Particle Deposition Velocity Research Articles

PARTICLE DEPOSITION IN A NEARLY DEVELOPED TURBULENT DUCT FLOW WITH ELECTROPHORESIS

This study is concerned with deposition of neutral and charged particles in nearly developed turbulent duct flows. The cases that the duct is vertical or horizontal and when the particles carry Boltzmann, static electrification, as well as saturation charge distributions are analyzed. The mean turbulent flow field is evaluated with the aid of the FLUENT code, using the Reynolds stress transport model. Deposition rate of particles in the size range of 0.01–100 μm are studied and the effects of electric field intensity on particle deposition velocity are evaluated. The simulation results are compared with the available experimental data, the earlier numerical results and those obtained from empirical equations for fully developed duct flows. It is shown that the electrostatic effect significantly increases the particle deposition rate.

Speciated particle dry deposition to the sea surface: results from ASEPS ’97

It has been postulated that atmospheric pathways may comprise a significant source of nitrogen for aquatic ecosystems and excess atmospheric deposition to coastal areas may be a major cause of eutrophication. Dry deposition of nitrogen containing particles is a potential, but poorly quantified pathway, for atmospheric nitrogen flux. This pathway is not well quantified because deposition velocities for particles are difficult to calculate and incorporate substantial uncertainties. Herein we employ an amended version of the Hummelshøj et al. (1992, Proceedings of the 5th International Conference on Precipitation Scavenging and Atmosphere–Surface Exchange Processes. AMS, Richland, Washington, USA, 12pp.) model to calculate size-segregated dry deposition of particle inorganic nitrogen compounds to the western Baltic during the late Spring of 1997 based on data collected as part of the Air–Sea Exchange Process Study (ASEPS). The results show that over a 15 d period in April and May dry deposition fluxes varied between 30 and 400 μg m -2 d -1 for nitrate and 1 and 120 μg m -2 d -1 for ammonium. Sensitivity analyses run to assess the potential bounds on actual dry deposition indicate that, for reasonable variation of model parameters and formulation, particle nitrogen dry deposition may be varied by up to an order of magnitude. The primary sources of uncertainty are identified and are discussed in the context of alternative model formulations.

A multilayer model for inferring dry deposition using standard meteorological measurements

In this paper, we describe the latest version of the dry deposition inferential model, which is used to estimate the deposition velocities (Vd) for SO2, O3, HNO3, and particles with diameters less than 2 μm. The dry deposition networks operated by the National Oceanic and Atmospheric Administration (NOAA) and the Environmental Protection Agency (EPA) use this model to estimate dry deposition on a weekly basis. This model uses a multilayer approach, discretizing the vegetated canopy into 20 layers. The use of canopy radiative transfer and simple wind profile models allows for estimates of stomatal (rs) and leaf boundary layer (rb) resistances to be determined at each layer in the plant canopy for both sunlit and shaded leaves. The effect of temperature, water stress, and vapor pressure deficits on the stomatal resistance (rs) have been included. Comparisons of modeled deposition velocities are made with extensive direct measurements performed at three different locations with different crops. The field experiment is discussed in some detail. Overall, modeled O3 deposition velocities are in good agreement with measured values with the average mean bias for all surfaces of the order of 0.01 cm/s or less. For SO2, mean biases range from −0.05 for corn to 0.15 cm/s for soybeans, while for HNO3, they range from 0.09 for corn to 0.47 cm/s for pasture.

Deposition of heavy metals and ionic aerosol species at a scottish moorland site

?While estimates of heavy metal deposition are often inferred from moss analyses (e.g. NORD, 1996) there are only few direct measurements of heavy metal loads. This is partly due to the size-dependent particle deposition velocity (v&) being still highly uncertain (Gallagher et al., 1997). Here micrometeorological measurements of v& are combined with particle size distributions to estimate the dry deposition of both ionic aerosols species and heavy metals at a remote, clean Scottish moorland site.

