Air Pollution Calculations: Quantifying Pollutant Formation, Transport, Transformation, Fate and Risks

By Daniel A. Vallero

Air Pollution Calculations: Quantifying Pollutant Formation, Transport, Transformation, Fate and Risks, Second Edition enhances the systems science aspects of air pollution, including transformation reactions in soil, water, sediment and biota that contribute to air pollution. This second edition will be an update based on research and actions taken since 2019 that affect air pollution calculations, including new control technologies, emissions measurement, and air quality modeling. Recent court cases, regulatory decisions, and advances in technology are discussed and, where necessary, calculations have been revised to reflect these updates. Sections discuss pollutant characterization, pollutant transformation, and environmental partitioning.

Air partitioning, physical transport of air pollutants, air pollution biogeochemistry, and thermal reactions are also thoroughly explored. The author then carefully examines air pollution risk calculations, control technologies and dispersion models. The text wraps with discussions of economics and project management, reliability and failure, and air pollution decision-making.

• Provides real-life current cases as examples of quantitation of emerging air pollution problems
• Includes straightforward derivation of equations, giving practitioners and instructors a direct link between first principles of science and applications of technologies
• Presents example calculations that make scientific theory real for the student and practitioner

• Language: English
• Publisher: Elsevier Science
• Release date: Sep 17, 2023
• ISBN: 9780443139888

Daniel A. Vallero

Professor Daniel A. Vallero is an internationally recognized author and expert in environmental science and engineering. He has devoted decades to conducting research, teaching, and mentoring future scientists and engineers. He is currently developing tools and models to predict potential exposures to chemicals in consumer products. He is a full adjunct professor of civil and environmental engineering at Duke University’s Pratt School of Engineering. He has authored 20 environmental textbooks, with the most recent addressing the importance of physical principles in environmental science and engineering. His books have addressed all environmental compartments and media within the earth’s atmosphere, hydrosphere, lithosphere, and biosphere.

Book preview

Front Cover for Air Pollution Calculations - Quantifying Pollutant Formation, Transport, Transformation, Fate and Risks - 2nd Edition - by Daniel A. Vallero

Air Pollution Calculations

Quantifying Pollutant Formation, Transport, Transformation, Fate and Risks

Second Edition

Daniel A. Vallero

Department of Civil and Environmental Engineering, Duke University Pratt School of Engineering, NC, United States

