Health Physics: Particulate Pollution in Cities

Introduction

An aerosol is a substance that presupposes the existence of a gaseous medium. It comprises tiny particles that can be either liquid or solid. They are known as particulate matter (PM), elements that pollute the air. The general public is aware of the fact that their health depends on the quality of air, but they cannot control it. The most vulnerable population includes people living in urban areas, as they are under the constant influence of air contamination after the industrial revolution. The number of motor vehicles increases, making issues related to urbanization and the industrial revolution even more critical than they used to be. As a result, much attention is paid to Particulate matter (PMs) pollution. Those professionals who follow environmental policy discuss PM thoroughly. They believe that it is the cause of air quality issues. In addition to that, the adverse influences of PM on people’s lives within urban territories became more critical as contamination became more criminal. Among the main reasons for this tendency is the fact that the number of exhaust emissions increased because people cannot imagine their lives without a car. However, less than 50 years ago, the situation seemed to be better. It was not because of the better technology but due to the reduced pollution. Currently, various locations are aware of airborne PM and the fact that their size distribution and composition (altered in greater oxidative cellular damage and toxicological effects. The data revealed in the current studies is rather crucial. Even though more and more professionals become interested in the topic, as they find out the rates of mortality and morbidity improves in this way.

Regardless of the fact that governmental bodies of many countries developed legislation to control the increase of the airborne PM and minimalize it, this issue is still observed in many cities. What is more, scientists, including Longhurst, claim that the ways to cope with air pollution were outlined years ago, but no expected outcome was reached. Recently, the European Environment Agency stated that a lot of countries it deals with have problems with the quality of air. It also emphasized that such conditions had an adverse influence on people’s health. A range of targets was developed for the countries to fulfill step by step and provide the citizens with the opportunity to live in a safe environment. However, professionals from the UK believe that expected results can be observed no earlier than in 2025. Such tendency can be expected by the existence of multiple sources of PM pollution in the UK, particularly traffic contamination. The gravity of the problem is proved by The Committee on the Medical Effects of Air Pollution and WHO, as they state that NO2 and PM2.5 particles turned out to trigger the deaths of numerous human beings. In fact, about 40,000 deaths annually are reported. The representatives of the healthcare bodies claim that the majority of health issues are caused not by diseases but by air pollution. It is currently criticized that the proposed initiatives will be enough to overcome this problem in the UK. However, the fact that effective measures are to be defined is undeniable.

PM pollution can be addressed in several ways. First of all, governments can initiate the reduction of the use of big vehicles. They can also demand minimalizing of the use of manufacturing machines and physical separation of zoning. Unfortunately, it can be rather difficult to implement such changes because both developed and developing countries tend to grow chaotically. That is why individuals can hardly be expected to keep to these restrictions. They are more likely to start breathing masks to prevent direct exposure to PM. Therefore, the best solution to protect people from PM pollution is to find a new method to eliminate PMs directly from the urban area.

Goals and Objectives

The issue of PMs’ management can be investigated in different directions. Mainly, it is advantageous to pay attention to environmental and public health perspectives. Recent research studies state that PM2.5 and PM10 levels are much higher than those defined in standards as permitted ones. The most vulnerable population tends to include the representatives of the developing nations. A solution that can cope with this issue presupposes a critical scientific challenge and economic challenge. Those locations that apply the ‘lax’ policies have a limited amount of options that are rather complex and costly. Therefore, it is significant to approach PM pollution in two ways. Firstly, research should be conducted to quantify chemicals on the basis of their size and total mass concentrations. Secondly, a range of cost-effective recommendations should be developed to benefit the representatives of the general public, ensuring their decent health condition. Thus, the research study discussed in this paper is targeted at finding a solution that can help to reduce PM concentrations in urban areas so that they meet regulatory standards.

Justification of the Current Study

The current study is likely to be rather beneficial because its results will be focused on the direct removal of PMs from the urban environment. It will be possible to point out how air quality alters in different seasons of the year. Various meteoroidal conditions will be aligned with the levels of PM distribution. On the basis of this information, those locations that have the highest concentrations of PM will be pointed out approximately. Thus, it will be possible to implement the required to eliminate the problem measures and to make sure that initial predictions are right. It is also expected that this study will assist professionals in finding the most advantageous places for PM filters. City planners are likely to be highly interested in the results of research because they will help to plan projects for buildings and roads. Those engineers who deal with ventilation systems will also benefit. They will be able to use them as a guide for HVAC inlets and outlets implementation.

