Transport Of Airborne Contaminants Research Articles

Rapid simulation of airborne contaminant transport: Coupling concentration response factor method into a Markov chain model

The Markov chain model has been proved viable for the airborne contaminant transport simulation. However, its performance should be further validated and improved when there is a contaminant source in the airflow field. This study proposed a novel method that couples the concentration response factor method into a Markov chain model, termed the CRFMC method, to simulate the release and transport of airborne contaminant. The accuracy of the proposed CRFMC method was validated based on two example flow fields, i.e., a 2D isothermal case and a 3D ventilated chamber case. Different contaminant source types, including point, line, area, and volume source (in 3D case only) with constant, linear, exponential, and periodic release strengths were considered. Furthermore, the influence of the pulse type in the CRFMC method on accuracy was evaluated. Results revealed that the CRFMC method is sufficiently accurate for the airborne contaminant transport simulation with a source release. The L2 relative error between the CRFMC method and scalar transport simulation ranges from 9.4% to 16.1% in 2D cases and from 14.9% to 33.4% in 3D cases. This study can aid the development of a Markov chain model for airborne contaminant transport.

Transient tracer gas measurements: Development and evaluation of a fast-response SF6 measuring system based on quartz-enhanced photoacoustic spectroscopy.

This study aims to develop a fast-response sulfur hexafluoride (SF6 ) measuring system, and evaluate its performance in tracer gas measurements for studying transient airborne contaminant transport. The new system is based on a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor using a quantum cascade laser. Transient SF6 tracer gas measurements were carried out in an environmental chamber with an instantaneous source using both the QEPAS system and a traditional commercial instrument. Real-time SF6 concentrations, peak SF6 concentrations and average SF6 concentrations for one room time constant under two air change rates obtained by the two instruments were compared. The results show that the QEPAS system, which features a 0.4 s data acquisition interval, can provide detailed real-time SF6 concentrations even when the concentration is changing rapidly. The QEPAS system successfully captured the peak SF6 concentrations for all the studies cases, while commercial instrument failed in most studied cases. In most of the cases, the two instruments obtained similar average SF6 concentrations for one room time constant. However, when the concentration was in rapid change, the two systems would report significantly different results. The QEPAS system can be potentially applied in transient tracer gas measurements under complex scenarios.

Eulerian numerical modeling of contaminant transport in Lower Manhattan, New York City, from a point-source release under the dominant wind condition: Insights gained via LES

Released pollutants, biological agents, and poisonous chemicals in populated urban areas can spread via wind flow and affect the safety and welfare of those close to the source and downstream. Being able to predict the transport of airborne contaminants in such urban areas could help stakeholders to plan emergency responses. Airflow due to the wind in urban areas with closely packed buildings results in a highly turbulent flow field around buildings that results in the transport of contaminant plumes. We conduct high-resolution large-eddy simulation (LES) of airflow of the dominant wind in the south to north direction with a mean-flow velocity of 3.58 m/s in a portion of Manhattan, New York, which is 2.5 km long in the north-south direction, 1.8 km wide in the east-west direction, and 0.6 km high, located at its southern tip. The study area is a highly-populated area with densely packed buildings in Manhattan. The LES captures turbulent flow dynamics around the buildings, which vary in height from several stories to skyscrapers, and, consequently, the transport of contaminant plume propagating downstream from a source point, which is located on the east side of the New York Stock Exchange building at 30 m above ground. The airflow tends to align with the streets that are reasonably parallel to the wind direction, creating high-velocity core regions. For as long as the source of contamination exists, the pollution plume rapidly travels through the streets, reaching downwind regions of the study area in about 20 min. Once the pollutant source is removed, the pollution plume propagates out of the study area. However, it takes about 40 min for the pollution to completely exit the study area due to the contaminants trapped in the low-momentum urban regions, such as open areas between tall buildings and narrow streets, which take longer to either exit the study area.

Isotopic Analyses Fingerprint Sources of Polycyclic Aromatic Compound-Bearing Dust in Athabasca Oil Sands Region Snowpack.