Hypersonic plasma particle deposition of nanostructured silicon and silicon carbide

A new process for the production of nanostructured materials, hypersonic plasma particle deposition (HPPD), is experimentally investigated. In HPPD, vapor phase precursors are injected into a flowing plasma generated by a DC arc. The plasma undergoes a supersonic expansion into a deposition chamber, with the pressure dropping across the nozzle from ∼500 Torr to ∼2 Torr. Ultrafine particles nucleate in the nozzle, accelerate in the hypersonic free jet downstream of the nozzle, and deposit by inertial impaction onto a temperature-controlled substrate. The low particle residence time (∼50 μs) minimizes particle agglomeration, while the high particle deposition velocity (∼1 km s -1) results in the formation of a partially consolidated coating. We have characterized silicon, carbon and silicon carbide coatings produced by injecting vapor-phase SiCl 4 and hydrocarbon (CH 4 and C 2H 2) precursors into an Ar–H 2 plasma. The silicon coatings are polycrystalline, while the carbon and silicon carbide deposits are amorphous and hydrogenated. Both Si and SiC coatings had nanostructured regions with grain sizes on the order of 20–30 nm, reasonably close to the diameters of impacting particles measured using an extractive aerosol probe coupled to a scanning electrical mobility analyzer.

Airborne Concentrations and Dry Deposition Fluxes of Particulate Species to Surrogate Surfaces Deployed in Southern Lake Michigan

Dry deposition flux measurements to surrogate surfaces and airborne concentration measurements of Zn-containing, S-rich, and soil particles (analyzed by scanning electron microscopy) and Al, Ba, Br, Ca, Cl, Cu, K, Mg, Mn, Na, Ti, and V (analyzed by neutron activation analysis) were made over southwestern Lake Michigan in July 1994 and January 1995 to determine atmospheric inputs of pollutants to the lake. Samples collected in the summer show that despite relatively low airborne concentrations of particles with physical diameters >8 μm, these particles account for the majority of the dry deposition mass flux. However, this sharp contrast is not found during January when particles with physical diameters of 4−8 μm dominate both the airborne concentration and the flux. Dry deposition velocities (flux divided by airborne concentration) for particles are found to range from 0.0062 cm/s for 0.75-μm particles to 5.4 cm/s for 24-μm particles.

Atmospheric deposition of nitrogen compounds in the Czech Republic

This study presents estimates of dry and wet deposition of nitrogen compounds in the Czech Republic. Deposition has been estimated from monitored and modelled concentrations in the atmosphere and in precipitation, where the most important nitrogen compounds are nitrogen oxides and ammonia, and their reaction products. Measured atmospheric concentrations of NO x, NH 3 and of aerosol particles (NO 3 − and NH 4 +), along with measured concentrations of NO 3 − and NH 4 + in precipitation, weighted by precipitation amounts, were interpolated with a properly used Kriging technique on a 10 × 10 km grid covering the whole Czech Republic. Wet deposition was derived from concentration values for NO 3 − and NH 4 + in precipitations and from precipitation amounts. Dry deposition was derived from concentrations of gaseous components and aerosol in the air, and from their deposition velocities. A resistance model was used for calculating NO x, NH 3 deposition velocities; deposition velocities of HNO 3 were derived from the literature and deposition velocities of particles were parameterized. The annual average NO y deposition in the Czech Republic in 1994 was estimated at 972 eq H + ha −1 a −1 and the annual average NH x deposition at 887 eq H + ha −1 a −1.

Chemical Dynamics of Persistent Organic Pollutants: A Sensitivity Analysis Relating Soil Concentration Levels to Atmospheric Emissions

A dynamic modeling framework is presented that provides a method to estimate deposition of air emissions in soil, based on either single or multiple time-point soil measurements. These dynamic estimates, made in the context of the uncertainty and variability associated with environmental systems, differ significantly from those based on steady-state single-valued assumptions. A case study on the global mass balance for polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) illustrates that, by explicitly representing the uncertainty of the parameters used to assess mass balance, there exists a discrepancy between deposition and source emissions of between 6- and 20-fold. The uncertainty in these soil deposition estimates was most strongly dependent on two landscape properties (the deposition velocity of air particles and the thickness of the atmosphere) and three chemical properties (the organic-carbon partition coef ficient, chemical degradation within the atmosphere, and vapor pressure). Usin...