Table of Contents

Cover image

Title page

Inside Front Cover

Copyright

Dedication

Preface

Chapter 1. Introduction

Abstract

1.1 What is air pollution?

1.2 A bit of history

1.3 The science of air pollution

1.4 The atmosphere

1.5 Recent trends

1.6 Spatial scale

1.7 Stressor–receptor paradigm

1.8 Calculation basics

1.9 Systems thinking and sustainability

1.10 Conclusions

References

Chapter 2. Characterizing air pollutants

Abstract

2.1 The periodic table

2.2 Empirical calculations

2.3 Health characterization

2.4 Formation reactions

2.5 Reaction types

2.6 Pollutant formation processes: introduction to chemodynamics

2.7 Nonchemical concentrations

References

Chapter 3. Pollutant transformations

Abstract

3.1 Introduction

3.2 The compound

3.3 Air pollution kinetics

3.4 Energy in atmospheric transformations

3.5 Atmospheric kinetics calculations

3.6 Transformation thermodynamics

References

Chapter 4. Environmental partitioning

Abstract

4.1 Introduction

4.2 Equilibrium

4.3 What is the Ksp for cadmium fluoride?

4.4 Partitioning and transport

4.5 Integrating inherent properties and substrate characteristics

4.6 Movement from liquid to solid phase

4.7 The octanol–water coefficient

4.8 Partitioning between air and tissue

4.9 Environmental persistence

4.10 Conclusions

References

Chapter 5. Air partitioning

Abstract

5.1 Introduction

5.2 Henry’s law revisited

5.3 Movement into the atmosphere

5.4 Interfaces

5.5 Partitioning between air and tissue

5.6 Monod equation

5.7 Phytopartitioning

5.8 Conclusions

References

Further reading

Chapter 6. Physical transport of air pollutants

Abstract

6.1 Introduction

6.2 Partitioning meets the laws of motion

6.3 Diffusion and biological processes

6.4 Partitioning, transport, and kinetics

References

Chapter 7. Water and the atmosphere

Abstract

7.1 Introduction

7.2 The water molecule as an agent of change

7.3 The hydrosphere

7.4 Ionization, acidity, and alkalinity

7.5 Pollutant deposition from the hydrosphere

7.6 Intercompartmental exchange to and from the hydrosphere

7.7 Water, oxygen, and energy relationships

7.8 Reactions with water

References

Further reading

Chapter 8. Air pollution biogeochemistry

Abstract

8.1 Introduction

8.2 Spheres and cycles

8.3 Carbon equilibrium and cycling

8.4 Nutrient cycling

8.5 Metal and metalloid cycles

8.6 Biogeochemical cycles and decision-making

References

Chapter 9. Thermal reactions

Abstract

9.1 Introduction

9.2 Air and combustion

9.3 Volatility of combustion products

9.4 Flue gas

9.5 Thermal pollutant destruction

9.6 Activation energy

9.7 Nitrogen and sulfur

9.8 Metals in thermal reactions

References

Chapter 10. Air pollution phases and flows

Abstract

10.1 Introduction

10.2 Gas-phase pollutants

10.3 Air pollution fluid dynamics

10.4 Aerosols

10.5 Concentration

References

Chapter 11. Sampling and analysis

Abstract

11.1 Introduction

11.2 Quality assurance

11.3 Data quality objectives

11.4 Sampling air pollutant emissions and releases

11.5 Ambient air pollution monitoring

11.6 Ambient air pollutant sampling

11.7 Gas- and vapor-phase measurement methods

11.8 Methods for specific pollutants

11.9 Particulate matter

11.10 Measuring gas and particulate phases together

11.11 Laboratory analysis

References

Further reading

Chapter 12. Air pollution risk calculations

Abstract

12.1 Introduction

12.2 Accountability based on risk reduction and sustainability

12.3 Defining risk

12.4 Dose–response curves

12.5 Uncertainty and factors of safety

12.6 Exposure estimation

12.7 Direct cancer risk calculations

12.8 Risk-based treatment targets

12.9 Causality

References

Chapter 13. Air pollution control technologies

Abstract

13.1 Introduction

13.2 Particulate matter controls

13.3 Forces

13.4 Particle morphology

13.5 Mechanisms of particulate removal

13.6 Aerosol removal technologies

13.7 Removal of liquid droplets and mists

13.8 Gas-phase pollutant controls

13.9 Air pollutant treatment technologies

13.10 Removal of odors

References

Further reading

Chapter 14. Air pollution dispersion models

Abstract

14.1 Introduction

14.2 Meteorological data and graphics

14.3 Gaussian models

14.4 Stability

14.5 Eulerian models

14.6 Model levels

14.7 Model uncertainty

14.8 Links to exposure and dose models

14.9 Recent advances

References

Chapter 15. Economics and project management

Abstract

15.1 Introduction

15.2 Emission strategies

15.3 Engineering economics

15.4 Comparing alternatives

15.5 Replacement cost analysis

15.6 Life cycle comparisons

References

Chapter 16. Reliability and failure

Abstract

16.1 Introduction

16.2 Failure classification

16.3 Margins of safety

16.4 Success

16.5 Systems engineering

References

Chapter 17. Air pollution decision-making

Abstract

17.1 Introduction

17.2 Data-based decision-making

17.3 Ethics

17.4 Ethical analysis of decisions and actions

17.5 Linking causes and outcomes

17.6 Analysis of complex air pollution problems

References

Appendix A. Key equations

A.1 Atmospheric chemistry

A.2 Atmospheric physics

A.3 Partitioning

A.4 Meteorology

A.5 Biogeochemistry

A.6 Air pollution engineering

A.7 Geometry

A.8 Statistics

A.9 Air pollution risk

A.10 Dispersion modeling

A.11 Reliability engineering

A.12 Air pollution economics

Appendix B. Abbreviations and symbols

Index

Inside Front Cover

From Science Notes http://sciencenotes.org.

Copyright

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Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

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Dedication

To Samuel, Alexander, Chloe Jayne, Amelia, Michael, Daniel Joseph, and Janis, May you live in an atmosphere of faith, hope, and love.

–Daniel Vallero

Preface

Daniel A. Vallero

The recent derailment of rail tanker cars in East Palestine, Ohio, and the devastating wildfires in Maui, Canada, and Europe were stark reminders that air quality can change in an instant from breathable and healthy to contaminated and dangerous. We take for granted that our air is clean, at least until we lose confidence that it is. As of this writing, we are all waiting for better data on what actually entered the air because all forms of pollution are dynamic. A dangerous chemical such as vinyl chloride or hydrochloric acid, under the right conditions, can be converted to an even more hazardous substance such as phosgene or a chlorinated dioxin. More common pollutants from the wildfires like particulate matter can threaten health, diminish visibility that threatens the safety air travel, and even change weather patterns. Such events are also reminders that air pollutants can, over time, become water and soil pollutants and can build up and change in the environment and even within organisms, including humans. Short-term mistakes can become persistent problems if groundwater and soil become contaminated. To paraphrase my late friend and colleague, Aarne Vesilind, in environmental science and engineering, everything matters.

A lot has happened since the first edition. Concerns about PFAS, the so-called forever chemicals, have found their way from peer-reviewed literature to the evening news. The pandemic made everyone a little smarter about aerosols, be they viruses or the water droplets that carry them. Green energy and electric vehicles appear increasingly in TV commercials and YouTube ads. Even technical terms such as controlled burns are used by the lay public and politicians. All of these reflect concern and interest of the larger public but need a more thorough and objective treatment from scientists. Science is never settled.

As I wrote in the preface of the first edition, nonfiction books that deal with environmental subject matter are of three basic types: descriptive, quantitative, and quantitatively descriptive. Descriptive books usually have the broadest readership, often for a general readership. They are devoid of equations and symbols. Instead, they rely on narratives about particular events and policies. Barry Commoner’s and Rachel Carson’s books fall into this category. They often have a few major points they want to make, such as the need to take actions to prevent cancer, protect endangered species, or restore and prevent the loss of habitat.

In descriptive books, the authors have information and knowledge that they want to share in order to build or enhance the reader’s awareness of a particular environmental or public health problem, such as global warming or asbestos in consumer products. Their principal focus is on a single or just a few topics and do not worry too much about all the other aspects involved. In this sense, they are often better reads at the beach or over the morning coffee. In my opinion, the principal driving force behind this type of environmental book is awareness.

Quantitative books are most popular with scientists and, especially, engineers. They are also descriptive but describe phenomena and processes using numbers, symbols, equations, and technical jargon. In addition to the attention to detail, they are usually much more expansive in their treatment of detailed environmental subject matter. I believe this is because books are not the principal means by which specific scientific information is shared. This is mainly done by peer-reviewed journal articles. As such, quantitative environmental books are the beneficiaries of the massive amount of scientific findings in ongoing, rigorous scientific research. Air pollution books, for example, do not usually present the results of the author’s own experiments for the first time. If they do present such results, the author has already published them in a peer-reviewed journal article. And, quite likely, most of the results presented are those of other scientists. The book author has chosen them to illustrate a point and to use the equations and coefficients derived by these legions of researchers and professionals who have made the effort to submit their findings to the scrutiny of the scientific community.

In this sense, the author of a quantitative environmental book is a messenger. The principal driving force for a quantitative book is the need to explain how things work. The author, hopefully, has much experience in applying the methods and is sharing this with the readership. To do so, it requires that the reader begin with a threshold of knowledge, usually much higher than what is required of the descriptive environmental book reader. I liken this to a prerequisite college course. Taking an air pollution modeling course based on a particular software would be very difficult if the student had not already completed the software course. Similarly, reading a quantitative atmospheric book would be very difficult without a modicum of knowledge of algebra and physics. Usually, however, the book will build knowledge, not only of the subject matter but also of new applications, both mathematical and physical. That is, the atmospheric science book not only improves one’s knowledge of the atmosphere but also enhances the general mathematical and scientific acumen of the reader. I believe the best way to learn science is experiential and the quantitative book is a virtual way to experience environmental phenomena and processes.

The third type of the book, the hybrid, combines narrative descriptions with quantitation. A phenomenon is described but is explained further using symbolic and technical tools. For example, completely describing a disaster requires descriptive information, for example, time lines, historical events, key players, and adverse outcomes. Not only describing the toxic plume in Bhopal, India, requires this type of information, but also it needs chemical information, for example, physical and chemical factors of the released chemical, methyl isocyanate, its biological and toxicological information, geographic and spatially explicit information, chemical concentration data, estimates of doses, and other quantitative information.

