High School Earth Science/Reducing Air Pollution

The Clean Air Act of 1970 and the amendments since then have done a great job in requiring people to clean up the air over the United States. Emissions of the six major pollutants regulated by the Clean Air Act, carbon monoxide, lead, nitrous oxides, ozone, sulfur dioxide, and particulates, have decreased by more than 50%. Cars, power plants, and factories individually release less pollution than they did in the mid-20^th century. But there are many more cars, power plants and factories. Many pollutants are still being released and some substances have been found to be pollutants that were not known to be pollutants in the past. There is still much work to be done to continue to clean up the air.

Lesson Objectives

Describe the major ways that energy use can be reduced.
Discuss new technologies that are being developed to reduce air pollutants, including greenhouse gases.
Describe the difference between placing caps on emissions and reducing emissions.

Ways to Reduce Air Pollution

Air pollution can be reduced in a number of ways. Using less fossil fuel is one way to lessen pollution. People use less fuel by engaging in conservation, which means not using a resource or using less of it. For example, riding a bike or walking instead of driving doesn't use any fossil fuel. Taking a bus uses less than driving or riding by yourself in a car, as does carpooling. If you need to drive, buying a car that has greater fuel efficiency is important.

You can conserve electricity (and thus fossil fuels) at home by turning off light bulbs and appliances when they are not in use, using energy efficient light bulbs and appliances, and even buying less things. All these actions reduce the amount of energy that power plants need to produce.

There are many reasons for people in North America and Europe to try to reduce their use of fossil fuels. As you have already seen, air pollution has tremendous health and environmental costs. There are other reasons as well. Much of the oil we use comes from the Middle East, which is a politically unstable region of the world. Also, fossil fuels are running out, although some will run out sooner than others. The most easily accessible fossil fuels are mostly already gone and harder to use or recover fuels are now being used. There are other types of fossil fuels that can eventually replace coal and petroleum, such as tar sands and oil shale. But these have even more environmental problems than traditional fossil fuels have: mining them from the ground causes severe environmental damage and burning them releases pollutants, including greenhouse gases.

Alternative energy sources are important. They currently are not a large part of the energy supply, but they will increase rapidly over the coming years and decades. Several sources of alternative energy, including solar and wind are not currently being used much because the technologies are not well enough developed. Converting sunlight into usable solar power, for example, is still very expensive relative to using fossil fuels. For solar to be used more widely, technology will need to advance so that the price falls. Also, solar power is not practiced in all parts of the United States because some areas get low amounts of sunlight. These locations will need to develop different power sources. While the desert Southwest will need to develop solar, the Great Plains can use wind energy as its energy source. Perhaps some locations will rely on nuclear power plants, although current nuclear power plants have major problems like safety and waste disposal.

Some pollutants can be filtered out of the exhaust stream before they are released into the atmosphere. Other pollutants can be broken down into non-toxic compounds before they are released. Some of these technologies will be described in the following sections.

Reducing Air Pollution from Vehicles

Reducing air pollution from vehicles can be done in a number of ways. Pollutants can be broken down before they are released into the atmosphere. The vehicles can be more fuel efficient. New technologies can be developed so that they do not rely on fossil fuels at all.

Motor vehicles emit less pollution than they once did due to catalytic converters (Figure 22.12). Catalytic converters are placed on modern cars in the United States. These devices reduce emissions of nitrous oxides, carbon monoxide and VOCs. A catalyst speeds up chemical reactions without being used up in the reaction itself. For nitrous oxides, the catalyst breaks the nitrogen and oxygen atoms apart. The nitrogen then combines with another nitrogen ion to form nitrogen gas (N₂) and the oxygen forms O₂. VOCs and CO are similarly broken apart into the greenhouse gases H₂O and CO₂. Catalytic converters only work when they are hot, so a lot of exhaust escapes as the car is warming up.

Figure 22.12: A large catalytic converter on an SUV.

