.     

COAGULATION OF CIGARETTE SMOKE PARTICLES

Experimental measurements on the deposition of cigarette smoke particles (CSP) in the human airways have produced results that are inconsistent with typical deposition data based on particle size. Previous work relating to hygroscopic growth indicates that hygroscopicity alone can not account for this discrepancy. The present study investigates coagulation of CSP modeled as a polydisperse-charged aerosol as a possible explanation. The results of the model more accurately predict the experimental coagulation data for mainstream CSP than models that treat CSP as a monodisperse or polydisperse-uncharged aerosol. An aerosol with an initial charge distribution based on Boltzmann equilibrium yields slightly larger coagulation rates than the mainstream CSP polydisperse-charged model. The numerical results indicate that the size and charge distribution of sidestream CSP, with a concentration of 10⁶ particles cm^-3, remain stable. In 2 s, the size distribution of mainstream CSP, with a concentration of 10⁹ particles cm^-3, shifts to a larger size while becoming flatter and wider. The diameter of average mass increases from 0.29 to 0.5 μm. Numerical results confirm experimental reports for mainstream CSP, which indicate that the total number of charged particles increases with time and, in the early stages of coagulation, the amount of charge per particle cannot be estimated based on the particle size. This study shows that polydisperse-charged CSP, allowed to coagulate for 2 s in the mouth, will not produce size distributions that yield the observed deposition of CSP. However, additional coagulation will take place as the CSP travels through the respiratory tract, which will be investigated in future work.     

Experimental determination of the respiratory tract deposition of diesel combustion particles in patients with chronic obstructive pulmonary disease

Background

Air pollution, mainly from combustion, is one of the leading global health risk factors. A susceptible group is the more than 200 million people worldwide suffering from chronic obstructive pulmonary disease (COPD). There are few data on lung deposition of airborne particles in patients with COPD and none for combustion particles.

Objectives

To determine respiratory tract deposition of diesel combustion particles in patients with COPD during spontaneous breathing.

Methods

Ten COPD patients and seven healthy subjects inhaled diesel exhaust particles generated during idling and transient driving in an exposure chamber. The respiratory tract deposition of the particles was measured in the size range 10?C500?nm during spontaneous breathing.

Results

The deposited dose rate increased with increasing severity of the disease. However, the deposition probability of the ultrafine combustion particles (< 100?nm) was decreased in COPD patients. The deposition probability was associated with both breathing parameters and lung function, but could be predicted only based on lung function.

Conclusions

The higher deposited dose rate of inhaled air pollution particles in COPD patients may be one of the factors contributing to their increased vulnerability. The strong correlations between lung function and particle deposition, especially in the size range of 20?C30?nm, suggest that altered particle deposition could be used as an indicator respiratory disease.     

纤维过滤滤饼孔隙率计算机模拟

基于受限扩散模型,建立纤维介质表面粉尘颗粒沉积形成滤饼的模拟模型。粉尘颗粒在远离过滤介质的控制面上释放,假设对流扩散运动为颗粒输送和沉积的主要机理。定义Pe数为颗粒对流作用与扩散运动的比值,讨论不同Pe数对滤饼孔隙率的影响。对Pe数范围为0.25~1600进行模拟计算结果表明,Pe数对滤饼的孔隙率影响显著。当颗粒的扩散运动强于对流运动,颗粒沉积形成的滤饼为较为松散的结构,当Pe数较大时,颗粒沉积形成的滤饼为较为紧密的堆积结构。此外,考虑到实际过滤过程中粉尘颗粒大多为非等径粒子,也对多分散粒子对滤饼形成及孔隙率的影响作了分析。     

Study of dynamic multi-dimensional temperature and concentration distributions in liquid-sprayed fluidized beds

A physically based model was developed for heat and mass transfer processes in liquid-sprayed fluidized beds. Such fluidized beds are used for granulation, coating and agglomeration. The complex correlations of a number of microprocesses, spraying, wetting, drop deposition, heat transfer, drying and mass transfer were studied, and transient three-dimensional distributions of air humidity, air temperature, particle wetting efficiency, liquid film temperature, particle temperature, local liquid loading and local evaporation rate were calculated. For the evaluation of the model, the stationary spatial air temperature distributions were measured at a fluidized bed pilot plant of the institute. The fluidized bed of monodisperse wood- or glass beads was sprayed with clear water. Conclusions are drawn on the relevance of particle dispersion, spraying and drying to simulating temperature and concentrations distributions.     