My book, Fundamentals of Air Pollution, falls into this category. Actually, the book had three previous editions before my fourth and fifth editions. When I reviewed the third edition, I found it to be an excellent documentation of the progress in addressing air pollution in the United States. It certainly had a wealth of quantitative information, but much of the book was highly descriptive. I attempted to add quantitation in the fourth edition but still thought it needed substantial quantitative information to explain the many air pollution processes, including the diseases and other outcomes, air pollution control technologies, and spatial and temporal complexities. I completely revamped the book in the fifth edition, including a substantial amount of quantitative information.

After this effort, explaining air pollution problems requires more than a cursory combination of descriptive and quantitative answers to questions at the end of each chapter of an air pollution book. In fact, instructors who use my book asked specifically for quantitative answers, which I take to mean a dedicated resource. In my opinion, I suspect those of others who teach air pollution courses and who practice in a field that addresses air pollution would appreciate such a resource. This suspicion led to the book you are reading. It is a companion to Fundamentals of Air Pollution, but also to any air pollution book. In this sense, its primary aim is to make the concepts and topics more relevant and practical to instructors, students, professionals, and others who work to address various aspects of air quality.

I am told that most people, including engineers and scientists, are predominantly experiential learners. We learn very little by simply reading about or seeing something and even less hearing about it. We are empathic readers who must insert ourselves into a situation. We learn more effectively by experiencing, at least virtually, than by merely watching. And since mathematics and quantification are the tools of engineering and science, the examples in this book provide a step toward experience in the matters of air pollution. The experience is vicarious, but the problems presented are exactly the types being confronted daily by decision-makers, scientists, and engineers. Indeed, it is possible that I have borrowed, and hopefully acknowledged, one or a few of your own experiences. If so, I am grateful to your contributions to clean air. If, on the other hand, reading this book is your first step, I look forward to your future contributions to clean air, with a hope that what I have written will have added to your air quality knowledge.

Chapter 1

Introduction

Abstract

This chapter introduces air pollution, including the need for meaningful calculations. This begins with the historical context of air pollution problems and controls.This is followed by a discussion of the scientific method and credible engineering as they apply to air pollution. This includes physical, chemical, and biological principles, with in-depth discussions and example calculations of atmospheric structure and composition. The mathematical and statistical tools need for air pollution calculations are introduced. The chapter ends with a discussion of air pollutants and the damage they can cause, that is, stressor–receptor linkages.

Keywords

Air pollution definition; air pollution history; black smoke; aerosols; mixing ratio; stratospheric ozone; ultraviolet light; troposphere; environmental scale and complexity; air quality statistics; greenhouse effect; global greenhouse gas (GHG); standard conditions; atmospheric pressure; stressor; receptor; air quality index

1.1 What is air pollution?

Air pollution can be studied from numerous perspectives. Researchers often choose a very specific aspect of air pollution and delve deeply into its meaning. They need to understand mathematics and the basic sciences to learn what is already known and to extend this knowledge to their needs.

Air pollution is commonly defined as the presence of contaminants or substances in the air that interfere with human health or welfare or produce other harmful environmental effects [1]. Put simply, anything in the air that is or can be harmful. However, harm is value-laden [2–5], and there is a threshold between desired and undesired states, that is, the difference between clean and polluted air. These socially acceptable thresholds vary according to cultural norms. Thus air pollution definition requires context, that is, something is an air pollutant if it causes conditions to deviate from a desired state. Thus an air pollutant interferes a benchmark of a desired condition that, at a minimum, provides for air quality that supports human and other life. However, the definition goes beyond health and extends to welfare, including ecological condition and societal well-being. From a systems perspective, the sum of these desired states is, in fact, a single desired state. Pope Francis put this succinctly in his encyclical letter, Laudato Sí [6]:

Some forms of pollution are part of people’s daily experience. Exposure to atmospheric pollutants produces a broad spectrum of health hazards, especially for the poor, and causes millions of premature deaths. People take sick, for example, from breathing high levels of smoke from fuels used in cooking or heating. There is also pollution that affects everyone, caused by transport, industrial fumes, substances which contribute to the acidification of soil and water, fertilizers, insecticides, fungicides, herbicides and agrotoxins in general.

Unfortunately, even when the general population accepts a condition as acceptable does not make it so. The determination of acceptability must be based in objective science. For example, for many years, the majority of people in many societies considered tobacco smoke to be acceptable [7,8]. This did not mean that the population was not being harmed by smoking. Similarly, most air pollutants are not detectable with unaided human senses until they reach very toxic levels. Detecting these substances at or below thresholds of toxic concentrations requires sophisticated equipment. Too often, people who have been exposed at harmful levels may not even know they have been exposed until disease symptoms arise and epidemiological studies link them to the presence of contaminants. Indeed, exposure to pollutants that cause cancer and other chronic diseases usually occur many years before the diseases themselves symptomatically. This is known as the latency period [9–14]. Thus, for air pollution, perception is not reality.

Thermodynamically speaking, an air pollutant, or any pollutant, is the result of inefficiency. The pollutant is mass or energy that is exiting a control volume. In trying to produce something of value, a waste product is released to the atmosphere. Not all scientists are researchers. Indeed, some of the best scientists are those who apply what researchers have discovered. Such scientists work in national, state, provincial, and local government agencies with missions to protect the environment. They write regulations, rules, and guidance and issue permits and licenses on emissions and ambient air quality protections. Other scientists work in the private sector or in environmental and health advocacy groups. Even when they all adhere to the scientific method and avail themselves to the same data, there is always various levels of certainty about environmental harm. Thus establishing air quality standards or deciding on the best air pollution control approach is an iterative and multifactorial process [15–20].

Often in combating air pollution, the researchers, regulatory scientists, and engineers work closely together. For example, in the United States, the Environmental Protection Agency (EPA) has three basic functions: (1) To conduct research; (2) To enforce environmental rules and regulations; and (3) To provide funding for other entities to help with the first two functions. EPA’s air pollution enforcement and compliance work are conducted by the Office of Air Quality Planning and Standards (OAQPS) and its 10 regional offices, and its research is conducted by the Office of Research and Development (ORD). To demonstration the collaboration between research, development, technology transfer, and enforcement of air pollution laws, OAQPS and ORD’s air programs are conducted on the same campus in Research Triangle Park, North Carolina. ORD funds and collaborates on air quality research with numerous universities and institutions.

There are other perspectives in addition to research and enforcement, including teaching, journalism, and policymaking. Those who educate others, write about, and promulgate polices and laws need must understand air pollution in different ways, but with a modicum of science.