There are several simple ways to make a vehicle more fuel efficient. Lighter vehicles need less energy to move. Streamlined vehicles experience less resistance from the wind. So, small, lightweight, streamlined cars get much better gas mileage than chunky, heavy SUVs. Hybrid vehicles are among the most efficient vehicles that are now widely available. Hybrids have a small internal combustion engine that works like an ordinary car. They also have an electric motor and a rechargeable battery. During braking, a normal car loses the energy it has because it is in motion. In a hybrid, that energy is instead funneled into charging the battery. When the car accelerates again, it uses the power stored in the battery. The internal combustion engine only takes over when power in the battery has run out. Hybrids get excellent gas mileage in cities where the vehicle frequently stops and starts. Hybrid vehicles also have catalytic converters: the battery preheats the converter so that it begins to work much sooner after the car is turned on. Hybrids can reduce auto emissions by 90% or more. Unfortunately, in many hybrid vehicles the hybrid technology is used to improve acceleration more than gas mileage.

A new technology that is in development is a plug-in hybrid. The vehicle is plugged into an electricity source when it is not in use, perhaps in a garage. The car uses the power stored in that battery when it is next used. Plug-in hybrids are less polluting than regular hybrids, since they can run for a longer time on electricity. Automakers expect that plug-in hybrids will become available around 2010.

Fuel cells are another technology that is in development (Figure 22.13). Hydrogen fuel cells harness the energy released when hydrogen and oxygen come together to create water. Fuel cells are extremely efficient and they produce no pollutants. But developing fuel cell technology has its problems. The oxygen the fuel cell uses comes from the atmosphere, but there is no easy source of hydrogen. Natural gas is a source, but converting it into usable hydrogen decreases the efficiency of the fuel cells and increases pollution, including greenhouse gases. Natural gas also has other important uses. A few fuel cell cars are now being produced as models. Right now these cars are extremely expensive and fueling stations are rare. Some automakers say that for fuel cell vehicles to become widespread the cost of production must decrease to 1% of its current price.

Figure 22.13: A hydrogen fuel cell car looks like a gasoline-powered car.

Reducing Industrial Air Pollution

Pollutants are removed from the exhaust streams of power plants and industrial plants before they enter the atmosphere. Particulates can be filtered out, while sulfur and nitric oxides are broken down by catalysts. Removing these oxides reduces the pollutants that cause acid rain.

Particles are relatively easy to remove from emissions. Baghouses work like a giant vacuum cleaner bag, filtering dust as it streams past. Baghouses collect about 98% of dry particulates. Cyclones are air streams that rotate quickly through a container shaped like a cylinder or a cone. Large particles are forced toward the edges of the air stream. When they hit the outside wall of the container, they fall to the bottom and are swept up. Smaller particles can be picked up as the radius of the cyclone is reduced. Particles can also be collected and removed by static electricity. These electrostatic precipitators are useful for removing materials from very hot gases.

Figure 22.14: Diagram of one type of scrubber.

Scrubbers remove particles and waste gases from exhaust (Figure 22.14). Wet scrubbers use a liquid solution to scrub pollutants. Dry scrubbers use alkali or other materials to neutralize acid gas pollutants. Other techniques are used to eliminate other toxic gases. Nitrogen oxides, for example, can be broken down at very high temperatures.

Gasification is a developing technology. This method removes some of the toxins present in coal before they are released into the atmosphere. In gasification, coal is heated to extremely high temperatures. The gas that is produced is filtered and the energy goes on to drive a generator. About 80% less pollution is released over regular coal plants, and greenhouse gases are also lower. Clean coal plants do not need scrubbers or other pollution control devices. Although the technology is ready, clean coal plants are more expensive to construct and operate and so they are seldom built. Also, heating the coal to high enough temperatures uses a great deal of energy, so the technology not very energy efficient. In addition, large amounts of the greenhouse gas CO₂ are still released even with clean coal technology.

Reducing Ozone Destruction

One success story in reducing pollutants that harm the atmosphere concerns ozone-destroying chemicals. In 1973, scientists calculated that CFCs could reach the stratosphere and break apart. This would release chlorine atoms, which would then destroy ozone. Based only on their calculations, the United States and most Scandinavian countries banned CFCs in spray cans in 1978.

More confirmation that CFCs break down ozone was needed before more was done to reduce production of ozone-destroying chemicals. In 1985, members of the British Antarctic Survey reported that a 50% reduction in the ozone layer had been found over Antarctica in the previous three springs. Two years later, the Montreal Protocol on Substances that Deplete the Ozone Layer was ratified by nations all over the world.