Requirements for contamination control in the gigabit era

The effect of contamination, especially particles and organic materials in cleanrooms, on future device manufacturing was estimated. The number of particles deposited on wafer surfaces was calculated based on particle size distribution in real cleanrooms and the reported data on particle deposition velocity. DRAM yield trend was then calculated taking only particles from the cleanroom environment into account. Killer particle size is assumed to be one-third of the feature size according to the SIA roadmap. In the Gigabit era, a class 0.1-0.01 level environment will be necessary even with a shortened TAT (turn around time). The lower limit of particle density in conventional cleanroom air was estimated to be class 0.1-1 Level by particle generation by people, suggesting the use of a mini-environment fab system to produce high-grade cleanliness. A mini-environment system is also preferable for eliminating other contaminants such as organic compounds. However, box material and additions must be reexamined from the viewpoint of organic contamination control because some organic materials generated from wafer box easily adsorb on silicon surfaces and change the surface conditions.

Dry Deposition of Nonspherical Particles on Natural and Surrogate Surfaces

ABSTRACT Deposition velocities of prolate and oblate spheroids and dust particles to a smooth surrogate surface, a surface with microroughness and to a sea surface are obtained by a two-layer model, consisting of constant flux layer and deposition layer submodels. The deposition layer model includes the mechanism of interception. Interception depends on particle shape and aspect ratio. Deposition velocities of nonspherical particles and of the equivalent spheres and the ratio between these velocities, the “shape effect ratio,” were calculated for a range of aerodynamical diameters (0.001 to 100 /μm), particle shapes (particle aspect ratio from 3 to 10), particle density (0.1–10 g/cc), wind speed (5–20 μs), and height of microroughness elements (0–22 μm). Interception is most significant for particles with large particle aspect ratios depositing to a smooth surface, under conditions of strong turbulence. Spray droplets from white caps on a sea surface or microroughness on a land surface reduce the effect o...

Temporal and spatial distributions of dust and its deposition to the China Sea

Atmospheric concentrations of aluminium, an indicator of dust substances, have been determined in a set of high-volume aerosol particle samples collected at different locations over continental China and over the China Sea. High concentrations of dust were observed in northern continental China, and at certain locations such as Beijing dust may include an anthropogenic fraction. The mass particle-size distributions of dust varied depending on its distance from source regions, with the mass median diameter for Al of ~1.6-5.9 μm at Beijing in northern China and ~ 1.9 μm over off-shore areas of the East China Sea. Model-predicted mean dry deposition velocities of dust particles are from 1.4 to 4.8 cm s-1 over northern continental China and from 1.4 to 2.1 cm s-1 over the China Sea. Atmospheric deposition models have been applied to estimate the atmospheric fluxes and deposition of dust at different locations. The estimated atmospheric flux of dust at Xi’an of the Loess Plateau is 25 (4.9 to 44) g m-2 mo-1 which is the highest among the regions we studied. The estimated present-day dust flux is comparable to the late quaternary records of eolian dust accumulation at this site. The total atmospheric deposition of dust to the China Sea is 67 Tg yr-1, accounting for 14% of the total atmospheric deposition of dust to the entire North Pacific. With such a high deposition rate, Asian dust may play an important rôle in biogeochemical cycles of trace substances in the Asia/North Pacific region.

Technique for measuring the deposition velocity of particulate matter to building surfaces

The deposition velocity of particles to a model surface has been determined experimentally in a wind tunnel by producing an aerosol consisting of monodisperse spheres containing a fluorescent substance. Simultaneously to the deposition a diluted sample of the aerosol is produced on a Nuclepore filter. By counting the particles on the deposition surface and the filter the deposition velocity can easily be determined.