Understanding the principles of air pollution and the calculations involved is something we all share, although the extent and specificity of these calculations vary. For example, a research scientist may need to completely explain all the reactions in a combustion facility that lead to the generation of a toxic compound, for example, benzo(a)pyrene. The air emissions permit the writer may not need to know all the reactions to the extent of the research, but enough to know which ones may be most difficult to change. The high school chemistry teacher may need to know only the most basic chemical reactions, but the air pollution engineering college professor may need to know just as much as the researcher to tailor their lectures properly to the target student. The journalist who can do the math and grasp the scientific principles has an advantage over her colleagues who only know the essence of the science. The local air pollution authority manager may need to know as much as the permit writer, since he supervises the permit writers. The politician who serves on a technical committee responsible for writing air pollution legislation will need to know more than the other members of the legislature, perhaps even grasp the math and science as well as the permit writer and the researcher, at least to the extent of knowing whether to increase or decrease funding.

Therefore, every air pollution perspective requires an aptitude and appreciation for calculations. This book attempts to provide examples of most of the types of calculations one may encounter, whether to succeed in one’s career or simply to trust what one reads in a report or article.

1.2 A bit of history

Not long ago, making environmental decisions based on and underpinned by sound physical and biological science was not the norm compared with those made for other reasons [21]. At first, the weight of environmental factors was based on theory and limited, isolated studies. Indeed, science-based air pollution decision-making tracked directly with the experiences of contagious diseases. Even after germ theory took hold in the biological research community, there was substantial time before public health decisions became predominantly based on contagion data. A rather large body of theory testing occurred before John Snow was able to make the famous decision to intervene and insist on infection controls by banning the use of contaminated Thames River water to abate the outbreak of the Broad Street cholera epidemic in the mid-19th century [22,23].

Concerns about air pollution predated Snow’s work. For centuries, people have known intuitively that something was amiss when their air was filled with smoke or when they smelled an unpleasant odor. But, for most pollutants, those that were not readily sensed without the aid of sensitive equipment, a baseline had to be set to begin to act. One way to look at the interferences mentioned in the definition is to place them within the context of harm. Harm can be acceptable, so long as it is sufficiently unavoidable and arises only due to obtaining a more favored good or service. The objects of the harm have received varying levels of interests. In the 1960s, the perception of harm to ecosystems grew as larger number of people perceived that the very survival of certain biological species was threatened and that these threats extended to humans. Indeed, the direct harm to humans was perceived earlier than those to ecosystems, especially in terms of diseases directly associated with obvious episodes, such as respiratory diseases and even death associated with combinations of weather and pollutant releases [24].

Myriad pollutant emissions have accompanied the industrial and technological eras. Nuclear power plants are associated with the possibilities of meltdown and the release of airborne radioactive materials. Burning fossil fuels releases products of incomplete combustion (PICs), for example, polycyclic aromatic hydrocarbons. Even complete combustion releases the greenhouse gas (GHGs), carbon dioxide (CO2). Leaks from chemical, pesticide, and other manufacturing facilities may cause exposures to toxic substances, such as the release of methyl isocyanate in Bhopal, India. In the last quarter of 20th century, these apprehensions led to the public’s growing wariness about toxic chemicals added to the more familiar conventional pollutants such as soot, carbon monoxide (CO), and oxides of nitrogen and sulfur. Toxic chemical exposures were increasingly linked to cancer and threats to hormonal systems in humans and wildlife, neurotoxicity (notably from lead and mercury exposure in children), and immune system disorders.

Some substances thought to be almost exclusively water or food contaminants were found in the air. For example, the source blood lead (blood-Pb) levels in children were initially thought to be pica, that is, eating chips of lead paint. However, the Pb concentrations declined precipitously in children living near roadways following the decreased use of tetraethyl-lead and other Pb organometallic compounds in gasoline, which may have accounted for more than half of blood-Pb in the 1970s [25].

Growing numbers of studies have provided evidence linking disease and adverse effects to extremely low levels of certain particularly toxic substances. For example, exposure to dioxin at almost any level above what science could detect could be associated with numerous adverse effects in humans.

At the threshold of the new millennium, the need to consider air pollution within the context of larger-scale environmental systems became obvious, including the loss of aquatic diversity in lakes due to deposition of acidic precipitation, so-called acid rain. Acid deposition was also being associated with the corrosion of materials, including some of the most important human-made structures, such as the pyramids in Egypt and monuments throughout the world. Presently, global pollutants have become the source of public concern, such as those that seemed to be destroying the stratospheric ozone layer or those that appeared to be affecting the global climate. This escalation of awareness of the multitude of pollutants complicated matters.

To set the stage for air pollution calculations, let us consider to some air pollution events to begin to understand why we need sound approaches that have evolved from lessons learned over time [26]. In addition to the need to hunt and gather, early people were nomadic because they wanted to exit the stench of the animal, vegetable, and human wastes that they generated. Within enclosed dwellings air pollutants accumulated. For centuries the open fire emissions created smoked that lingered. Thus among the first air pollution control technologies was the chimney, which helped to remove the combustion products and cooking odors from the living quarters. This was a boon to indoor air quality, but also an early lesson in the cumulative sources’ impact on ambient air. This was aptly state in AD 61 by the Roman philosopher Seneca:

As soon as I had gotten out of the heavy air of Rome and from the stink of the smoky chimneys thereof, which, being stirred, poured forth whatever pestilential vapors and soot they had enclosed in them, I felt an alteration of my disposition [27].

Eleanor of Aquitaine, the wife of King Henry II of England, moved from her castle in the year 1157 because wood smoke was generating so much air pollution [24]. In 1306, Edward I issued a royal proclamation enjoining the use of sea coal in furnaces in London. Elizabeth I barred the burning of coal in London when Parliament was in session. Perhaps as an early lesson in the difference between promulgating air pollution law and the ability to enforce it, coal continued to be burned despite these royal edicts. As evidence, by 1661, London’s air had become sufficiently polluted that John Evelyn submit to King Charles II and Parliament a brochure entitled, Fumifugium, or the Inconvenience of the Aer, and Smoake of London Dissipated (together with some remedies humbly proposed) [28]. This lays out several proposed actions that are viable and used today [29].

The principal industries associated with the production of air pollution in the centuries preceding the Industrial Revolution were metallurgy, ceramics, and preservation of animal products. In the bronze and iron ages, villages were exposed to dust and fumes from many sources. Native copper and gold were forged, and clay was baked and glazed to form pottery and bricks before 4000 B.C. Iron was in common use, and leather was tanned before 1000 B.C. Most of the methods of modern metallurgy were known before AD 1. They relied on charcoal rather than coal or coke. However, coal was mined and used for fuel before AD 1000, although it was not made into coke until about 1600; and coke did not enter metallurgical practice significantly until about 1700. These industries and their effluents as they existed before 1556 are best described in the book De Re Metallica published in that year by Georg Bauer, known as Georgius Agricola. It was translated into English by President (and engineer) Herbert Clark Hoover and his wife [30]. Combustion in kilns generated air pollution even before the industrial revolution. For example, ceramics [31] and animal product preservation industries [32] burned various materials to heat kilns.

