Air Pollution Perspective

Air pollution is woven throughout the fabric of our modern life. A by‐product of the manner in which we build our cities, air pollution is waste remaining from ways we produce our goods, transport ourselves, and generate the energy to heat and light the cities where we live, work, and play. The major cause of all air pollutants is combustion which is an essential part of our civilization. When perfect or theoretical combustion occurs, the hydrogen and carbon in the fuel combine with oxygen from the air to produce heat, light, carbon dioxide, and water vapor. However, impurities in the fuel, poor fuel‐to‐air ratio, or too high or too low combustion temperatures cause the formation of such by‐products as carbon monoxide, sulfur dioxide, nitrogen oxides, fly ash, and unburned hydrocarbons – all air pollutants.

Air pollution is of public health concern on the micro, meso, and macroscales (Section 4.11). Industrial and mobile sources contribute to mesoscale air pollution and contaminates the ambient air that surrounds us outdoors. Macroscale (or global) effects include transport of ambient air pollutants over large distances. Examples of macroscale effects include acid rain, ozone, and dust or particulate matter (PM) pollution. Global effects of air pollution results from sources that may potentially change the upper atmosphere, examples of which include depletion of good ozone layer and global warming. Although microscale and macroscale effects are of concern, our focus is predominantly on mesoscale air pollution in the troposphere where we live.

The 1970 Clean Air Act (CAA) required the USEPA to investigate and describe the environmental effects of any air pollutant emitted by stationary and/or mobile sources that may adversely affect human health and the environment. The EPA used these studies to establish the National Ambient Air Quality Standards (NAAQS). The NAAQS have been established for six criteria air pollutants – five primary (meaning emitted directly) and one secondary pollutant (because it is formed in the lower atmosphere by chemical reactions among primary pollutants).

These standards are for the ambient air, that is the outdoor air that normally surrounds us. The EPA calls the pollutants listed in Table 3.9 criteria pollutants because they were developed on health‐based criteria. The primary standard was established to protect human health with an “adequate MOS.” The secondary standards are intended to prevent environmental and property damage. In a conflict between ambient air quality and anticipated economic growth, regulations were proposed in August 1974 to prevent serious deterioration of air quality in areas where the air is already cleaner than required by federal standards. A threefold classification was put into effect with a goal that in no circumstances the ambient air quality of an area violate federal primary and secondary standards. The classifications are as follows:

In 1987 the EPA revised the NAAQs by replacing the standard for hydrocarbons (or VOCs) with total suspended PM with an aerodynamic diameter less than or equal to 10 μm. This standard is referred to as the PM₁₀ standard. In 1997 a new PM_2.5 standard was added. Figure 4.13 should help you visualize the size of a micrometer or micron. Note that a hair has an average diameter of approximately 80 μm.

Causes, Sources, and Effects

In this section, we will briefly discuss some of the major pollutants with respect to their causes, sources, and effects. A cause is distinguished from a source in that a cause is fundamental, whereas a source is locational; that is, a cause explains why or how a pollutant is formed, whereas a source identifies what type of process, industry, or device discharges a particular pollutant.

Criteria air pollutants, or criteria pollutants, are a set of air pollutants that cause smog, acid rain, and other health hazards. These pollutants are typically emitted from many sources in industry, chemicals, mining, transportation, electricity generation, refineries, smelter, pharmaceuticals, agriculture, and others. In most cases, they are the products of the combustion of fossil fuels or of industrial processes. The sources and the effects on public health and the environment were discussed and presented in Table 3.8.

The five primary criteria pollutants are PM less than 10 μm in diameter and PM less than 2.5 μm in diameter (PM −10 and PM −2.5), sulfur dioxide (SO₂), nitrogen dioxide (NO₂), carbon monoxide (CO), and particulate lead (Pb); the secondary criteria pollutant is ozone (O₃). Among these six pollutants, the first four are emitted in the United States (and other large industrialized countries) in quantities measured in millions of metric tons per year and are sometimes called major primary pollutants. Another class of pollutant – volatile organic compounds (VOCs) – though not a criteria pollutant, is recognized as a major primary pollutant because of its large emissions (both anthropogenic and biogenic sources) and its importance in the atmospheric reactions that form ground level ozone.

Table 4.7 presents data on emission rates from major sources, including industry, stationary, and mobile, for five major primary pollutants in the United States in 2006 and in 2016. We can see from the Table 4.7 that emissions from automobiles, trucks, buses, motorboats, lawns and garden equipment, construction vehicles, and other sources account for approximately 61 and 42% of all the NO _x and VOC emissions, respectively, and over 80% of all the CO emissions in the United States. Internationally, with continuing population growth particularly in Asia and South America, significance of mobile source pollution has been growing at an even faster rate than industrial pollution. Hence, the air pollution from the mobile sources is more severe each year. In various countries around the world, motor vehicles account for some 10–60% of total air pollution emissions. Here our focus is on air emissions from stationary sources and its industrial‐scale best available control technologies (BACT) for all major primary pollutants (also see Section 6.7.1).