The reduction of air pollution was addressed previously as well. For example, in successful reduction of this issue was reported in the 1960s and 1970s. New regulations and the abatement of pollutants, such as Sox (Sulfur Dioxide) and NOx (Nitrogen Oxide), helped for a while, but adverse health effects and deaths were still observed, according to Holland et al. (1979). However, not all scholars support the idea that high levels of PM damage people’s health. In fact, a lot of them proved that even at relatively low concentrations, PMs could lead to adverse outcomes (Shy 1979).

Air filtration is considered to be one of the most effective ways to ensure decent health conditions. In addition to that, it reduces productivity loss. Fisk and Rosenfeld (1979) proved that by using upgraded filters in an office building, expenditures could be reduced. Even though energy penalties and total costs for them tend to be rather high, they are not critical at all when being compared to the money spent by healthcare facilities on a regular basis. All in all, improved filtration is likely to minimalize hospitalization costs. The protection of the individual components of HVAC systems can also be advantageous.

Therefore, the overall benefits of air filtration overcome its costs. Much money can be saved due to decreased morbidity and mortality, but other benefits depend on the stakeholder and the valuation approach. It is presupposed that the advantages of the filtration of outdoor PM are almost similar to those revealed by indoor filtration research. Finally, outdoor air filtering has positive effects on the condition of indoor air.

PM Dispersion Modeling

When analyzing the influence provided by the quality of air, different modeling approaches can be used. An air quality model consists of those elements that are connected with some management or scientific problems. In general, models are often used to develop predictions and solutions for particular issues, such as PM pollutions. They are aligned with the national scale and/or resort to dispersion algorithms. With their help, physical and chemical processes that influence PMs are triggered. Today, these models are usually computerized. Thus, the concentration of PMs can be easily calculated, emission rates, characteristics of the source, and topography identified.

These models are mainly used while working with primary pollutants (PM2.5) that spread in the atmosphere. Secondary pollutants (PM10) that are formed within the atmosphere can also be sometimes analyzed in this way. Models that analyze air quality are critical for the management system, as they allow to control pollution, find its source, and develop strategies to reduce the amount of PMs. They can be used to discuss the effectiveness of the programs developed to ensure human and environmental safety. In order to maintain an effective dispersion modeling study, professionals need to select a proper tool. Choosing a model, professionals need to pay attention to:

• The complexity of dispersion.
• Potential scale.
• The significance of potential effects.

Not so long ago, operational models based on the Gaussian dispersion model were created to analyze air quality. Still, only an integral nature of atmospheric dispersion was taken as a basis for such kind of research. They are known as fast response models that can be easily altered to meet different purposes and maintain various formal evaluations. They allow calculating multiple cases one by one (Hall et al. 2002). The US Environmental Protection Agency (US EPA) creates or just allows to use of these models. It also groups them into four categories:

Ø Recommended or refined models. They are rather detailed and need precise data. They can be used to focus on physical and chemical atmospheric processes. Spatially and concentration can be estimated. Refined models give authoritative and accurate information about the source impact and the effectiveness of control strategies. AERMOD, for example, can be used to analyze different PMs. It is a steady-state plume dispersion model that simulates transport and dispersion. Current information about the atmospheric boundary layer is taken as the basis. Multiple sources from rural or urban areas are considered. Receptors can be in various terrains. The PRIME building downwash algorithms are used for this model. Meteorological data is gathered hourly so that concentrations of PMs are regularly measured. AERMOD can be used for:

• Point, volume, and area sources.
• Buoyant elevated line sources.
• Mobile (line) sources.
• Surface, near-surface, and elevated releases.
• Rural or urban areas.
• Simple and complex terrain.
• Transport distances up to 50km (Cimorelli et al. 2005).

Except for AERMOD, other models can be used:

• CALPUF.
• BLP.
• CALINE3.

1. Alternative models. It is possible to choose the most beneficial model depending on a particular analysis and its purposes. For example, ADAM can be used when investigating the characteristics of buoyant or neutrally buoyant emissions. They can be both gaseous and particulate. In addition to that, ADMS-Urban traffic model can be used.
2. Screening models. They are created to give conservative data about concentrations. They are screening versions of the most appropriate models. For example, the AERMOD model is aligned with a screening version AERSCREEN. Except for that, CTSCREEN can be used.
3. Related programs. This type of program is used when one of the previous models is already utilized. They tend to be a tool of support. The main examples of related programs are AERMAP and AERSURFACE.