The Industrial Revolution soon began after it was found that harnessing of steam could provide power to pump water and to move heavy objects. Early in the 18th century, Savery, Papin, and Newcomen designed their pumping engines and culminated in 1784 in Watt’s reciprocating engine. The reciprocating steam engine was ubiquitous in industry until it was displaced by the steam turbine in the 20th century. During most of the 19th century, coal was the principal fuel used to make steam, although some oil was used for this purpose late in the century. Thus the predominant air pollution problem of the 19th century was smoke and ash from the burning of coal or oil in the boiler furnaces of stationary power plants, locomotives, and marine vessels, and in-home heating fireplaces and furnaces. Great Britain took the lead in addressing this problem, and in the words of Sir Hugh Beaver [33]:

By 1819, there was sufficient pressure for Parliament to appoint the first of a whole dynasty of committees to consider how far persons using steam engines and furnaces could work them in a manner less prejudicial to public health and comfort. This committee confirmed the practicability of smoke prevention, as so many succeeding committees were to do, but as was often again to be experienced, nothing was done.

In 1845, during the height of the great railway boom, an act of Parliament required that locomotives consume their own smoke and 2 years later applied the same requirement to factory furnaces. These and later laws were passed with limited success. Air pollution from the emerging chemical industry was considered a separate matter and was made the responsibility of the Alkali Inspectorate created by the Alkali Act of 1863. As opposed to the kingdom-wide proclamations in Britain, the United States, considered smoke abatement (as air pollution control was then known) to be a municipal responsibility. No federal or state smoke abatement laws or regulations were promulgated until the next century. The first municipal ordinances and regulations limiting the emission of black smoke (BS) and ash appeared in the 1880s and were directed toward industrial, locomotive, and marine rather than domestic sources. By the end of the 19th century, smelting of sulfide ores was identified as the source of crop damage.

Air pollution control technology in the 19th century began to look like those in use today, including improved fuel feed technologies, especially stokers for mechanical firing of coal, scrubbers to remove acid gases from effluent gas streams, cyclones, and fabric filters (baghouses) to collect particulate matter (PM). Also, with the growth of scientific knowledge, physical and chemical principles began to replace, or at least augment, trial and error and intuitive processes in pollution control design.

The first quarter of the 20th century experienced dramatic changes in the technology of both the production of air pollution and its engineering control. However, this was not accompanied by substantial changes in legislation, regulations, understanding of the problem, or public attitudes toward the air quality. As cities and factories grew, the severity of the pollution problem increased.

A big technological change was the replacement of the steam engine by the electric motor as the means of operating machinery and pumping water. This substantially transferred the smoke and ash emission from the boiler house of the factory to the boiler house of the electric generating station. At the start of this period, coal was hand-fired in the boiler house; by the middle of the period, it was mechanically fired by stokers; but by the end of the period, pulverized coal, oil, and gas firing had begun to dominate. Each form of firing produced its own characteristic emissions to the atmosphere.

Steam locomotives started to enter central business districts in the heart of the larger cities. Later, the urban terminals of many railroads had been electrified, thereby transferring much air pollution from the railroad right-of-way to the electric generating station. The replacement of coal by oil in many applications decreased ash emissions from those sources. There was rapid technological change in industry. However, the most significant change was the rapid increase in the number of automobiles from almost none at the turn of the century to millions by 1925. The principal technological changes in the engineering control of air pollution were the perfection of the motor-driven fan, which allowed large-scale gas-treating systems to be built; the invention of the electrostatic precipitator, which made particulate control in many processes feasible; and the development of a chemical engineering capability for the design of process equipment, which made the control of gas and vapor effluents feasible.

The first large-scale surveys of air pollution were undertaken—Salt Lake City, Utah (1926) [34]; New York City (1937) [35]; and Leicester, England (1939) [36]. By mid-20th century, major air pollution became almost ubiquitous in urban areas. Public health disasters, including air pollution related deaths, occurred in the Europe and North America. The Meuse Valley, Belgium, episode occurred in 1930 [37]; the Donora, Pennsylvania, episode occurred in 1948 [38]; and the Poza Rica, Mexico, episode in 1950 [39]. Smog appeared in Los Angeles in the 1940s. In response, the first National Air Pollution Symposium in the United States was held in Pasadena, California, in 1949 [40] and the first United States Technical Conference on Air Pollution was held in Washington, DC, in 1950 [41].

Building of natural gas pipelines resulted was rapid displacement of coal and oil as home heating fuels with dramatic improvement in air quality. As shown in Fig. 1.1, cities throughout the United States and Europe commonly suffered extended and dangerous air pollution episodes, but began to see the much-publicized decreases in concentrations of BS in their central business districts [24]. The diesel locomotive began to displace the steam locomotive, thereby slowing the pace of railroad electrification. The internal combustion engine bus started its displacement of the electrified streetcar. The love for the automobile continued to grow and the numbers on the road proliferated.

Figure 1.1 (A) Pittsburgh after the decrease in black smoke. (B) Pittsburgh before the decrease in black smoke. Used with permission from: Allegheny County, Pennsylvania and A.C. Stern, H.C. Wohlers, R.W. Boubel, W.P. Lowery, Fundamentals of Air Pollution, Academic Press, New York and London, 1973.

During this period, no significant national air pollution legislation or regulations were adopted anywhere in the world. Indeed, industrial operations in and around urban centers were vastly increased to support the war efforts in the late 1930s and early to mid-1940s. Refining iron ore to make steel emitted large amounts of PM and oxides of sulfur into populated areas [42]. Shortly after, however, in 1947, California became the first state in the United States to promulgate an air pollution law in the United States [43], soon followed by other states and nations [44].

1.2.1 Black smoke

Great Britain experienced a major air pollution disaster in London in 1952, which led to the passage of the Clean Air Act in 1956 and an expansion of the authority of the Alkali Inspectorate [45,46]. The new law target home heating, which had been mainly done by burning soft coal on grates in separate fireplaces in each room. This led to a successful substitute of much lower-smoke fuels and central or electrical heating instead of fireplace heating.