Fugitive dust associated with surface mining activities is one of the principal vectors for transport of airborne contaminants in Canada's Athabasca oil sands region (AOSR). Effective environmental management requires quantitative identification of the sources of this dust. Using natural abundance radiocarbon (Δ14C) and dual (δ13C, δ2H) compound-specific isotope analysis (CSIA), this study investigated the sources of dust and particulate-bound polycyclic aromatic compounds (PACs) deposited in AOSR lake snowpack. Lower Δ14C values, higher particulate and PAC loadings, and lower δ13C values for phenanthrene and C1-alkylated phenanthrenes/anthracenes (C1-Phen) at sites closer to the mining operations indicated unprocessed oil sand and/or petroleum coke (petcoke-a byproduct of bitumen upgrading) as major sources of anthropogenic fugitive dust. However, a Bayesian isotopic mixing model that incorporated both δ13C and δ2H could discriminate petcoke from oil sand, and determined that petcoke comprised between 44 and 95% (95% credibility intervals) of a C1-Phen isomer at lakes <25 km from the heart of the mining operations, making it by far the most abundant source. This study is the first to demonstrate the potential of CSIA to provide accurate PAC source apportionment in snowpack and reveals that petcoke rather than oil sand is the main source of mining-related particulate PACs deposited directly to AOSR lakes.

Calculation method of state transfer matrix in Markov chain model for airborne contaminant transport: Investigation and improvement

Rapid contaminant transport simulation is important in emergency scenarios, and Markov chain models have shown promise in this regard. The state transfer matrix (STM) is the core of the Markov chain model and determines the simulation accuracy and computing cost. However, existing methods to calculate the STM result in significant errors or large computing costs. Using set theory, the characteristic form of the STM was investigated according to the continuity equation for incompressible fluids. Based on this form, the calculation method of the STM when the initial contaminant is distributed uniformly was improved. The performances of the original and modified methods were compared via a case study. In addition, the influence of underlying airflow grid resolution on model performance was analyzed. Finally, sensitivity analysis was conducted to determine the dominant factor. The results revealed that the STM should be constructed as an approximate doubly stochastic matrix; however, this is problematic due to the discrete underlying airflow and the associated interpolation. As an alternative, a left stochastic matrix is appropriate, with higher accuracy when the initial contaminant is uniformly distributed over a large area. Increasing the underlying airflow grid resolution can improve the model accuracy; however, a minimum grid resolution with a credible velocity field is sufficient, especially considering the computing cost. Sensitivity analysis showed that the Markov state size is the dominant factor for the accuracy and computing cost, and it should be carefully selected. This study can aid the development of a Markov chain model for airborne contaminant transport.

Validation and assessment of the CFD-0 module of CONTAM software for airborne contaminant transport simulation in laboratory and hospital applications

Hospital and laboratory buildings are designed with intrinsic features for infection control, and are related to an intensive energy use. Although many studies suggest that insufficient ventilation increases the risk of disease transmission in the indoor environment, significant questions still remain on the ventilation requirements for airborne infection control. ASHRAE proposes high research priority on the study of indoor flow regimes, room pressurization and filtration, for hospitals and laboratories where infectious diseases agents are handled. The objective of this paper is to promote an assessment and validation of the CFD-0 module of the CONTAM software, for the simulation of airborne contaminant transport in hospital and laboratory applications. These objectives are justified by the fact that this public domain code, supported by the NIST, may be an important tool for studying the role of ventilation parameters in infection control for hospital and laboratory settings. Three benchmarks were selected for that task: the ASHRAE-RP 1271, geared to challenge the code ability in simulating the complex indoor airflow features, such as jets, separation, impingement, and thermal plumes, that arise in room transitional non-isothermal confined flow; A benchmark that resembles real laboratory spaces; and a third benchmark, that resembles real hospital isolation rooms. Results demonstrated that CFD-0 provided, at least, a marginally acceptable performance, for mixed and displacement ventilation modes. The prediction results are more meaningful than those using the “perfect mixture” assumption, for assessments on ventilation performance and personnel exposure to hazardous substances in these spaces.

