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In our discussion of energy and energy choices, we saw that sustainability is best achieved by considering conservation and efficiency first. Similarly, if we can address the problems of pollution by seeking ways to avoid creating it in the first place, we will require less energy and fewer resources to clean it up. Preventing pollution is usually much less expensive and energy intensive then controlling it. Unfortunately, pollution prevention is not always possible.

Learning Objectives

After reading this module, you should be able to

As with other types of pollution, the best way to decrease air pollution emissions is to avoid them in the first place. This can be achieved through the use of fuels that contain fewer impurities. Coal and oil, for example, both occur naturally with different sulfur concentrations and are available for purchase at a variety of sulfur concentrations. In addition, during refining and processing, the concentration of sulfur can be reduced in both fuels.

Use of a low-sulfur coal or oil is certainly one of the best means of controlling air pollution, although typically low-sulfur coal or oil is more expensive to purchase than coal or oil containing higher sulfur concentrations. As we discussed in Chapters 12 and 13, other ways to reduce air pollution include increased efficiency and conservation: Use less fuel and you will produce less air pollution. While these measures reduce emissions by a certain amount, wherever fuel is combusted, pollution will be emitted. So ultimately, most attempts to reduce air pollution will depend on the control of pollutants after combustion. There are a number of approaches to controlling emissions, as we will discuss in the next section.

Control of Sulfur and Nitrogen Oxide Emissions

Sulfur and nitrogen oxides are common air pollutants in the United States and they cause a variety of environmental problems, including acid deposition that we described in the previous module. A substantial number of air pollution control measures have been directed toward sulfur and nitrogen oxides. Sulfur dioxide emissions from coal exhaust can be reduced by a process known as fluidized bed combustion. In this process, granulated coal is burned in close proximity to calcium carbonate. The heated calcium carbonate absorbs sulfur dioxide and produces calcium sulfate, which can be used in the production of gypsum wallboard, also known as sheetrock, for houses. Some of the sulfur oxide that does escape the combustion process can be captured by other methods after combustion.

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The atmosphere of Earth is 78 percent nitrogen gas and, as a result, nitrogen oxides are produced in virtually all combustion processes. Hotter burning conditions and the presence of oxygen allow proportionally more nitrogen oxide to be generated per unit of fuel burned. In order to reduce nitrogen oxide emissions, burn temperatures must be reduced and the amount of oxygen must be controlled—a procedure that is sometimes utilized in factories and power plants by means of certain air pollution control technologies. However, lowering temperatures and oxygen supply can result in less-complete combustion, which reduces the efficiency of the process and increases the amount of particulates and carbon monoxide. Finding the exact mix of air, temperature, oxygen, and other factors is a significant challenge.

Nitrogen oxide emissions from automobiles have also been reduced significantly in the United States over the last 35 years. Beginning in 1975, all new automobiles sold in the United States were required to include a catalytic converter, which reduces the nitrogen oxide and carbon monoxide emissions. In order to operate properly, the precious metals in the catalytic converter (mostly platinum and palladium) cannot be exposed to lead. Therefore gasoline could no longer contain lead. As FIGURE 46.6 illustrates, the change in gasoline formulation caused a significant reduction in emissions of lead from automobiles and in lead concentrations in the atmosphere. At the same time, improvements in the combustion technology of power plants and factories also reduced emissions of nitrogen oxides.

Control of Particulate Matter

The removal of particulate matter is the most common means of pollution control. Sometimes the process of removing particulate matter also removes sulfur. There are a variety of methods used to remove particulate matter. The simplest is gravitational settling, which relies on gravity as the exhaust travels through the smokestack. The particles simply settle out to the bottom. The ash residue that accumulates must be disposed of in a landfill. Depending on the fuel that was burned, the ash may contain sufficiently high concentrations of metals that require special disposal. This subject will be covered in more detail in Chapter 16 on solid waste.

Pollution control devices remove particulate matter and other compounds after combustion. Each has its advantages and disadvantages and all of them use energy—most commonly electricity—which generates additional pollution. Fabric filters are a type of filtration device that allow gases to pass through them but remove particulate matter. Often called baghouse filters, certain fabric filters can remove almost 100 percent of the particulate matter emissions. Electrostatic precipitators use an electrical charge to make particles coalesce so they can be removed. Polluted air enters the precipitator and the electrically charged particles within are attracted to negative or positive charges on the sides of the precipitator. The particles collect and relatively clean gas exits the precipitator. A scrubber, shown in FIGURE 48.1, uses a combination of water and air that actually separates and removes particles. Particles are removed in the scrubber in a liquid or sludge form and clean gas exits. Borrowing from the concept utilized in the electrostatic precipitator, particles are sometimes ionized before entering the scrubber to increase its efficiency. Scrubbers are also used to reduce the emissions of sulfur dioxide. All three types of pollution control devices, because they use additional energy and increase resistance to air flow in the factory or power plant, require the use of more fuel and result in increased carbon dioxide emissions.