The outcome was a decrease in smoke concentration. This was determined by measuring the interference of visible light, referred to as blackness of paper filters that collect air. The opacity of the filter is proportional to the amount of soot and dust on the filter. These opacity readings improved from 175 µg m−3 in 1958 to 75 µg m−3 in 1968 [47]. Today, the filters are weighed before and after passage of air, but the percentage of black carbon is still of interest. Thus, variations on this British method, now known as the BS method, are still in use. The updated BS method, known as the filter smoke method, draws air through a pipe at constant temperature (See Fig. 1.2) [48]. It typically requires 26 impacting samples from a 4.5 µm inlet, through which the air to be measured flows onto white filter paper, which is stained. The stained filter is analyzed for blackness of the stain, which is 27 as measured by light absorption. The blackness or darkness of the stain is measured using a reflectometer. Smoke particles composed of elemental carbon typically contribute the most stain darkness.

Figure 1.2 Apparatus for black smoke (BS) measurement method. The effective sampling length is the sampling length is 405 mm. The dead volume is the volume of gas away from the sampling point in the pipe up to the filter paper, which includes the probe and lines. The opacity of the filter area is measured using a reflectometer [49]. Adapted from A.L. GmbH, Smoke Value Measurement with the Filter Paper Method, Application Notes. Graz, Austria. Report No. AVL1007E, 2005.

The mix of ambient particles varies widely spatially and seasonally. Thus the correlation between BS measurements and PM mass is highly specific to the place and time of measurement, making comparisons difficult [50]. Thus the conversion of the opacity readings mentioned earlier is conversion from blackness on paper filters to mass per volume of soot and dust. However, before the end of the 20th century, opacity readings were mainly used to quantify PM. As mentioned, PM is now mainly quantified using monitors that differentiate particle diameters, especially 10 and 2.5 microns, by mass. So, this begs the question, how were these early BS method results converted to specific values of 175 µg m−3 in 1958 to 75 µg m−3 in 1968?

The answer is that empirically derived conversions were applied. For reflectometer readings of 40–99, the following formula is used [51]:

Equation

(1.1)

where C=concentration in µg m−3; V=volume of air sampled, in cubic feet (m³ is converted to cubic feet by multiplying by 35.315); R=reflectometer reading; and F=a factor relating to the sampler clamp size:

0.288 for 0.5-inch clamp

1.00 for 1-inch clamp

3.68 for 2-inch clamp

12.80 for 4-inch clamp.

This formula represents the calibration curve to within ± 1.3% over the range of reflectometer readings between 40 and 90. When used to calculate concentrations from reflectometer readings between 91 and 98, the results may be underestimated by as much as 6%. For darker stains with reflectometer readings between 40 and 20, the formula used is:

Equation

(1.2)

Thus the 175 and 75 values could have been reported as 175±2 and 75±1, respectively. Such ratios are important because we need to compare readings over decades, or even centuries, during which time measurement technologies continue to improve and methods are replaced. Thus there have been several studies attempting to compare BS method results with later, direct PM measurements [48,52–54].

1.2.2 Recent history

In the middle of the 20th century, almost every country in Europe, as well as Japan, Australia, and New Zealand, experienced serious air pollution in its larger cities, with each enacting national air pollution control legislation. By 1980, air pollution institutions emerged, including Warren Springs Laboratory, Stevenage, England; the Institut National de la Santé et de las Recherche Medicale at Le Visinet, France; the Rijksinstituut Voor de Volksgezondheid, Bilthoven and the Instituut voor Gezondheidstechniek-TNO, Delft, The Netherlands; the Statens Naturvardsverk, Solna, Sweden; the Institut für Wasser-Bodenund Luft-hygiene, Berlin; and the Landensanstalt für Immissions und Bodennutzungsshutz, Essen, Germany, along with many in Japan.

Smog in the United States continued to worsen, notably in Los Angeles, but extending to large cities throughout the nation. In 1955, the first federal air pollution legislation was enacted, which funded research, training, and technical assistance. The Public Health Service of the United States Department of Health, Education, and Welfare oversaw these efforts until 1970, when it was transferred to the newly formed EPA. The initial federal legislation was amended and extended several times between 1955 and 1980, greatly increasing federal authority, particularly in emissions control [55]. The automobile continued to proliferate, peaking in 2005 and declining substantially in 2009 and 2010, likely due to the economic recession. However, the rest of the world has continued to increase vehicle ownership. Global production of motor vehicles increased from approximately 54 million in 1997 to 95 million in 2016. In fact, China produced twice as many vehicles as either the United States or Japan in 2016 (over 28 million compared to 12 million in the United States and 9 million in Japan) [56].

A major event that raised awareness of environmental pollution occurred on June 22, 1969, near Cleveland, Ohio [57]. Like many cities in North America and Europe, Cleveland suffered from air, water, and land pollution resulting from almost unbridled industrial production, but lacking sufficient regulations and authority to curb the release of pollutants into the Cuyahoga River, one of the most polluted rivers in the world. At about noon, oil debris between two wooden trestles in the river became ignited by a passing train’s sparks, beneath two wooden trestles, in Southeast Cleveland. The fire burned for about a half-hour, reaching heights over nearly 20 m. Seemingly defying logic, the river had caught fire at least 13 times since 1868. Indeed, previous fires led to loss of life and major property damage, but were not covered as extensively by the news media as the 1969 fire. Other rivers had previously caught fire due to large discharges of chemicals and sewage, including the Chicago River in Illinois, Buffalo River in New York, and the River Rouge in Detroit [58].

The Cuyahoga fire actually was a major impetus behind passage of the National Environmental Policy Act signed into law on January 1, 1970. The law created the Council on Environmental Quality in the President’s office [59] and required that environmental impact statements be written by all US federal agencies for major projects that could significantly affect the quality of the environment [60,61]. It was also a major impetus for establishing the EPA, as well as other environmental agencies around the world and in states, provinces, and local jurisdictions. In subsequent years, many new and amended laws were passed to address pollution, including the Clean Air Act Amendments (CAAA) of 1970 and 1977 [62–64]. Interestingly, water pollution concerns, such as burning rivers, were among the major reasons for the passage of laws and regulations to address air pollution problems more aggressively in the 1970s. Certainly seeing the plumes rise from flowing water was an amazing site, but it was the awareness that waters were so polluted to be flammable that was the real call for action.

In the 20th century, sanitary engineer became a separate branch of civil engineering, mainly to design structures for treating water pollution. For example, in 1909, sanitary engineer William Paul Gerhard (1854–1927) observed that microbes were effective in degrading pollutants [65]:

The purifying action, which takes place in the two stages of treatment mentioned, is due to the different kinds of bacteria, known respectively as the aerobic, the anaerobic and the facultative bacteria. None of the methods of treatment are automatic, and all require intelligent management and constant supervision.