Numerical investigation of airborne contaminant transport under different vortex structures in the aircraft cabin

Airborne contaminants such as pathogens, odors and CO2 released from an individual passenger could spread via air flow in an aircraft cabin and make other passengers unhealthy and uncomfortable. In this study, we introduced the airflow vortex structure to analyze how airflow patterns affected contaminant transport in an aircraft cabin. Experimental data regarding airflow patterns were used to validate a computational fluid dynamics (CFD) model. Using the validated CFD model, we investigated the effects of the airflow vortex structure on contaminant transmission based on quantitative analysis. It was found that the contaminant source located in a vorticity-dominated region was more likely to be “locked” in the vortex, resulting in higher 62% higher average concentration and 14% longer residual time than that when the source was on a deformation dominated location. The contaminant concentrations also differed between the front and rear parts of the cabin because of different airflow structures. Contaminant released close to the heated manikin face was likely to be transported backward according to its distribution mean position. Based on these results, the air flow patterns inside aircraft cabins can potentially be improved to better control the spread of airborne contaminant.

Experimental analysis of the transport of airborne contaminants between adjacent rooms at different pressure due to the door opening

Today it is common practice to try and obtain airborne contamination control through pressurization-depressurization areas. Moreover, there is some qualitative evidence that turbulence induced by the operation of a door between different pressure areas, could overcome the differential pressurization effect, and cause a pouring effect between zones, and consequently a contamination. The paper investigates the described matter through an experimental setup, with a scale physical model. Obtained results confirm that door operation is able to produce a dirty air transfer in the clean room, and that transfer entity is almost independent from differential pressure and flow rate imbalance, at least for the experimentally tested values, while it appears strongly related to air volume displaced in the door opening operation, and has the same order of magnitude of it.

Contaminant source identification within a building: Toward design of immune buildings.

The level of protection of a building against the intentional or accidental release of chemical agents is crucial. Both scenarios could endanger life and safety of the buildings occupants. Equipping buildings with appropriate chemical sensors can alert the building occupants about the contaminant release. The readings of these sensors can be employed to trace the location of release, and help to take the appropriate actions to minimize the casualties. However, only a limited number of them can be installed due to their initial and operating cost. Moreover, there is no information about the source strength, release time and possible source location. This paper reports the development of a methodology to identify the source location using sensors reading from limited locations. The methodology uses the artificial neural network (ANN) as a statistical analysis integrated with a multi-zone airborne contaminant transport model, CONTAM. To evaluate the applicability of this method, the contaminant dispersion within a building was modeled and the results were integrated to an ANN for the source identification. The prediction made by the trained ANN was then evaluated by predicting the source of the contaminant in 40 extra cases, which had not been seen by the network during the training session. The model was able to predict the source location in more than 90% of the cases when the building was monitored by three or more sensors. The results show that the method can be used to help building designers decide the optimum configuration of the sensors required for a space based on the accuracy level of the source detection.

The Mathematics of Atmospheric Dispersion Modeling

The Gaussian plume model is a standard approach for studying the transport of airborne contaminants due to turbulent diffusion and advection by the wind. This paper reviews the assumptions underlying the model, its derivation from the advection-diffusion equation, and the key properties of the plume solution. The results are then applied to solving an inverse problem in which emission source rates are determined from a given set of ground-level contaminant measurements. This source identification problem can be formulated as an overdetermined linear system of equations that is most easily solved using the method of least squares. Various generalizations of this problem are discussed, and we illustrate our results with an application to the study of zinc emissions from a smelting operation.