Models for PM2.5

Ambient PM2.5 is a mixture of two components: primary and secondary (they were discussed earlier). PM2.5 is spread exactly in the atmosphere while PM10 is formed within it.

One of the modeling systems discussed previously, AERMOD, for example, is used to work with PM2.5 emissions. However, depending on the necessities of the study, other models can be used (EPA 2014). It is significant to remember that AERMOD is not utilized for the secondary formation of these PMs. The way NOx and/or SO2 precursor emission affect secondary PM2.5 can be:

• Qualitative.
• Hybrid (qualitative and quantitative assessments).
• Quantitative (EPA 2014).

Models for PM10

Screening models and refined models are mainly used when dealing with PM10 modeling. Fugitive emissions occur from the dust that is blown by the wind in the atmosphere. As a rule, it comes from fields, dirt roads or deserts, etc. These emissions are caused by industrial processes. Being not captured and vented, they gather from various locations and turn out to be rather complex. That is why some extraordinary cases can be observed that require the design of a separate model. There is a necessity to address appropriate reviewing authority in order to obtain vital information for those cases when modeling was required. As it is rather complicated to characterize and module this kind of dust and PM, such an approach can turn out to be rather efficient and effective. Line, area, or volume sources of emission can be found in arid areas. Mainly, they come from vehicles that drive over dirt and dust-covered roads. It is critical that emission rates may be based on specific information or values discussed in the literature. General requirements for such types of models should be aligned with PM10.

Computational Fluid Dynamics

Computational Fluid Dynamics (CFD) urges the utilization of such a technique that makes professionals practice and assists in the prediction of engineering flows. It was firstly used in the second half of the 20th century. During the last 20 years, CFD simulation has become a technique that professionals often utilize to analyze PM dispersion. The focus is mainly made in urban areas. The CFD simulation method is rather complex and consuming. For example, the transport equation of concentration is to be discussed. This method can be used to gather information regarding the flow and concentration within a particular location. However, if the time for research is limited, it will be more advantageous to resort to the previously discussed methods. It is also rather complicated to implement dispersion processes (such as chemistry) without operational models.

The CFD approaches are separated into such categories:

• Direct Numerical Simulation (DNS).
• Reynolds Averaged Navier-Stokes (RANS) equation modeling.
• Large Eddy Simulation (LES) (Xie & Castro 2009).

Dispersion of PMs around buildings has been studied actively during the last several decades. In this way, a lot of attention was paid to CFD, but its strengths and limitations could have been hardly assessed in the framework of PM dispersion in urban areas because scientists focused on different purposes, conditions, approaches, etc. It is significant to take into consideration the fact that it is tightly connected with two types of flows. The atmospheric flow and the one that exists around buildings interact with one another, involving meteorological and building aerodynamic aspects. That is why those studies that dealt with near-field dispersion have been connected with each of them. Hence, it cannot be denied that such kind of research that deals with all discussed aspects are needed to analyze the issue. Those urban areas that are extremely polluted are typically represented by street canyons. Adverse influences that can be experienced by people living in such areas are rather critical. That is why it is vital to develop a model that can be used to reduce them. Only by knowing contaminant dispersion properties will professionals be able to overcome this issue.

A lot of research studies were conducted by scientists who used modeling and A 3-D street canyon. They consisted of at least two building blocks and were discussed according to the characteristics of the flow regime. Unlike a 2-D canyon, except for height (H) and width (W), length (L) was considered. In this way, the flow formed in this way was more complex and provided an opportunity to obtain data that was more accurate. According to Li et al. (2006), 3-D street simulations allowed the professionals to discuss 3-D flows and influence the mixing of PM concentrations. A range of studies that are focused on arrayed obstacles that are treated as urban buildings exists today (Yee et al. 2006). It was proved that a Gaussian distribution is the most appropriate tool to be used when creating the plume profile of them. In their study, Riddle et al. (2004) found out that the CFD could provide more accurate data related to complex geometry due to the ADMS model in comparison to the operational models. But even the capability of CFD in the framework of other situations should be assessed additionally. For example, dispersion should be calculated within atmospheric stability conditions. Some similar works have been performed already. For instance, studies were conducted with the focus on model performance using the RANS model.

