Because people spend most of their time indoors, it is useful to examine the composition and quality of indoor air. 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 and indoor emissions. Outdoor air contains the constituents of smog, but some of these constituents dissipate quickly indoors because of the lack of ultraviolet (UV) radiation to reproduce them indoors. Major indoor sources of pollution include stoves, heaters, carpets, fireplaces, tobacco smoke, motor vehicle exhaust from garages, building materials, and insulation. Whereas indoor air pollution in the United States is 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

In the United States, about 89 percent of people's time is spent indoors, 6 percent is spent in vehicles, and 5 percent is spent outdoors (Robinson et al., 1991). Another study found that in nonindustrialized countries, people in urban areas spend 79 percent of their time indoors and those in rural areas spend 65 percent of their time indoors (Smith, 1993). Because people breathe indoor air more than outdoor air, an examination of indoor air is warranted.

Table 9.1 identifies major pollutants in indoor air and their primary sources. Many of the pollutant gases in indoor air are also found in outdoor air.

Visibility, ultraviolet (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 through several optical processes, including reflection, refraction, diffraction, dispersion, scattering, absorption, and transmission. These processes are described next.

PROCESSES AFFECTING SOLAR RADIATION IN THE ATMOSPHERE

The solar spectrum is divided into UV (0.01 to 0.38 μm), visible (0.38 to 0.75 μm), and near-infrared (IR) (0.75 to 4.0 μm) wavelength ranges (Section 2.2).

In 1666, Sir Isaac Newton (1642–1727; Fig. 7.1), an English physicist and mathematician, showed that when white, visible light passed through a glass prism, each wavelength of the light bent to a different degree, resulting in the separation of the white light into a variety of colors that he called the light spectrum. Although the spectrum is continuous (the eye can distinguish 10 million colors), Newton discretized the spectrum into seven colors: red, orange, yellow, green, blue, indigo, and violet, to correspond to the seven notes on a musical scale. When the colors of the spectrum were recombined, they reproduced white light.

Anthropogenic pollution problems result from the enhancement of gas and aerosol-particle concentrations above background concentrations. In this chapter, the evolution of the background atmosphere is discussed. 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 soil-dust emission. 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 that energy originates from the sun's surface, not from its interior. The reason for this is discussed as follows.

About 15 billion years ago (b.y.a.), all mass in the known universe may have been compressed into a single point, estimated to have a density of 109 kg m−3 and a temperature of 1012 K (kelvin). With the “Big Bang,” this point of mass exploded, ejecting material in all directions. Aggregates of ejected material collapsed gravitationally to form the earliest stars.

In this chapter, the structure and composition of the present-day atmosphere are described. The structure is defined in terms of the variation of pressure, density, and temperature with height. Pressure and density are controlled by gases in the air, the most abundant of which are molecular nitrogen and oxygen. The temperature structure is controlled by the distribution of gases that absorb ultraviolet (UV) and thermal-infrared (IR) radiation. Pressure, density, and temperature are interrelated by the equation of state. Gases in the air include fixed and variable gases. In the following, the structure of the atmosphere in terms of pressure, density, and temperature variations with height is discussed. The main constituents of the air are then examined in terms of their sources and sinks, abundances, health effects, and importance with respect to different air pollution issues.

AIR PRESSURE AND DENSITY STRUCTURE

Air consists of gases and particles, but the mass of air is dominated by gases. Of all gas molecules in the air, more than 99 percent are molecular nitrogen or oxygen. Thus, oxygen and nitrogen are responsible for the current pressure, temperature, and density structure of the Earth's atmosphere. Air pressure, the force of air per unit area, can be calculated as the summed weight of all gas molecules between a horizontal plane and the top of the atmosphere and divided by the area of the plane. Thus, the more oxygen and nitrogen present, the greater the air pressure.

The two major global-scale environmental issues of international concern since the 1970s have been the threats of global stratospheric ozone reduction and global warming. As discussed in Chapter 11, the threat to the global ozone layer has been controlled to some degree because national and international regulations have required the chemical industry to use alternatives to chlorocarbons and bromocarbons, the chemicals primarily responsible for much of the ozone-layer reductions. Regulations are similarly responsible for improvements in air quality and acid deposition problems in the United States and many western European countries since the early 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 Clean Air Act Amendments and the 1979 Geneva Convention on Long-Range Transboundary Air Pollution led to reductions in sulfur emissions and an amelioration of some acid deposition problem in the 1980s and 1990s). The second major issue of international concern, the threat of global warming, has been addressed at international meetings, but progress in regulating emissions responsible for the problem has been slow. In this chapter, global warming is discussed and distinguished from the natural greenhouse effect. Climate responses to enhanced gas and aerosol particle concentrations are examined. Historical and recent temperature trends, both in the lower and upper atmosphere, are described. National and international efforts to curtail global warming are also discussed, as are potential effects of global warming.

