Air Quality

Wayne T. Davis, Joshua S. Fu, Thad Godish

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000346107
• Edition: 6
• Topic: Ciencias biológicas
• Subtopic: Conservación y Protección del Medio Ambiente

The sixth edition of a bestseller, Air Quality provides students with a comprehensive overview of air quality, the science that continues to provide a better understanding of atmospheric chemistry and its effects on public health and the environment, and the regulatory and technological management practices employed in achieving air quality goals. Maintaining the practical approach that has made previous editions popular, the chapters have been reorganized, new material has been added, less relevant material has been deleted, and new images have been added, particularly those from Earth satellites.

New in the Sixth Edition

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• New graphics, images, and an appended list of unit conversions

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• New problems and questions

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• Presents all-new information on the state of air quality monitoring

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• Provides the latest updates on air quality legislation in the United States

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• Updates the effects of air pollution and CO2 on climate change

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• Examines the effects of the latest changes in energy production and the related emissions and pollutants

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• Offers broadened coverage of air pollutant emissions and air quality in a global context

This new edition elucidates the challenges we face in our efforts to protect and enhance the quality of the nation's air. It also highlights the growing global awareness of air quality issues, climate change, and public health concerns in the developing world. The breadth of coverage, review questions at the end of each chapter, extensive glossary, and list of readings place the tools for understanding into your students' hands.

1 Atmosphere

1.1 Chemical Composition

1.1.1 General Composition

The Earth’s atmosphere is a mixture of gases and particulate-phase substances. The most abundant of these, nitrogen (N₂) and oxygen (O₂), comprise ~78% and 21%, respectively, of atmospheric mass and volume. A number of trace gases make up the remaining 1%. Average concentrations (with the exception of stratospheric ozone; O₃) are reported in Table 1.1. These include gases present in essentially constant concentrations: N₂, O₂, argon (Ar), neon (Ne), helium (He), krypton (Kr), hydrogen (H₂), and xenon (Xe). Others vary temporally and spatially. These include water vapor (H₂O), carbon dioxide (CO₂), carbon monoxide (CO), methane (CH₄), O₃, the nitrogen oxides (nitrous oxide [N₂O], nitric oxide [NO], and nitrogen dioxide [NO₂]), ammonia (NH₃), formaldehyde (HCHO), sulfur dioxide (SO₂), a number of reduced sulfur compounds (dimethyl sulfide [[CH₃]₂S], carbon disulfide [CS₂], carbonyl sulfide [COS], and hydrogen sulfide [H₂S]), and odd hydrogen species (hydroxyl radical [OH], hydroperoxyl radical [HO₂], and hydrogen peroxide [H₂O₂]). In addition to these gas-phase substances, the atmosphere contains trace quantities of particulate nitrate $\left({\text{NO}}_3^{-}\right)$, ammonium $\left({\text{NH}}_4^{+}\right)$, and sulfate $\left({\text{SO}}_3^{2-}\right)$.

TABLE 1.1
Atmospheric Gases

Chemical Species	Symbol	Concentration (%, ppmv, ppbv, pptv)
Nitrogen	N2	78.084%
Oxygen	O2	20.948
Argon	Ar	0.934
Water vapor	H2O	0.1–30,000 ppmv
Carbon dioxide	CO2	~412
Neon	Ne	18.18
Ozone (stratosphere)	O3	0.5–10
Helium	He	5.24
Methane	CH4	~1.87
Krypton	Kr	1.14
Hydrogen	H2	0.50
Xenon	Xe	0.09
Nitrous oxide	N2O	~0.33
Carbon monoxide	CO	110 ppbv (50–200)
Ozone	O3	20
Ammonia	NH3	4
Formaldehyde	HCOH	0.1–1
Sulfur dioxide	SO2	~1
Nitrogen dioxide	NO2	~1
Carbonyl sulfide	COS	500 pptv
Carbon disulfide	CS2	1–300
Dimethyl sulfide	(CH3)2S	10–100
Hydrogen sulfide	H2S	~50
Nitric oxide	NO	~50
Hydroxyl radical	OH	0.1–10

Although N₂ is the most abundant constituent of the atmosphere, it has a relatively limited direct role in atmospheric and life processes. It serves as a precursor molecule for the formation of ${\text{NO}}_3^{-}$, from which plant processes synthesize amino acids, proteins, chlorophyll, and nucleic acids (organic molecules that are directly or indirectly essential to all living things). The conversion of N₂ to ${\text{NO}}_3^{-}$ occurs as a result of atmospheric and symbiotic biological processes.

