The study of air pollution begins with the study of chemicals that comprise the air. These chemicals include molecules in the gas, liquid, or solid phases. Because the air contains so many different types of molecules, it is helpful to become familiar with the more important ones through the history of their discovery. Such a history also gives insight into characteristics of atmospheric chemicals and an understanding of how much our knowledge of air pollution today relies on the scientific achievements of alchemists, chemists, natural scientists, and physicists of the past. This chapter begins with some basic definitions, and then examines historical discoveries of chemicals of atmospheric importance. Finally, types of chemical reactions that occur in the atmosphere are identified, and chemical lifetimes are defined.

Basic Definitions

Air is a mixture of gases and particles, both of which are made of atoms. In this section, atoms, elements, molecules, compounds, gases, and particles are defined.

Basic Definitions

Air is a mixture of gases and particles, both of which are made of atoms. In this section, atoms, elements, molecules, compounds, gases, and particles are defined.

Atoms, Elements, Molecules, and Compounds

In 1913, Niels Bohr (1885–1962), a Danish physicist, proposed that an atom consists of one or more negatively charged electrons in discrete circular orbits around a positively charged nucleus. Each electron carries a charge of –1 and a tiny mass. The nucleus of an atom consists of 1 to 118 protons and 0 to 165 neutrons. Protons have a net charge of ₊1 and a mass 1,836 times that of an electron. Neutrons have zero net charge and a mass 1,839 times that of an electron. For the net charge of an atom to be zero, the number of electrons must equal the number of protons. Positively charged atoms have fewer electrons than protons. Negatively charged atoms have more electrons than protons. Positively or negatively charged atoms are called ions.

Anthropogenic air pollution has affected the environment since the development of the first human communities. Today, pollution arises due to the burning of wood, vegetation, coal, natural gas, oil, gasoline, kerosene, diesel, liquid biofuels, waste, and chemicals. Two general types of urban-scale pollution were identified in the twentieth century: London-type smog and photochemical smog. The former results from the burning of coal and other raw materials in the presence of a fog or strong temperature inversion, and the latter results from the emission of hydrocarbons and oxides of nitrogen in the presence of sunlight. In most places, urban air pollution consists of a combination of the two. In this chapter, urban outdoor gas-phase air pollution is discussed in terms of its history, early regulation, and chemistry.

History and Early Regulation of Outdoor Urban Air Pollution

Before the twentieth century, air pollution was treated as a regulatory or legal problem rather than a scientific problem. Because regulations were often weak or not enforced and health effects of air pollution were not well understood, pollution problems were rarely mitigated. In this section, a brief history of air pollution and its regulation until the 1940s is discussed.

People spend most of their time indoors, so the composition and quality of indoor air has a significant impact on human health. Because people's time is often divided between home and work, it is important to examine air quality in both residences and workplaces. Sources of indoor air pollution include outdoor air that infiltrates indoors and indoor emissions. Outdoor air contains the constituents of smog, but some of these constituents dissipate quickly indoors because of the lack of UV radiation to regenerate them indoors. Major indoor sources of pollution include stoves, heaters, carpets, fireplaces, tobacco smoke, motor vehicle exhaust from garages, building materials, and insulation. In developing countries, major sources of indoor air pollution include the products of solid biofuel and coal combustion for home heating and cooking. Such pollution is responsible for significant premature mortality worldwide. Whereas indoor air pollution is often regulated in workplaces, it is not regulated in residences. In this chapter, characteristics, sources, and regulation of indoor air pollution are discussed.

Pollutants in Indoor Air and Their Sources

Throughout the world, people spend most of their time indoors. One study found that, in the United States, about 89 percent of time was spent indoors, 6 percent was spent in vehicles, and 5 percent was spent outdoors (Robinson et al., 1991). A study of less developed countries showed that people in urban areas spent about 79 percent of their time indoors, and those in rural areas spent about 65 percent of their time indoors (Smith, 1993). Because people breathe indoor air more than outdoor air, an examination of indoor air is warranted. Nearly 1.6 million people worldwide die each year prematurely from indoor air pollution (not including smoking), making it one of the leading causes of death worldwide (World Health Organization (WHO), 2005). Most of these deaths are due to indoor burning of solid biofuels and coal for home heating and cooking with inefficient cook stoves and heaters.