The stratospheric ozone layer began to form soon after the onset of oxygen-producing photosynthesis, about 2.3 billion years ago (b.y.a.). It probably did not develop fully until at least 400 million years ago (m.y.a.), when green plants evolved and molecular oxygen mixing ratios began to approach their present levels. Absorption of ultraviolet (UV) radiation by ozone is responsible for the temperature inversion that defines the present day stratosphere. This absorption is critical for preventing UV radiation from reaching the surface of the Earth, where it can harm life. The anthropogenic emission of long-lived chlorine- and bromine-containing compounds into the air since the 1930s and the slow transfer of these compounds to the stratosphere has caused a nontrivial reduction in the global stratospheric ozone layer since the 1970s. In addition, during September, October, and November each year since the early 1980s, up to 70 percent of the ozone layer has been destroyed over the Antarctic. Lesser reductions have occurred over the Arctic in March, April, and May each year. Recent international cooperation has helped reduce emissions and slow further ozone loss. In this chapter, the natural stratospheric ozone layer, global ozone reduction, and Antarctic/Arctic ozone destruction and regeneration are discussed.

STRUCTURE OF THE PRESENT-DAY OZONE LAYER

About 90 percent of all ozone molecules in the atmosphere reside in the stratosphere; most of the remaining molecules reside in the troposphere.

Acid deposition occurs when sulfuric acid, nitric acid, or hydrochloric acid, emitted into or produced in the air as a gas or liquid, deposits to soils, lakes, farmland, forests, or buildings. Deposition of acid gases is dry acid deposition, and deposition of acid liquids is wet acid deposition. Wet acid deposition can be through rain (acid rain), fog (acid fog), or 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. In the air, acids in high concentrations are harmful to humans. Acid deposition problems have existed since coal was first combusted, but were exacerbated during the Industrial Revolution in the eighteenth century. The problem became more severe with the growth of the alkali industry in 19th-century France and the United Kingdom. In this chapter, the history, science, and regulation of acid deposition problems are discussed.

HISTORICAL ASPECTS OF ACID DEPOSITION

Acid deposition is caused by the emission or atmospheric formation of gas- or 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 produces sulfur dioxide gas [SO2(g)] and hydrochloric acid gas [HCl(g)]. SO2(g) produces gas-and aqueous-phase sulfuric 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).

Until the 1940s, efforts to control air pollution in the United States were limited to a few municipal ordinances and state laws regulating smoke (Chapter 4). In the United Kingdom, regulations were limited to the Alkali Act of 1863 and some relatively weak Public Health Bills designed to abate smoke. Regulations in other countries were similarly weak or nonexistent. The main reason for the lack of regulation in polluted cities was that the coal, oil, chemical, and auto industries, the ultimate sources of much of the pollution, had political power and used it to resist efforts of government intervention (e.g., Section 4.1). Because the long-term health effects of pollutants in outdoor concentrations were not well known at the time, it was also difficult for public health agencies to recommend the banning of a pollutant, particularly in the face of political pressure from industry and arguments that such a ban would hurt economic growth (e.g., Midgley's defense of tetraethyl lead in Section 3.6.9). In the late 1940s through mid-1950s, damage due to photochemical smog in the United States was sufficiently apparent that the federal government decided to take steps to address the problem. Similarly, deadly London-type smog events in the United Kingdom spurred government legislation in the 1950s. Today, many countries have instituted air pollution regulations. Nevertheless, regulations in most countries are still weak, resulting in severe pollution problems. Many countries, for instance, continue to allow tetraethyl lead in their gasoline and do not require catalytic converters in automobiles.

In this chapter, the effects of meteorology on air pollution are discussed. The concentrations of gases and aerosol particles 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 large-scale regions of high and low pressure. Small-scale weather systems are controlled by ground temperatures and small-scale variations in pressure. The first section of the chapter examines the forces acting on air. The second section examines how forces combine to form winds. The third section discusses how radiation, coupled with forces and the rotation of the Earth, generates the global circulation of the atmosphere. Sections 6.4 and 6.5 discuss the two major types of large-scale pressure systems. Section 6.6 discusses the effects of such pressure systems on air pollution. The last section focuses on the effects of local meteorology on air pollution.