This was a big step in applying biological process for pollution control and treatment, especially microbial oxidation, reduction, and hydrolysis [66–69]. Microbes like the soil bacterium Pseudomonas spp. can be coaxed to use pollutants as their exclusive food source by withholding other preferred food sources, making the microbes to provide their energy by using their metabolic processes [70]. Of course, the prominent applications of microbial treatment have traditionally been water, sediment, and soil treatment, but numerous air pollutant control technologies now deploy Gerhard’s observations [71–75].

In the second half of the 20th century, sanitary engineering programs in many universities were transformed into environmental engineering programs. This was a move beyond sanitary engineering’s primary focus on structures, for example, design of sanitary landfills, sewage treatment facilities, drinking water systems, and pollution control equipment to additional attention to air quality and ecosystems, as well as applying engineering expertise in support of environmental assessments and impact statements [59,60,76–81]. It was also a first move to toward systems engineering, especially what is now known as green engineering and sustainable design, including air quality models, including physical or dynamic models, computational fluid dynamics models, exposure and risk models, and optimization and predictive models to compare and chose among different outcomes expected to result from alternative actions and pollution control options [82–101].

Air pollution technologies grew rapidly from the 1950s through the 1970s, focusing predominantly on automotive air pollution and its control, as well as oxides of sulfur (SOx) pollution and its control by sulfur oxide removal from flue gases and fuel desulfurization, and on control oxides of nitrogen, especially nitrogen dioxide (NO2) produced in combustion processes [24].

Environmental protection became almost universally accepted in the 1970s. Organizationally, this has taken the form of departments or ministries of the environment in governments at all levels throughout the world. In addition to national agencies, states, provinces, counties, and cities took on greater responsibility for air and water quality, sold waste sanitation, noise abatement, and control of the hazards associated with radiation and the use of pesticides. This is paralleled in industry, where formerly diffuse responsibility for these areas is increasingly the responsibility of an environmental protection departments. Similar changes became evident in research and education institutions.

New professions were formed commensurate with air pollution awareness. Indeed, air pollution meteorology was becoming a distinct academic and professional discipline [102]. By 1980, mathematical models of the pollution of the atmosphere were being energetically developed. A start had been made in elucidating the photochemistry of air pollution. Air quality monitoring systems became operational throughout the world. A wide variety of measuring instruments became available, many of which are very similar to those being used today.

Much of the early air pollution research was devoted to localized effects, but later began to be increasingly directed to the larger scale problems. Notably, the buildup of GHGs in the atmosphere, depletion of the stratospheric ozone layer by chlorofluorocarbons (CFCs), long-range transport of persistent bioaccumulating toxics (PBTs) persistent organic pollutants (POPs), prevention of significant deterioration of air quality due to PM and sulfur dioxide (SO2), and acidic deposition increasingly attracted media and scientific attention. More recently, disasters have highlighted the importance of air pollution in long-range pollutant transport [103–112]. For example, the meltdown of the Fukushima nuclear power facility in Japan was feared to have potential for attendant migration of radioisotopes created across the ocean, even as far as the West Coast of North America [113–115]. Like the PBT pollution of Inuit peoples [116–121], Fukushima served as another reminder that pollutants can travel thousands of miles in winds aloft. Wildfires in North and biomass burning in South America [122,123] and dust transported from Saharan Africa have released large amounts of PM and other air pollutants [124,125]. Even what would be considered water disasters, the huge oil spills in the Gulf of Mexico and around the world had to include measurements of volatile organic compounds (VOCs) and other potential airborne contaminants [126,127].

As we shall see later when discussing combustion efficiencies, one of the indicators of success is the production of carbon dioxide (CO2). The presence of water and CO2 means we have complete combustion and have avoided producing many very toxic and persistent air pollutants, such as CO and polycyclic aromatic compounds, known as PICs. More recently, however, the product of complete combustion, CO2, is also a GHGs.

Air pollution problems are not solely the result of industry. In fact, a problem that originally plagued residences in London and other developed nations centuries ago remains among the worst air pollution problems worldwide. Open-fire and other solid fuel cookstoves may be responsible for 4 million premature deaths annually, especially in developing nations [128].

There are many successes in air pollution prevention and control over recent decades, but industrial pollution continues to be a worldwide and growing problem. Hopefully, developing nations will not repeat the 20th century problems experienced by Western nations and will apply lessons without having to suffer the consequences of side effects from economic development. Sound air pollution practice and planning will be needed. Air quality programs will need [73]:

• to describe and document atmospheric conditions accurately and precisely;

• to consider the central factors leading associated with and causing air pollution events;

• to evaluate the data and modeling results and to articulate completely the decisions to be made based on sound science; and

• to uphold objectivity to arrive at comprehensive, fair, and consistent decisions that lead to suitable air pollution control and prevention actions, outcomes, and consequences.

1.3 The science of air pollution

Understanding air pollution begins with the physical sciences. The atmosphere is one of the spheres of importance to environment. It surrounds the globe and interacts with and connects the earth’s surface to space. As such, all physical processes are at work in the atmosphere, including gravity, pressure, friction, and exchanges of matter and energy. The atmosphere is the major system by which substances are transported and transformed into either essential or detrimental compounds that we breath.

Like most air pollution texts, chemical air pollutants dominate this book. However, pollutants can also be physical and biological. The energy from ultraviolet (UV) light is an example of a physical stressor. Although exposure is to the physical contamination, that is, energy at this wavelength, the exposure has been indirectly increased by chemical contamination. For example, the chemicals released into the atmosphere in turn react with ozone in the stratosphere, decreasing the ozone concentration and increasing the amount of UV radiation at the earth’s surface. This has meant that the mean UV dose in the temperate zones of the world has increased. This has been associated with an increase in the incidence of skin cancer, especially the most virulent form, melanoma.

Air pollutants may also be biological, as when bacteria and viruses are released to the atmosphere from medical facilities. Other biological air pollutants include irritants and allergens, such as pollen and molds (i.e., bioaerosols).

Thus calculations related to the structure of the atmosphere are crucial to science that underpins air pollution decisions. The troposphere, that is, the lowest atmospheric layer, is the major location of most air pollution. However, the stratosphere is also important, as it contains the ozone layer, which absorbs much of the incoming UV light. When the stratospheric ozone (O3) concentrations fall, exposures to UV increase, which damages skin cells and can lead to chronic problems, notably increased incidence of melanoma, the most virulent form of skin cancer.