Material and Methods

The research methodology for the discussed study includes several stages. In general, they can be represented as:

• Modeling
• Survey
• Analysis

The attention is paid to the outdoors PM mainly. However, gaseous pollution levels are also considered. In addition to that, those events that affect them are also pointed out. First of all, the characteristic of PM contamination will be discussed. The attention will be paid to both physical and chemical ones. The software will be tested to be sure that the created PM dispersion model is appropriate. The filter technology will also be used due to the necessity to ensure the efficiency of this activity. Statistical software will be needed as well. The analysis of distributions and their levels will be maintained with its help. Finally, the technology will be tested experimentally, and observed results will be compared.

Site Description

Air pollution is a problem that is critical for big cities mainly because they have heavy road traffic. Trying to protect citizens, governmental bodies tend to ensure that the ways to get to the center are limited.

In Aberdeen, three Automatic Urban and Rural Network (AURN) stationary precision monitors are used. The air quality monitoring station (AQMS) in the city center is connected with Union Street. It is the major route that leads to this location, so it is often rather busy. The AQMS can be found at the intersection of two streets, which allows gathering more information. The study revealed that almost 25000 vehicles pass Union Street every day. The majority of them are buses. Similar results are obtained from King Street, which gives an opportunity to approach the center of the city.

Simulation Model

The dynamics of external flows can be installed in different ways. Several patterns exist only within CFD models. SOLIDWORKS Flow Simulation can be used to quantify equations and parameters. In this particular case, the Reynolds-Averaged Navier- Stokes (RANS) equations will be considered. Except for that, PM, ideal gas, and some other elements will be discussed. The improved k-ε turbulence model will be added (SolidWorks flow simulation technical paper 2015). The SolidWorks CFD program will be used to analyze air pollution in the framework of street canyons.

I order to obtain the most accurate results, buildings used in the model will have the same characteristics as those that exist in real life. They will also be positioned accordingly. Thus, it will be advantageous to resort to satellite images obtained from ACC. The ground plate is mentioned just for the picture to look completed. It is not a part of the computational domain.

The computational domain is 1000×554×200m. It is represented in the form of rectangular. The boundaries and coordinate system are aligned with the wind direction that is usually observed in the selected location. A developed turbulent flow profile allows deepening the knowledge of wind tunnel experiments. On the basis of previous studies, it can be claimed that the target building and should be a maximum of 3% with a boundary distance of a minimum of five heights. The outflow boundary, on the other hand, is expected to reach no less than ten heights (Tominaga et al., 2008).

The study considers average seasonal wind directions. Small obstacles and plants are not treated as critical elements and are mainly neglected. It is believed that their influence on the distribution and its effects are not that crucial as those provided by the buildings. Enough space for the flow to adjust is provided. The boundaries are approximately 10 meters away from the buildings. What is more, some streets are included in the model. In fact, these are those that have dimensions of 1000x8m and 200x7m.

Particulate Matter Pollution Studies

Marine aerosols and vehicular exhaust are among the main sources of air pollution. Being related to traffic, it turns out to be a complex mixture of gasses and PM. NOx and SOx air pollution is considered in this framework. The first one consists of NO and NO2 that are initially produced by vehicles. Still, it is critical to pay attention to the fact that NO can oxidize to NO2 rather quickly. This kind of emission is about 7.6×10-5 kg/s. The second air pollution source mentioned lately includes SO and SO2, with emissions of about 7.6×10-5 kg/s. The PM studies paid attention to different particles and related characteristics, such as temperature. Tracer gas emissions observed within this research are also discussed.

Boundary Conditions

According to Barton and Babister (2012), boundary conditions are “a condition that is required to be satisfied at all or part of the boundary of a region in which a set of differential equations is to be solved.” Those models that are used for CFD are mainly aligned with the computational domain that consists of three main elements. These are upstream, central, and downstream parts. Boundaries are mainly of two types: lateral and top ones. For both of them, Franke et al. 2007 offer to resort to symmetry boundary conditions. They believe that in this way, a parallel flow can be strengthened as the velocity component is moved towards the boundary.