Particulate Respiratory Dose to Indian Women from Domestic Cooking

Particulate mass size distributions were measured during cooking and non-cooking periods in three Indian urban household kitchens with Liquefied Petroleum Gas as fuel. Based on the measured mass size distributions, fraction of particulate deposition in the respiratory system were calculated for a healthy Indian female using a deterministic lung deposition model. Respiratory physiological data of Indian women were collected from the published data. These physiological parameters were incorporated in the model to determine the particulate deposition in the respiratory system. The cooking generated very high concentration of particles 4 to 5 times more than the non-cooking background periods. Particulate size distributions in both cooking and non-cooking periods showed bimodal characteristics. Cooking process generated particles predominantly in accumulation mode (0.1–0.3 μ m) whereas during non-cooking periods particulates are found in coarse mode (1.0–2.0 μ m). Also, during frying process, the particulates were found to have a predominant coarser/droplet mode 0.7–1.0 μ m. The highest deposition was observed in pulmonary region during cooking periods. The study shows that the daily particulate dose to the urban Indian women from domestic cooking is comparable with the dose resulting from outdoor particulate exposure.     

提升管内颗粒团聚行为的离散颗粒模拟

采用计算流体力学和离散单元方法对二维提升管气固流动过程进行了数值模拟。采用大涡模拟方法模拟气体湍流流动,采用离散单元方法模拟颗粒碰撞过程。基于网格内流体能量守衡,提出了动能加权平均法,实现Euler坐标与Lagrange坐标的耦合与分解。模拟计算得到的瞬态流型揭示了颗粒聚团的合并和破碎过程及颗粒的详细运动行为,定性地揭示了提升管内气体-颗粒两相流动过程。     

A Potential Soot Mass Determination Method from Resistivity Measurement of Thermophoretically Deposited Soot

Miniaturized detection systems for nanometer-sized airborne particles are in demand, for example in applications for onboard diagnostics downstream particulate filters in modern diesel engines. A soot sensor based on resistivity measurements was developed and characterized. This involved generation of soot particles using a quenched co-flow diffusion flame; depositing the particles onto a sensor substrate using thermophoresis and particle detection using a finger electrode structure, patterned on thermally oxidized silicon substrate. The generated soot particles were characterized using techniques including Scanning Mobility Particle Sizer for mobility size distributions, Differential Mobility Analyzer—Aerosol Particle Mass analyzer for the mass–mobility relationship, and Transmission Electron Microscopy for morphology. The generated particles were similar to particles from diesel engines in concentration, mobility size distribution, and mass fractal dimension. The primary particle size, effective density and organic mass fraction were slightly lower than values reported for diesel engines. The response measured with the sensors was largely dependent on particle mass concentration, but increased with increasing soot aggregate mobility size. Detection down to cumulative mass as small as 20–30 μg has been demonstrated. The detection limit can be improved by using a more sensitive resistance meter, modified deposition cell, larger flow rates of soot aerosol and modifying the sensor surface.     

Passive Filtration of Airborne Particles from Buildings Ventilated by Natural Convection: Design Procedures and a Case Study at the Buddhist Cave Temples at Yungang,China

Air exchange between interior spaces and the outdoor atmosphere can occur due to a variety of processes, including wind-driven flows and natural convectiondriven flows. As air is exchanged with the outdoors, airborne particles can be brought inside. Depending on the use of the indoor space, the presence of particles in indoor air could be a nuisance to the occupants or could be damaging to materials kept indoors. While one obvious solution to such problems is to install a mechanical air filtration system, that is not always practical. In particular, the character of some historical houses and some archaeological sites would be degraded by the presence of a mechanical air distribution system, and in some parts of the world the reliable electrical power supply needed for such a filtration system may not be available. In the present paper we consider principles for the design of passive filtration systems in which air motion through the filter material is induced by a natural convection flow rather than by a mechanical fan. A fluid mechanical model first is described for predicting the air flow through an interior space that acts as a thermal siphon. The effect of placing filter material in the path of such air flows is examined next. The indoor-outdoor air quality model of Nazaroff and Cass (1989a) is matched to the natural convection air exchange model, and calculations are performed to determine the relationship between the outdoor particle size distribution and indoor particle size distributions and particle deposition rates given a passive filtration system. Example calculations are worked for the case of a passive particle filtration system that could be installed to protect the interior of the Buddhist cave temples at Yungang, China. These are a collection of manmade cave temples dating from the 5th century AD, now situated in the middle of one of China's largest coal-mining regions with its accompanying air pollution problems.