Nitrogen reacts with O₂ to produce nitrogen oxides (NO_x), which include N₂O, NO, NO₂, gas-phase nitric acid (HNO₃), and short-lived substances such as dinitrogen pentoxide (N₂O₅) and nitrate radical (NO₃). Concentrations of these compounds or substances, unlike their precursors (N₂ and O₂), vary significantly in time and space. Nitrous oxide (N₂O), a relatively inert gas commonly referred to as “laughing gas,” was, until several decades ago, thought to be present in the atmosphere at constant levels. It is one of a number of substances whose concentrations are increasing as a result of human activities.

The evolution of free atmospheric O₂ at elevated concentrations set the stage for the evolution of oxidative metabolism, the series of energy-transferring chemical reactions that sustain most life-forms. Oxygen, as a consequence, is vital to almost all living things. It is a precursor for the production of O₃ and the development of the O₃ layer, the stratospheric region that absorbs high-energy ultraviolet (UV) light radiation streaming into the Earth’s atmosphere from the sun. By absorbing most of the UV radiation incident on the Earth and its atmosphere, the O₃ layer shields most organic materials and living things from UV’s destructive energy.

On average, background O₃ levels in the troposphere are ~20 parts per billion by volume (ppbv). Ozone concentrations vary, and there is evidence that average levels are increasing as a result of human activities. At heights of 10–50 km (6.2–31 mi.), O₃ concentrations increase dramatically, with peak mixing ratio (parts per million by volume [ppmv]) concentrations (defined in Chapter 7) observed at 35 km (21.7 mi.; Figure 1.1) and concentrations based on partial pressure or molecular density peak at 25 km (15.5 mi.). At ~15 km (9.3 mi.), O₃ concentrations are ~0.1 to 0.5 ppmv; at 20 km (12.4 mi.), 3 ppmv; at 35 km, 8–10 ppmv; and at 50 km, 2 ppmv.

FIGURE 1.1 Atmospheric O₃ concentrations expressed as mixing ratios (ppmv) and partial pressure (nbars) as a function of height. (From National Research Council, Rethinking the Ozone Problem in Urban and Regional Air Pollution, National Academy Press, Washington, DC, 1991. With permission.)

In addition to these altitude-based differences, stratospheric O₃ concentrations vary from day to day, from one season to another, and latitudinally. There are also significant variations that result from quasi-biennial oscillations (air movement in the stratosphere), the 11-year sunspot cycle, and volcanic eruptions.

Ozone concentrations are also measured and reported as total column O₃ (vertical sum, reported as Dobson units). Mean monthly column O₃ concentrations are illustrated as a function of latitude and time of year in Figure 1.2. Column O₃ concentrations are highest at latitudes where O₃ production is relatively low. The highest O₃ values are found at high latitudes in winter and early spring; the lowest values are found in the tropics. Most O₃ production occurs near the equator where high levels of solar radiation are received; the observed stratospheric O₃ distribution reflects a strong poleward transport during winter.

FIGURE 1.2 Total column O₃ (Dobson units) averaged over the period 1979–1986 reflecting latitudinal and seasonal differences. (From WMO, Report of the International Ozone Trends Panel: 1988, World Meteorological Organization, Geneva, 1990.)

In contrast to N₂ and O₂, the atmospheric concentration of CO₂ is relatively low, ~0.041% or 412 ppmv. Carbon dioxide is enormously important. It is one of the two principal raw materials from which green plants (during photosynthesis) make food molecules on which most living things depend. Life is carbon based, and CO₂ is the source of that carbon. Carbon dioxide is also a major greenhouse gas and, because of its thermal absorptivity, is responsible in good measure for maintaining a favorable global heat balance. However, carbon dioxide is also increasing at a rate of ~2.5 ppmv/year and is also a major contributor to global warming. Other greenhouse gases are often referred to with respect to their global warming potential (GWP) when compared to CO₂ which, by definition, has a value of 1.0. The GWP is the ratio of a specific gas’s ability to trap heat when com...