Fast, Accurate Defense for Homeland Security: Bringing High-Performance Computing to First Responders

An urban-oriented emergency assessment system, called CT-Analyst ® was developed to evaluate airborne contaminant transport threats and to aid in making rapid decisions for complex-geometryenvironmentssuchascitieswherecurrenttransportanddispersionmethods are slow and inaccurate. Contaminant transport-Analyst was designed for the military prior to 9/11 to incorporate verbal reports, to treat systems with mobile sensors, and to function in realistic situations where the nature, amount, and source location of an airborne contaminant or a chemical, biological, or radiological agent is unknown. Thus contaminant transport-Analyst is well suited to urban defense in the Homeland Security context. Contaminant transport-Analyst gives good accuracy and much greater speed than possible with current alternatives because is it based on entirely new principles and designed to function in information-starved situations, characterizing the first few minutes of a terrorist or accident scenario, where alternate technologies do not. These advantages derive from pre-computed data structures based on three-dimensional large-eddy simulation computational fluid dynamics that includes solar heating and buoyancy, complete building geometry specification,trees,andimpressedwindfluctuations.Afewdetailedurbanaerodynamicssimulations are pre-computed for each coverage region when contaminant transport-Analyst is installed. These results extend to all wind directions, speeds, likely sources and source locationsthroughanewdatastructurecalledDispersionNomografs™.Thusahigh-performance computing based system can generate Nomografs for cities, military bases, industrial complexes,andotherpotentialdangerareaswellinadvance,removingtheneedfortheemergency first responders and warfighters to wait for supporting analyses. Furthermore, since the full power of high-performance computing is available for the pre-computations, contaminant transport-Analyst provides important, new, real-time, zero-latency functions such as sensor data fusion, backtracking to an unknown source location, and evacuation route planning directly to the first responder. Thus the Department of Homeland Security can avoid the delays and uncertainties of reachback to high-performance computing modeling resources and the associated support and communications infrastructure currently required during airborne contaminant transport emergencies.

Analysis of indoor air pollution trends and characterization of infiltration delay time using a cross-correlation method

High-density housing in close proximity to freeways in conjunction with high concentrations of traffic emissions may contribute to significant degradation of indoor air quality. Densely populated areas may also be targeted for intentional releases of biological or chemical agents because an urban release could result in higher morbidity and mortality from the attack. Since people tend to spend the majority of their time indoors, it is paramount to explore the relationships between outdoor and indoor air quality and, specifically, the time scales that characterize transport of airborne contaminants from outdoors to indoors. In the Brooklyn Traffic Real-Time Ambient Pollutant Penetration and Environmental Dispersion (B-TRAPPED) study, a three-story row house with a flat face and roof and multiple rooms was used to investigate outdoor-to-indoor contaminant time scales. The building was located in the Sunset Park neighborhood of Brooklyn, NY, USA, in the vicinity of a major expressway and a heavily trafficked arterial road. It was found that the building shell has a profound impact on the indoor concentrations. A strong hourly periodicity (see Eisner et al., this issue, DOI: 10.1039/b907132f) in concentration outside the building during the morning "rush hour" was used as evidence to suggest that indoor contaminants originated from outdoor air penetration. Although the indoor concentrations followed a similar pattern, indoor concentrations were found to be more persistent than outdoor concentrations. Stronger persistency is used here to describe the tendency of the indoor concentration to continue to rise even if the outdoor concentration has started to drop, or vice versa. This may be an important factor in assessing negative health risks to inhabitants or first responders. A cross-correlation technique was employed to study the correlation between outdoor and indoor time series. In the high-density housing residential building used in the study, it was found that a long lag time exists (11 min) before indoor and outdoor concentrations reach maximal correlation.

A one-dimensional analytical model for airborne contaminant transport in airliner cabins