Street canyons are one of the most polluted areas in big cities because they are connected with busy streets with heavy traffic. The variability of the pollution is associated with the differences in traffic, of course. However, the influence of the meteorological conditions should not be neglected. Noncomplex terrains are usually lower than 200m. They are often used to speak about the wind profile. For example, such formula can be used in engineering:

In this case, U means the speed of the wind at a particular height. Then, Ur can be treated as the speed at a reference height. P is a coefficient that is identified by a professional on the basis of the stability of the atmosphere. It is known as the power-law exponent that usually equals about 0.1-0.6. For the obtained data to be properly analyzed, it is critical to consider the fact that the increase of the exponent presupposes the increase of the speed gradient.

A range of equations and boundary conditions are used to build the model. There are a lot of premises, but the major ones include:

• Steady-state.
• Newtonian fluid.
• Incompressible flow.
• Turbulent flow.

Scientists should also pay attention to mass, momentum, and energy conservation laws when working with fluid regions. It would be advantageous to support the Navier-Stokes equations. SolidWorks Flow Simulation can be rather advantageous in this situation.

Mesh Sensitivity Analysis

When engineers work with CFD problems, they can resort to SolidWorks Flow Simulation. It is considered to be one of the most efficient but simple tools to overcome such an issue. A set of benchmark issues is used in order to ensure the validity of the solver. The quality of the obtained results is usually aligned with the mesh. That is why it plays a critical role in various types of simulation. The users obtain an opportunity to develop a suitable mesh. As a result, it will be possible for them to investigate a complex fluid dynamic behavior thoroughly. All in all, reasonable solution times are expected to be pointed out.

With the help of a volumetric approach, professionals can mesh various elements and surroundings. As a rule, the volume includes the greatest extents and fluid regions, but there is always a possibility to change it to meet the peculiarities of the case. Another approach that is used in SolidWorks Simulation is body fitting. In the framework of structural analysis, it allows fitting parts within solid bodies. The flow mesh consists of 3 main types of fluid cells. Their differentiation is maintained on the basis of the geometry of the assembly. Thus, attention is paid to:

• Fluid cells. They can be found in fluid regions;
• Solid cells. They can be found in solid bodies;
• Partial cells. They cross the fluid and solid regions.

The rectangular space which the mesh occupies (called the ‘computational domain’) is subdivided according to the geometry. This is carried out to refine the mesh at solid/fluid boundaries and to recognize important geometric features such as small holes through which fluid may flow or fins on a heat sink. Refinement is the key to obtaining a good mesh.

In order to make mesh fit the case, professionals can resort to the slider bar. It is the most fundamental tool that has an extended effect. Due to it, the number of cells can be altered without any extreme complications. Basically, scientists discuss simulations with levels 3-5, but for this very research study levels, 6-7 will be used. In fact, an initial local mesh will be defined to focus on a limited area.

Result

All in all, it can be concluded that air pollution and the distribution of PMs depend greatly on the characteristics of the street canyon. In order to define their characteristics, professionals designed a model that represents buildings that can be found in real life. Initially, it was presupposed that vortices, recirculation, and reduction in the wind velocity would be observed during the study. Gathering the information about the characteristics of the wind, the data that relates to all four seasons were taken into consideration and thoroughly analyzed. The wind streamlines were identified so that it became easier to discuss observed outcomes. As the results reveal, the expectations related to the wind alterations and the existence of vortices and recirculation turned out to be true to life. It is also proved that the reduction in the velocity was affected by buildings and, of course, the height, width, and length of the canyon. With the help of the model, professionals obtained an opportunity to prove that there is a massive airflow within the discussed area. According to the data gathered during the study, it usually follows the direction of the wind. Emissions were discussed in several cases for the result to be accurate and to obtain the opportunity to generalize them. In the framework of the selected model, a possibility to measure PM mass concentration between different cases can be obtained when assessing it in micrometer/m3. Finally, there is also a possibility to increase fractions due to the dispersed gas mixture. Such changes can be observed as the concentration contour cut plots at heights of more than 1.5 m.

Reference List

Barton & Babister 2012

Cimorelli et al. 2005

EPA 2014

Fisk & Rosenfeld 1979

Franke et al. 2007

Hall et al. 2002

Holland et al. 1979

Shy 1979

SolidWorks flow simulation technical paper 2015

Tominaga et al. 2008

Xie & Castro 2009

Yee et al. 2006