Devices such as the electrostatic precipitator and the scrubber have helped reduce pollution significantly before it is released into the atmosphere. It is much harder—if not impossible—to remove pollutants from the environment after they have been dispersed over a wide area.

Smog Reduction

We have seen that many cities in the United States and around the world continue to have smog problems. Because the main component of photochemical smog—ozone—is a secondary pollutant, control efforts must be directed toward reducing the precursors, or primary pollutants. Historically, most local smog reduction measures have been directed primarily at reducing emissions of VOCs in urban areas. As noted earlier, with fewer VOCs in the air there are fewer compounds to interact with nitrogen oxides, and thus more nitrogen oxide will be available to recombine with ozone. More recently, regional efforts to control ozone have focused on reducing nitrogen oxide emissions, which appears to be a more effective method of controlling smog in areas away from urban centers.

A number of cities around the world, including those in China, Mexico, and England, have taken innovative and often controversial measures to reduce smog levels. Municipalities have passed measures, for example, to reduce the amount of gasoline spilled at gasoline stations, restrict the evaporation of dry-cleaning fluids, or restrict the use of lighter fluid (a VOC) for starting charcoal barbecues. Both urban and suburban areas have taken additional actions such as calling for a reduction in the use of wood-burning stoves or fireplaces that would reduce emissions of not only nitrogen oxide but also particulate matter, VOCs, and carbon monoxide. A number of California municipalities even discussed reducing the number of bakeries within certain areas, as the emissions from rising bread contain VOCs. This proposal was not very popular, as you can imagine, but emissions from bakeries along with many other businesses are sometimes regulated by local air-quality ordinances.

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Since cars are responsible for large emissions of nitrogen oxides and VOCs in urban areas, and these two compounds are the major contributors to smog formation, some municipalities have tried to achieve lower smog concentrations by restricting automobile use. A number of cities, including Mexico City, have instituted plans permitting automobiles to be driven only every other day—for example, those with license plates ending in odd numbers may be used on one day and those with even-numbered license plates on alternate days. In China, during the 2008 Beijing Olympics, the government successfully expanded public transportation networks, imposed motor vehicle restrictions, and temporarily shut down a number of industries as a way to reduce photochemical smog and improve visibility (FIGURE 48.2).

Limiting automobile use has also helped to reduce other air pollutants. Carpool lanes, available in many areas, reduce the number of cars on the road by encouraging two or more people to share one vehicle. Improving the quality and accessibility of public transportation encourages people to leave their cars at home. A number of cities in England, including London, have been experimenting with charging individual user fees (tolls) for the use of roads at certain times of the day or within certain parts of a city as a way to reduce automobile traffic. Road user fees have been proposed for cities in the United States, including New York City, but none has yet been implemented.

In 1990 and again in 1995, scientists, policy makers, and academics collaborated on amendments to the Clean Air Act that would allow the free market to determine the least expensive ways to reduce emissions of sulfur dioxide. The free-market program was implemented in two phases between 1995 and 2000, and approximately 3,000 power plants are now covered under the Acid Rain Program of the act. So far, each phase has led to significant reductions in sulfur emissions.

One of the most innovative aspects of the Clean Air Act amendments was the provision for the buying and selling of allowances that authorized the owner to release a certain quantity of sulfur. Each allowance authorizes a power plant or industrial source to emit one ton of SO₂ during a given year. Sulfur allowances are awarded annually to existing sulfur emitters proportional to the amounts of sulfur they were emitting before 1990, and the emitters are not allowed to emit more sulfur than the amount for which they have permits. At the end of a given year, the emitter must possess a number of allowances at least equal to its annual emissions. In other words, a facility that emits 1,000 tons of SO₂ must possess at least 1,000 allowances that are usable in that year. Facilities that emit quantities of SO₂ above their allowances must pay a financial penalty.

Sulfur allowances can be bought and sold on the open market by anyone. If emitters wanted to exceed their allowance level—say, because they intended to increase their industrial output—they would be required to purchase more allowances from another source. If, on the other hand, a company decreased its sulfur emissions more than it needed to in order to comply with its allowance amount, it could sell any unused sulfur emission allowances. Over time, the number of allowances distributed each year has been gradually reduced: The total SO₂ emissions from all sources in the United States have declined from 23.5 million metric tons (26 million U.S. tons) in 1982 to 10.3 million metric tons (11.4 million U.S. tons) in 2008. “Do the Math: Calculating Annual Sulfur Reductions” shows you how to calculate these decreases as percentages. The overall economic cost for achieving these reductions has been about one-quarter of the original cost estimate. Global change researchers have used the sulfur allowance example as a model for the more recent experiments with buying and selling carbon dioxide allowances. We discuss this further in “Science Applied 8: Can We Solve the Carbon Crisis Using Cap-and-Trade?” following Chapter 20 on page 730.