Quick information on airborne infectious disease transmission in airliner cabins is essential to reduce the risk of infection of passengers and crew members. This investigation proposed a one-dimensional analytical model that can predict the longitudinal transmission of airborne contaminants or disease viruses inside an airliner cabin. The model considered both diffusive and convective transport of contaminants in the longitudinal direction of the cabin but assumed complete mixing of contaminants in the cabin cross-section. The effect of recirculation of the cabin air and efficiency of the high-efficiency particulate air (HEPA) filters is also considered in the model. The analytical solution for the one-dimensional contaminant transport model is obtained by using the principle of superposition and the method of separation of variables. The analytical solutions agree well with the computational fluid dynamics (CFD) results. The coupling of a CFD model with the one-dimensional analytical model could capture the impact of local airflow on contaminant transport. This analytical model has been used for analyzing contaminant transport in a 30-row all-economy-class airliner cabin with minimal computing effort. The paper presents a new one-dimensional analytical model that can provide quick information on global airborne contaminant transmissions in airliner cabins for effective response plans. The model can be used to study the effects of air exchange rates, recirculation, efficiency of the high-efficiency particulate air (HEPA) filters and longitudinal airflow on airborne contaminant transport in airliner cabins with minimal computing effort.

Large Scale Urban Contaminant Transport Simulations With Miles

Airborne contaminant transport in cities presents challenging new requirements for computational fluid dynamics. The unsteady flow involves very complex geometry and insufficiently characterized boundary conditions, and yet the challenging and timely nature of the overall problem demands that the turbulence be included efficiently with an absolute minimum of extra memory and computing time requirements. This paper describes the monotone integrated large eddy simulation methodology used in NRL’s FAST3D-CT (CT is contaminant transport) simulation model for urban CT and focuses on critical validation issues that need to be addressed to achieve practical predictability. Progress in validation studies benchmarking with flow data from wind-tunnel urban model simulations and actual urban field studies are reported. Despite inherent physical uncertainties and current model tradeoffs, it is clearly possible to achieve some degree of reliable prediction.

The effect of freestream turbulence on the transport of particles in the vicinity of a blunt flow obstacle

This paper describes the effects of intermediate and large freestream levels of turbulence (intensity and integral length scale) on the dispersion of fine particles in the near wake regions of long flat plates placed normal to the freestream. The work is particularly relevant to the transport of airborne contaminants in complex flows, especially like those encountered in many ambient and occupational atmospheric environments. An experiment was designed based on the release of fine aerosol particles upstream of the obstacle and the measurement of the concentration decay for the entrained aerosol in the near wake region after the source generation has been suddenly interrupted. Five grids were employed for generating specific levels of freestream turbulence characterized by the parameter, Λ f (defined as l f k 1/2 f /UD , where l f and k f are the length scale and the kinetic energy of the free stream turbulence, and U and D are the freestream velocity and the plate width, respectively). The investigation covered the ranges 1500⩽ Re⩽11,000 and 0.0015⩽ Λ f ⩽0.071. The dimensionless residence time, H (defined as τU/D, where τ is the time constant for the observed decay of the aerosol concentration in the near wake cavity), appeared to be independent of the freestream turbulence for values of Λ f between 0.0015 and 0.013, for which the average H-value was about 7.2±1.5. For larger freestream turbulence, corresponding to Λ f greater than 0.018, H increased up to about 10. A visualization procedure was used to capture some of the qualitative features of the dispersion process for aerosols in the near wake regions of the plates and provide some support for the physical mechanisms suggested by this work. In particular, by comparing the results with those of other studies, we note the marked contrast in the effects of the freestream turbulence for aerosol flows about two- and three-dimensional bodies, respectively.

Mass Balance between Emission and Deposition of Airborne Contaminants

Quantification of (1) the total mass of airborne pollutants deposited around pollution emitters, (2) the contaminated radius around these point sources, and (3) the proportion of emissions available for long-range transport remains chal lenging for environmental scientists. Here, we present a simple method for comparing yearly airborne emission figures from any point source with observed deposition estimates from the surrounding area. First, the deposition−distance function D(x) around the smokestack must be defined. This requires the empirical determination of total (water soluble + particulate) annual deposition at several locations away from the source and of the extent of the smokestack “shadow” zone. Secondly, the loading function L(x) is obtained by integrating D(x) around the point-source. This allows estimation of the yearly mass of a contaminant deposited within an area corresponding to that of a circle of radius x around the point source. Thus, emission, deposition, and (long-range) transport of airborne contaminants can be assessed quantitatively. Preliminary results from the Kola Peninsula, Russia, suggest that deposition of trace metals overwhelmingly occurs within 200 km from the source.