Anthropogenic pollution occurs when gas and aerosol particle concentrations rise above natural, background concentrations. This chapter examines the evolution of the background atmosphere. The discussion requires a description of the sun and its origins because sunlight has affected much of the evolution of the Earth's atmosphere. The description also requires a discussion of the Earth's composition and structure because the inner Earth affects atmospheric composition through outgassing, and the crust affects atmospheric composition through exchange processes, including wind-blown soil dust, volcanic, and sea spray emissions. Earth's earliest atmosphere contained mostly hydrogen and helium. Carbon dioxide replaced these gases during the onset of the Earth's second atmosphere. Today, nitrogen and oxygen are the prevalent gases. Processes controlling the changes in atmospheric composition over time include outgassing from the Earth's interior, microbial metabolism, and atmospheric chemistry. These processes still affect the natural composition of the air today.

The Sun and Its Origin

The sun provides the energy to power the Earth. Most of the sun's energy reaching the Earth originates from the sun's surface, not from its interior. The evolution, structure, and relevant radiation emissions from the sun are discussed here.

Visibility, UV radiation intensity, and optical phenomena are affected by gases, aerosol particles, and hydrometeor particles interacting with solar radiation. In clean air, gases and particles affect how far we can see along the horizon and the colors of the sky, clouds, and rainbows. In polluted air, gases and aerosol particles affect visibility, optical phenomena, and UV radiation intensity. In this chapter, visibility, optics, and UV transmission in clean and polluted atmospheres are discussed. An understanding of these phenomena requires a study of the interaction of solar radiation with gases, aerosol particles, and hydrometeor particles in light of several optical processes, including reflection, refraction, diffraction, dispersion, scattering, absorption, and transmission. This chapter describes such interactions and processes.

Processes Affecting Solar Radiation in the Atmosphere

The solar spectrum is comprised of UV (0.01–0.38 μm), visible (0.38–0.75 μm), and solar-IR (0.75–4.0 μm) wavelengths of light (Section 2.2). The UV portion of the spectrum drives most of the photochemistry of the atmosphere, controls the color of our skin, and causes most of the health problems, including skin cancer and cataracts, associated with solar radiation (Chapter 11). The visible portion of the spectrum provides most of the energy that keeps the Earth warm. It also affects the distance we can see and colors in the atmosphere. It is no coincidence that the acuity (keenness) of our vision peaks at 0.55 μm, in the green part of the visible spectrum, which is near the wavelength of the sun's peak radiation intensity. Our eyes have evolved to take advantage of the peak intensity in this part of the visible spectrum. The solar-IR portion of the spectrum is important primarily for heating the Earth, but not so much as is the visible portion of the spectrum.

The two major global-scale environmental threats of international concern since the 1970s have been global stratospheric ozone layer loss and global warming. As discussed in Chapter 11, the global ozone layer is expected to recover by the mid–twenty-first century because national and international regulations have required the chemical industry to use alternatives to chloro- and bromocarbons, which are the chemicals primarily responsible for stratospheric ozone loss. Regulations are similarly responsible for improvements in air quality and acid deposition problems in many parts of the world since the 1970s (e.g., the U.S. Clean Air Act Amendments of 1970 motivated U.S. automobile manufacturers to develop the catalytic converter in 1975, which led to improvements in urban air quality; regulations through the 1979 Geneva Convention on Long-Range Transboundary Air Pollution led to the amelioration of some acid deposition problems in the 1980s and 1990s). However, progress toward solving the second major issue of international concern, global warming, has been slow. In this chapter, global warming is described and distinguished from the natural greenhouse effect. In addition, historical and recent temperature trends, both in the lower and upper atmosphere, are addressed, and climate responses to increased pollution are examined. The chapter also discusses potential effects of global warming and international and national efforts to curtail it.