Numerical simulation of thermophoretic particle decontamination in clean rooms

In order to develop strategies for minimizing deposition of contaminant particles of diameters ranging from 0.1 to 1.0 μm on a wafer, the effect of thermophoresis on a particle deposition velocity was numerically studied. The angle between wafer surface and direction of free-stream flow was introduced as a system parameter. Convection, diffusion, sedimentation, and thermophoresis were included as particle transport mechanisms. Similarity transform was applied to the model equations and obtained equations with dimensions reduced by one. The results suggest that it is possible to enhance the removal of particles of diameter ranging from 0.1 to 1.0 μm by heating with a temperature difference of 10–30°C between wafer surface and the air stream. If the filter of a clean room removes well around 0.1 μm sized particles, the free-slream velocity or flow angle should be increased for the effective removal of particle by thermophoresis, but, if the filter is efficient in removing particles around 1 μm, the free-stream velocity or flow angle should be decreased.

Particle Deposition from Natural Convection Enclosure Flow Onto Smooth Surfaces

ABSTRACT Deposition can be an important fate for airborne particles in indoor environments. The effects of particle size (diameters: 0.1, 0.5, 0.7, 1.3, and 2.5 μm), surface orientation, and surface-air-temperature difference (±10 K and ±1.5 K) on particle deposition velocity have been studied experimentally in a 1.22 m × 1.22 m × 1.22 m aluminum chamber. Monodispersed ammonium fluorescein particles were deposited onto the chamber surfaces under natural convection flow conditions and then extracted to determine particle deposition at seven locations along each surface. For horizontal surfaces, gravitational settling was the dominant factor for particle diameters greater than 1 μm. For vertical surfaces, several factors significantly influenced deposition. Excluding near-corner areas, the average deposition velocities on cool vertical walls varied from a maximum of 5.8 × 10−5 m s−1 for 0.1 μm particles (surface-to-air temperature difference of -10 K) to a minimum of 5.3 × 10−7 m s−1 for 1.3 μm particles (-...

Numerical Analysis of Particle Deposition onto Horizontal Freestanding Wafer Surfaces Heated or Cooled

ABSTRACT Numerical analysis was performed to characterize the particle deposition behavior on a horizontal freestanding wafer with thermophoretic effect under the turbulent flow field. A low Reynolds number turbulent κ-e model was used to analyze the turbulent flow field around the water. The deposition mechanisms considered were convection, Brownian and turbulent diffusion, sedimentation, and thermophoresis. The averaged particle deposition velocities and their radial distributions for both the upper and lower surfaces of the wafer were calculated from the particle concentration equation in an Eulerian frame of reference. When the wafer is unheated, in the diffusion-controlled deposition regime with particle size dp < 0.1 μm the averaged particle deposition velocity under the turbulent flow was about 1.3 times higher than the laminar flow case, and the local deposition velocity near the center of the wafer was high equivalent to that near the edge. The particle deposition on the lower surface was compara...

Assessing Requirements for Wafer Environment Control

A methodology for assessing wafer environment control (WEC) needs based on process specific defect density (D0) goals, actual process and airborne particle levels, particle deposition velocity and wafer exposure time, as well as risks associated with changes in these factors is presented. The motivation is to ultimately determine cost-effective wafer isolation technology (WIT) strategies needed to achieve the required levels of particulate contamination at the wafer surface. The means of extending the methodology to other potentially yield-limiting nonparticulate airborne contaminants is also discussed.

Dry Deposition of Trace Elements to Lake Michigan: A Hybrid-Receptor Deposition Modeling Approach