FORCES

Winds arise due to forces acting on the air. Next, the major forces are described.

Pressure Gradient Force

When high air pressure exists in one location and low pressure exists nearby, air moves from high to low pressure. The force causing this motion is the pressure gradient force (PGF). The force is proportional to the difference in pressure divided by the distance between the two locations and always acts from high to low pressure.

Although most regulations of air pollution focus on gases, aerosol particles cause more visibility degradation and possibly more health problems than do gases. Particles smaller than 2.5 μm in diameter cause the most severe health problems. Particles enter the atmosphere by emissions and nucleation. In the air, their number concentrations and sizes change by coagulation, condensation, chemistry, water uptake, rainout, sedimentation, dry deposition, and transport. Particle concentration, size, and morphology affect the radiative energy balance in urban air and in the global atmosphere. In this chapter, compositions, concentrations, sources, transformation processes, sinks, and health effects of aerosol particles are discussed. The effects of aerosol particles on visibility are described in Chapter 7. Regulations relating to particles are given in Chapter 8.

SIZE DISTRIBUTIONS

Aerosol and hydrometeor particles are characterized by their size distribution and composition. A size distribution is the variation of concentration (i.e., number, surface area, volume, or mass of particles per unit volume of air) with size. Table 5.1 compares typical diameters, number concentrations, and mass concentrations of gases, aerosol particles, and hydrometeor particles under lower tropospheric conditions. The table indicates that the number and mass concentrations of gas molecules are much greater than are those of particles. The number concentration of aerosol particles decreases with increasing particle size. The number concentrations of hydrometeor particles are typically less than are those of aerosol particles, but the mass concentrations of hydrometeor particles are always greater than are those of aerosol particles.

Natural air pollution problems on the Earth are as old as the Earth itself. Volcanoes, fumaroles, natural fires, and desert dust have all contributed to natural air pollution. Humans first emitted air pollutants when they burned wood and cleared land (increasing windblown dust). More recently, the burning of coal; chemicals; oil, gasoline, kerosene, diesel, jet and alcohol fuels; natural gas; and waste and the release of chemicals have contributed to several major air pollution problems on a range of spatial scales. These problems include outdoor urban smog, indoor air pollution, acid deposition, Antarctic ozone depletion, global ozone reduction, and global warming.

Urban smog is characterized by the outdoor buildup of gases and particles emitted from vehicles, smokestacks, and other human sources, or formed chemically in the air from emitted precursors. Smog affects human and animal health, structures, and vegetation. Urban smog occurs over scales of tens to hundreds of kilometers.

Indoor air pollution results from the emission of pollutant gases and particles in enclosed buildings and the transport of pollutants from outdoors into buildings. Indoor air pollutants cause a variety of human health effects. Indoor air pollution occurs over scales of meters to tens of meters.

Acid deposition occurs when sulfuric acid, nitric acid, or hydrochloric acid in the air deposits to the ground as a gas or dissolved in rainwater, fogwater, or particles. Acids harm soils, lakes, forests, and structures. In high concentrations, they can harm humans. Acid deposition occurs over scales of meters to thousands of kilometers.

Urban air pollution problems have existed for centuries and result from the burning of wood, vegetation, coal, natural gas, oil, gasoline, kerosene, diesel, 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 inversion, and the latter results from the emission of hydrocarbons and oxides of nitrogen in the presence of sunlight. In most places, urban pollution consists of a combination of the two. In this chapter, gas-phase urban air pollution is discussed in terms of its early history, early regulation, and chemistry.

HISTORY AND EARLY REGULATION OF URBAN AIR POLLUTION

Before the twentieth century, air pollution was not treated as a science but as a regulatory or legal problem. Because regulations were often weak or not enforced and health problems associated with 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.

Before 1200

In ancient Greece, town leaders were responsible for keeping sources of odors outside of town. In ancient Rome, air pollution resulted in civil lawsuits. The Roman poet Horace noted thousands of wood-burning fires (Hughes, 1994) and the blackening of buildings (Brimblecombe, 1999). Air pollution events caused by emissions under strong inversions in Rome were called heavy heavens (Hughes, 1994).