Temperature on Earth in the Absence of a Greenhouse Effect

The natural greenhouse effect is the warming of the Earth's troposphere due to an increase in natural greenhouse gases. Greenhouse gases are gases that are largely transparent to the sun’s visible radiation but absorb and reemit the Earth's thermal-IR radiation at selective wavelengths. They cause a net warming of the Earth's atmosphere similar to the way in which a glass house causes a net warming of its interior. Because most incoming solar radiation can penetrate a glass house but a portion of outgoing thermal-IR radiation cannot, air inside a glass house warms during the day as long as mass (e.g., plant mass) is present within the house to absorb the solar radiation, to heat up the air, and to reemit thermal-IR radiation. The surface of the Earth, like plants, absorbs solar radiation and reemits thermal-IR radiation. Greenhouse gases, like glass, are transparent to most solar radiation but absorb a portion of the Earth's thermal-IR radiation at selective wavelengths.

Acid deposition occurs when an acid – primarily sulfuric acid, nitric acid, or hydrochloric acid – is emitted into or produced in the air and deposits to soils, lakes, grass, forests, or buildings. Acid deposition can be dry or wet. Dry acid deposition is the direct deposition of acid gases to surfaces. Wet acid deposition is the deposition to the surface of acids dissolved in rainwater (acid rain), fog water (acid fog), or liquid aerosol particles (acid haze). On the Earth's surface, acids have a variety of environmental impacts, including damage to microorganisms, fish, forests, agriculture, and structures. When breathed in, acids in high concentrations are harmful to humans and animals. Acid deposition problems have occurred since coal was first combusted and increased during the Industrial Revolution in the eighteenth century. The problems became more severe with the growth of the alkali industry in nineteenth-century France and the UK. In this chapter, the history, science, and regulatory control of acid deposition problems are discussed.

Historical Aspects of Acid Deposition

Acid deposition is caused by the emission or atmospheric formation of gas- and aqueous-phase sulfuric acid (H2SO4), nitric acid (HNO3), or hydrochloric acid (HCl). Historically, coal was the first and largest source of anthropogenically produced atmospheric acids. Coal combustion emits gas-phase sulfur dioxide [SO2(g)], hydrochloric acid [HCl(g)], and nitrogen oxides [NO x (g)]. SO2(g) oxidizes to gas- and aqueousphase sulfuric acid, and NO x (g) oxidize to nitric acid. Humans have combusted coal for thousands of years. In the 1200s, sea coal was brought to London and used in lime kilns and forges (Section 4.1). It was later burned in furnaces to produce glass and bricks, in breweries to produce beer and ale, and in homes to provide heat. Beginning with the eighteenth-century Industrial Revolution, coal combustion provided energy for the steam engine.

The concentrations of gases and aerosol particles in the air are affected by winds, temperatures, vertical temperature profiles, clouds, and the relative humidity. These meteorological parameters are influenced by large- and small-scale weather systems. Large-scale weather systems are controlled by vast regions of high and low pressure, whereas small-scale weather systems are controlled by ground temperatures, soil moisture, and small-scale variations in pressure. The first section of the chapter examines the forces acting on air, and the second section examines how forces combine to form winds. In the third section, the way in which radiation coupled with forces and the rotation of the Earth generates the global circulation of the atmosphere is discussed. Sections four and five discuss characteristics of the two major types of large-scale pressure systems. In the sixth section, the effects of such pressure systems on air pollution are addressed. The last section focuses on the effects of local meteorology on air pollution.

Forces

Winds arise due to forces acting on the air. The four major forces – pressure gradient force, apparent Coriolis force, friction force, and apparent centrifugal force – are described in this section.

Atmospheric chemistry, as a modern discipline, can be considered to have originated in 1931, when Sydney Chapman, distinguished British physicist, formulated a chemical mechanism for the formation of stratospheric ozone. The foundations of understanding tropospheric chemistry were laid in the early 1950s by Arie Haagen-Smit, a bioorganic chemist at the California Institute of Technology, who described ozone formation in the Los Angeles Basin as resulting from reactions involving volatile organic compounds and oxides of nitrogen. The essential reactive species in tropospheric chemistry remained unknown until the early 1970s, when the central role of the hydroxyl radical as the troposphere's “detergent” was revealed. The existence of particles in the air (aerosols) had long been recognized, but it was not until the past 50 years that instrumentation was developed that is capable of determining the size distribution and composition of atmospheric aerosols.