This book approaches air pollution from a question-and-answer perspective. Questioning is the very essence of air quality. Some of the questions go back to the earliest times of human existence, such as why does the air smell so bad or when I burn different things, why does the smoke look different? Modern questions are often different only in details, such as what chemicals make the smoke smell this way or what chemicals are in the fuel that were emitted and led to brownish smog?

There are myriad questions about air pollution. Some can be readily and completely answered. Most can be answered only under stipulated scientific conditions. Some have more than one answer. Some have answers that will differ depending on whom one asks. Each of these types of questions is asked in this book.

1.4 The atmosphere

The concentration of gases is often the first expression of atmosphere. At the global scale, the gas concentrations are usually described by volume, rather than by weight. The presence of water is always a consideration in environmental sciences and engineering. This is no different for atmospheric science. Water in its various physical states, that is, solid, liquid, or gas, is found at some concentration in most of the lower atmosphere. The vapor phase of water in the atmosphere is commonly referred to as humidity. Given the varying amounts of humidity, meteorologists and atmospheric scientists often express gas concentrations as dry volume, that is, no humidity. By dry volume, 99.997% of the atmosphere consists of four gases, molecular nitrogen and oxygen (N2 and O2, respectively), argon (Ar), and carbon dioxide (CO2). Chemically, Ar is inert (nonreactive) as it is a noble gas. The other three compounds are also very stable and nonreactive under atmospheric conditions of temperature and pressure, so they remain very stable components of the atmosphere. Approximately 99% of the mass of the atmosphere lies within 50 kms of the Earth’s surface, that is, in the troposphere (lowest level) and stratosphere. This is where most air pollution occurs. The major exception is the ozone layer, which is affected by human activities at the earth’s surface, especially the use of halogenated chemical compounds that find their way to the stratosphere [129–132].

The earth’s atmospheric temperature varies with altitude (Fig. 1.3), as does the density of the substances comprising the atmosphere. The earth is warmed when incoming solar radiation is absorbed at or near the earth’s surface and reradiated at longer electromagnetic wavelengths (infrared). In general, the air grows progressively less dense with increasing altitude moving upward from the troposphere through the stratosphere and the chemosphere to the ionosphere. In the upper reaches of the ionosphere, the gaseous molecules are few and far between as compared with the troposphere.

Figure 1.3 Temperature profile of the Earth’s atmosphere.

The ionosphere and chemosphere are of interest to space scientists because they must be traversed by space vehicles en route to or from the moon or the planets, and they are also regions in which satellites travel in the earth’s orbit. These regions are also of interest to communications scientists because of their influence on radio communications. However, these layers are of interest to air pollution scientists primarily because of their absorption and scattering of solar energy, which influence the amount and spectral distribution of solar energy and cosmic rays reaching the stratosphere and troposphere.

The stratosphere is of interest to a wide range of scientists and engineers. Aeronautical scientists and engineers see it as a layer to traverse when exiting the earth’s orbit, but also because it has been traversed by airplanes like the supersonic transport planes in the late 20th century. The stratosphere is also the venue of communications scientists who must consider ionization and other factors to provide reliable radio, television, and satellite communications.

Air pollution scientists and engineers must also be familiar with the stratosphere for numerous reasons. Chemical compounds are formed and transformed there. Persistent chemicals can move thousands of miles via global transport. For example, debris from aboveground nuclear tests and volcanic eruptions can be suspended for protracted periods of time within the stratosphere given the intense rates of absorption and scattering of solar energy. Arguably, the best-known reason for air pollution experts to care about the stratosphere is that the lower layers contain the ozone layer, which absorbs harmful UV solar radiation. The layer has been threatened by chemicals, especially the CFCs, several of which have been banned as a result. Global change scientists are also concerned about CFCs and other gases released at the earth’s surface or by high-altitude aircraft.

1.4.1 Introduction to tropospheric physics

Most air pollution occurs in the troposphere, so except for stratospheric ozone depletion and climate change, air pollution texts, including this one, almost exclusively address pollution near the earth’s surface. Atmospheric physics and chemistry will be covered in greater detail in subsequent chapters but need to be highlighted here to begin to explain air pollution from a scientific perspective. A good start is understanding the mechanisms responsible for gas and particle concentrations and mixing in the troposphere. These mechanisms are not only keys to estimating where pollutants form and move but also help to explain the likelihood that humans and ecosystems will be exposed to air pollutants. To prime the air pollution calculations pump, so to speak, let us pose and answer a few foundational questions about atmospheric gases.

What is the mixing ratio? What advantage is there to using this as opposed to concentrations of gases in the atmosphere?

The general definition of mixing ratio is the amount of a single substance divided by the total amount of all substances in a mixture. In air pollution and atmospheric sciences, this can be a mole ratio (ri):

Equation (1.3)

Where ni=the amount of the single substance and ntotal=the total amount of the mixture. The amount is usually expressed as the International System of Units unit, which is the mole. A mole, which is abbreviated mol is expressed using Avogadro’s number. That is, a mol contains 6.02214076 ×10²³ elementary entities. For air pollutants the elementary entity is usually a molecule or an atom, but can also include other entities, especially an ion. Thus, 3 mols of N2 would contain 3 × 6.02214076 ×10²³ molecules, which is about 1.81 ×10²³ molecules. In fact, 3 mols of O2 would also contain 1.81 ×10²³ molecules. 3 mols of argon (Ar) would contain 1.81 ×10²³ atoms, and so on.

The mixing ratio may also be expressed as a mass ratio (ζi):

Equation (1.4)

Where mi=the mass of the single substance, and mtotal=the total mass of the mixture.

So, then, why use something seemingly more complicated than simple chemical concentrations to express the amount of the various gases that make up the atmosphere? The principal reason lies in the fact that the density of air varies horizontally, vertically, and within air masses. Density is the volume of air divided by the mass of all constituent gases that make up the air. Thus, the mixing ratio has the advantage of remaining constant with changing air density, as opposed to listing concentrations. Concentration is expressed as volume of the individual gas per volume of air, for example, parts per million (ppm) or mass of the gas per volume of air, for example, milligrams per cubic meter (mg m−3). The mixing ratios of some important tropospheric gases are provided in Table 1.1.

Why is the mixing ratio below 100 km altitude in the atmosphere so highly variable for most substances other than molecular nitrogen and the noble gases?

Noble gases have their outer shell filled with electrons, so are chemically nonreactive. Thus their concentrations depend on sources entirely and do not diminish due to chemical reactions in the upper atmosphere like most other substances. Similarly, N2 is nonreactive except under extreme conditions, for example, very high temperatures and pressures like those