Field Evaluation of an Empirical—Conceptual Exposure Model

Abstract Empirical-conceptual models relate exposure to various process parameters responsible for the generation and transport of airborne contaminants. If the model includes all process parameters important to defining the exposure, it has the potential for a priori exposure prediction. This research tested the ability of an empirical-conceptual model of a spray painting task to predict worker exposures in the field. The model relates paint mist concentrations to spray booth ventilation rates and other process parameters, including spray gun nozzle pressure, paint viscosity, mist generation rates, and worker orientation to the booth freestream. Eight workers in a paint shop were sampled over a 5-week period. A total of 55 tasks were sampled; 40 percent of the measured task exposures are within the estimated experimental error of the model prediction and 71 percent are within a factor of three. Excluding tasks that either did not fit the model assumptions well or where a sampling error may have occurred ...

CFD experiment characterization of airborne contaminant transport for two practical 3-D room air flow fields

With the increasing concern for, and focus on, indoor air quality, determination of the distribution of airborne contaminants within rooms becomes of great interest. Computational fluid dynamics (CFD) techniques offer a means for simulation of contaminant transport processes, with quantitative richness of detail rarely possible in laboratory testing. Results of such “CFD experiments” are documented herein for two practical, low turbulence-level, thermal room air flow fields. The supply flow is tagged with a distinguishable species, and the resulting time-dependent concentration evolution throughout the room is predicted. The simulated full-scale room air flow field Reynolds numbers are 15,000 and 30,000, the Archimedes number ranges from 0.83 to 4.3, and simulation turbulent Schmidt number ranges an order of magnitude.

The transport and dispersion of airborne contaminants in boundary layers over the ocean and an isolated island cape

A series of sixteen atmospheric tracer experiments using sulfur hexafluoride (SF6), chemical smoke and meteorological balloons was conducted to explore the transport of airborne contaminants in the boundary layer over the ocean surface and in the separating boundary layer over an isolated island cape. The immediate objective of the tests was to determine the impact of local pollutant sources on a background air quality sampling program conducted in the South Pacific from elevated towers on Tutuila Island, American Samoa. In addition to satisfying this objective, the tests are of interest in that they illustrate the local behavior of pollutants in a complex natural atmospheric flow. Offshore tracer tests indicated that the crosswind dispersion of pollutants over the ocean surface can be approximately modeled using the simple Gaussian plume model. The observed crosswind dispersion of the tracer corresponded to that expected under neutrally stable atmospheric conditions, consistent with the near equilibration of the ocean surface and air temperature in the South Pacific. Local, or near-field, tests indicated that tracer released into the wake downwind of the leading edge of the cape mixed rapidly to a height of about 8 m above the surface (i.e., 30–40% of the cape height). Due to decoupling between the boundary layer over the cape and the freestream flow, however, very little of the tracer was observed above this height. This suggests that the impact of local pollutant sources (i.e., on the cape) would be minimized if the proposed sampling towers were elevated significantly above an 8 m altitude (e.g., twice that height).

SIGNIFICANCE AND CONTROL OF AIR-BORNE CONTAMINATION IN MILK AND FOOD PLANTS

Summary The control or elimination of air-borne contamination in the food industry provides many distinct advantages. Obtaining maximum shelf-life of packaged non-sterile food products, gaining full economic advantages of continuous sterilization of fluid food products and maintaining overall high quality levels are among the advantages. By control or elimination of sources such as floor drains ventilation systems and plant workers, populations of air-borne microorganisms can be reduced significantly. Recent results reveal that 68% of the air-borne bacteria counts in packaging areas of a dairy plant are greater than 6 per ft3. Nearly 45% of the air-borne bacteria are associated with particles between 2.0 and 5.5 microns. Contamination control can be accomplished by preventing transport of air-borne contaminants from source to point of contamination. Air turbulence is one of the primary mechanisms contributing to transport. Laminar flow of filtered air has provided effective control under experimental situations.