Threats to stratospheric ozone made headlines in the early 1970s, when Harold Johnston at the University of California, Berkeley, published calculations of the effect on stratospheric ozone of a proposed fleet of supersonic aircraft. Johnston's work was followed shortly thereafter by the revelation of the stratospheric chemical impact of chlorofluorocarbons, widely used as refrigerants and in consumer products, by F. Sherwood Rowland and Mario Molina of the University of California, Irvine. For their penetrating insights into atmospheric chemistry, Rowland, Molina, and Paul Crutzen of the Max Planck Institute for Chemistry in Mainz, Germany, received the 1995 Nobel Prize in Chemistry.

Ancient Greek philosophy was divided into three sciences: physics, ethics, and logic. This division is perfectly suitable to the nature of the matter, and there is no need to amend it, except perhaps just to add its principle, partly so as to assure oneself in this way of its completeness, partly to be able to determine correctly the necessary subdivisions.

All rational cognition is either material and considers some object, or formal and occupied merely with the form of the understanding and of reason itself, and with the universal rules of thinking as such, regardless of differences among its objects. Formal philosophy is called logic, whereas material philosophy, which has to do with determinate objects and the laws to which they are subject, is once again twofold. For these laws are either laws of nature, or of freedom. The science of the first is called physics, that of the other is ethics; the former is also called doctrine of nature, the latter doctrine of morals.

If so far we have drawn our concept of duty from the common use of our practical reason, it is by no means to be inferred from this that we have treated it as an experiential concept. Rather, if we attend to our experience of the behavior of human beings we meet frequent and, as we ourselves concede, just complaints that no reliable example can be cited of the disposition to act from pure duty; that, though much may be done that conforms with what duty commands, still it is always doubtful whether it is actually done from duty and thus has a moral worth. That is why there have been philosophers in every age who have absolutely denied the actuality of this disposition in human actions, and attributed everything to a more or less refined self-love, without however calling into doubt the correctness of the concept of morality because of this; rather, with intimate regret they made mention of the frailty and impurity of a human nature that is indeed noble enough to take an idea so worthy of respect as its prescription, but at the same time too weak to follow it, and that uses reason, which should serve it for legislation, only to take care of the interest of inclinations, whether singly or, at most, in their greatest compatibility with one another.

In fact, it is absolutely impossible by means of experience to make out with complete certainty a single case in which the maxim of an action that otherwise conforms with duty did rest solely on moral grounds and on the representation of one’s duty. For at times it is indeed the case that with the acutest self-examination we find nothing whatsoever that – besides the moral ground of duty – could have been powerful enough to move us to this or that good action and so great a sacrifice; but from this it cannot be inferred with certainty that the real determining cause of the will was not actually a covert impulse of self-love under the mere pretense of that idea; for which we then gladly flatter ourselves with the false presumption of a nobler motive, whereas in fact we can never, even by the most strenuous examination, get entirely behind our covert incentives, because when moral worth is at issue what counts is not the actions, which one sees, but their inner principles, which one does not see.

A life devoted to the pursuit of philosophical inquiry may be inwardly asfull of drama and event – of obstacle and overcoming, battle and victory,challenge and conquest – as that of any general, politician, or explorer,and yet be outwardly so quiet and routine as to defy biographical narration.Immanuel Kant was born in 1724 in Königsberg, East Prussia, to aPietist family of modest means. Encouraged by his mother and the familypastor to pursue the career marked out by his intellectual gifts, Kantattended the University of Königsberg, and then worked for a time as aprivate tutor in the homes of various families in the neighborhood, whilepursuing his researches in natural science. Later he got a position as a Privatdozent, an unsalaried lecturer who is paid by student fees, at theUniversity. There Kant lectured on logic, metaphysics, ethics, geography,anthropology, mathematics, the foundations of natural science, andphysics. In 1770, he finally obtained a regular professorship, the Chair ofLogic and Metaphysics, at Königsberg. Destined by limited means anduneven health never to marry or travel, Kant remained in the Königsbergarea, a quiet, hardworking scholar and teacher, until his death in 1804.