A hybrid-receptor deposition modeling approach was utilized to estimate the dry deposition flux of size segregated particles to Lake Michigan during the Lake Michigan Urban Air Toxics Study (LMUATS) from July 8 to August 9, 1991. The model estimated the flux of trace elements associated with particles in the fine (<2.5 μm) and PM10-coarse (2.5-10 μm) size range to Lake Michigan using ambient concentration measurements. The ensemble average deposition velocities for fine particles traversing Lake Michigan were found to vary considerably, by factors of 3-15. The average deposition velocities for PM10-coarse particles vary by factors of <1-3. The ratios of observed to estimated ambient concentrations ranged from 0.75 to 2 for fine fraction elements and from 0.8 to 3.6 for elements in the PM10-coarse fraction. The ratios of the estimated deposition fluxes of Ca and Pb with those measured by surrogate surfaces over Lake Michigan during the same period ranged from 0.75 to 1 for the fine fraction and from 1.4 to 2.2 for the PM10-coarse fraction. Si (517 x 10 3 kg) had the highest deposition flux among trace elements primarily of soil or crustal origin, followed by Ca (250 x 10 3 kg), Fe (208 x 10 3 kg), Al (143 x 10 3 kg), and K (73 x 10 3 kg). Trace elements primarily of anthropogenic origin had the following deposition fluxes : S, 53 x 10 3 kg ; Zn, 3.8 x 103 kg ; Mn, 2.4 x 10 3 kg ; Cu, 2.2 x 10 3 kg ; Pb, 1.01 x 10 3 kg ; Se, 1.0 x 10 3 kg ; Ni, 0.86 x 10 3 kg ; Cr, 0.5 x 10 3 kg ; and Br, 0.35 x 10 3 kg. This paper discusses the effect of mi-crometeorological parameters, particle size distributions, water wave dynamics, and the type and location of measurement sites on the estimate of dry deposition fluxes.

Measurements and Control of Particle Deposition Velocity on a Horizontal Wafer with Thermophoretic Effect

To investigate positive and negative thermophoretic effects for polystyrene latex (PSL) spheres of diameter between 0.3 and 0.8 μm, the average deposition velocity toward a horizontal wafer surface in vertical airflow is measured keeping the wafer surface temperature different from the surrounding air temperature. The temperature difference ranges from -10° to 4°C. The number of particles deposited on a wafer surface is obtained from the measurements by using a wafer surface scanner (PMS SAS-3600). Experimental data of particle deposition velocity are compared with those given by the prediction model to validate the model. Since thermophoresis changes greatly the particle deposition velocity, temperature difference necessary for the particle deposition velocity to remain under a given value is sought as a means to control the deposition velocity. The minimum temperature differences required to keep the average deposition velocity smaller than 10−4cm/s and 10−5 cm/s in a clean room environment are suggeste...

The Development of a New Thermophoretic Precipitator for Scanning Transmission Electron Microscope Analysis of Ultrafine Aerosol Particles

A thermophoretic precipitator specifically for collecting ultrafine aerosol samples onto electron microscope support grids has been designed, and built. The precipitator has been designed to deposit particles of diameter less than 100 nm discretely onto support grids, with uniform deposition velocity across the size range. In addition, it has been designed to be compact, and portable. Preliminary investigations indicate it to give discrete deposits suitable for single particle analysis. Qualitatively, particle deposition velocity appears uniform between 4 and 30 nm, with a slight decrease towards higher diameters, although this is yet to be confirmed by comparison with reference particle size distribution analysis methods. Particle distribution on the microscope grid was shown to be uneven on a millimeter scale, but relatively even on a micrometer scale, enabling good characterization of the deposit.

The Landes experiment: Biosphere‐atmosphere exchanges of ozone and aerosol particles above a pine forest

An experiment was conducted in a pine forest in southwestern France during late spring 1992. The aim was fourfold: testing various flux measurement methodologies for chemically reactive species; quantifying the exchanges between the forest and the atmosphere; analyzing the involved mechanisms; and studying their influence on the chemistry of the surface boundary layer. This paper presents preliminary results obtained on the dry deposition of ozone and submicronic aerosol particles, measured using eddy correlation. Once properly normalized, the spectra and cospectra of all scalar species exhibit universal shapes over the whole frequency range. However, evidence is provided that under some meteorological conditions the time series of turbulent variables can be affected by nonstationary trends, or low‐frequency fluctuations that do not contribute to vertical transfer but whose presence can induce large errors in the calculated fluxes. The time variations of the deposition velocities for ozone and aerosol particles are then presented over 2 days with different meteorological conditions. The deposition velocities are shown to be consistent with other reported studies. Dry deposition of ozone appears to be mainly governed by the stomatal resistance, whereas friction velocity and atmospheric instability in the boundary layer seem to govern the deposition of aerosol particles.