The Montreal Protocol controls the production and consumption of 96 chemicals that damage the ozone layer. Hazardous substances are phased out first by developed nations and one decade later by developing nations. More hazardous substances are phased out more quickly. CFCs have been mostly phased out since 1995, although some will be used in developing nations until 2010. The Protocol also requires that wealthier nations donate money to develop technologies that will replace these chemicals.

If CFCs were not being phased out, by 2050 they would have been probably been 10 times more abundant than they were in 1980. The result would have been about 20 million more cases of skin cancer in the United States and 130 million cases globally. Even though governments have acted to reduce CFCs, they take many years to reach the stratosphere and they can survive there a long time before they break down. So the ozone hole will probably continue to grow for some time before it begins to shrink. The ozone layer will reach the same levels it had before 1980 in around 2068 and 1950 levels in one or two centuries.

Reducing Greenhouse Gases

Reducing greenhouse gas emissions is related to air pollution control. Unlike many other air pollutants, climate change is a global problem. Climate scientists agree that all nations must come together to reduce greenhouse gas emissions. So far, this has not occurred.

The first attempt to cap greenhouse gas emissions was the Kyoto Protocol. The Kyoto Protocol limits greenhouse gas emissions for developed nations to below 1990 levels. Kyoto has not achieved the success of the Montreal Protocol for several reasons. The largest emitter of greenhouse gases, the United States, did not sign and was not bound by the agreement. Developing nations, most notably China, signed the treaty but are not obligated to make changes in their greenhouse gas emissions. Of the nations that agreed to reduce their emissions, few are on track to achieve their target. More importantly, several years have passed since this process was begun and climate scientists agree that the Protocol does not reduce emissions nearly enough. Some say that reductions 40 times those required by Kyoto are needed to avoid dangerous climate change. Plans are now being made to replace the Kyoto Protocol with a more effective treaty in 2012.

The Kyoto Protocol set up a cap-and-trade system. Each participating nation was given a cap on greenhouse gas emissions that it should not go over. If a nation is likely to go over its cap, it can buy credits from a nation that will emit less greenhouses gases than allowed by the cap. Cap-and-trade provides a monetary incentive for nations to develop technologies that will reduce emissions and to conserve energy. Some states and cities within the United States have begun their own cap-and-trade systems, since they believe that the federal government is not doing enough to address the problem of climate change.

However it is done, climate scientists and many others agree that greenhouse gas emissions must be lowered. The easiest and quickest way is to increase energy efficiency. A carbon tax can be placed on CO₂ emissions to encourage conservation. The tax would be placed on gasoline, carbon dioxide emitted by factories, and home energy bills to encourage conservation. For example, when people make a purchase of a new car, they will be more likely to purchase an energy efficient model. The money from the carbon tax can then be used for research into alternative energy sources. All plans for a carbon tax allow a tax credit for people who cannot afford to pay more for energy, so that they do not suffer unfairly.

More energy efficient vehicles and appliances can be developed. Some, like hybrid cars are currently available. Agricultural practices that lessen the amount of methane produced can be used.

Beyond increasing efficiency, new technologies can be developed. Alternative energy sources, like solar and wind can be developed and expanded. Biofuels can replace gasoline in vehicles, but they must be developed sensibly (Figure 22.15). So far much of the biofuel is produced from crops like corn. But when food crops are used for fuel, the price of food goes up. Also modern agriculture is extremely reliant on fossil fuels for pesticides, fertilizers and the work of farming. This means that not much energy is gained from using a biofuel over using the fossil fuels directly. More promising crops for biofuels are now being researched. Surprisingly, algae is being investigated as a source of fuel! The algae can be grown in areas that are not useful for agriculture, and it also contains much more usable oil than crops like corn.

Figure 22.15: A bus that runs on soybean oil shows the potential of biofuels.

Greenhouse gases can also be removed from the atmosphere after they are emitted. Carbon sequestration occurs when carbon dioxide is removed from the atmosphere. Carbon is sequestered naturally in forests, but unfortunately, more forest land is currently being lost than gained. Another idea is to artificially sequester carbon. For example, carbon can be captured from the emissions from gasification plants. That carbon is then stored underground in salt layers or coal seams, which keeps it out of the atmosphere. While some small sequestration projects are underway, no large-scale sequestration has yet been attempted. While it is a promising new technology, carbon sequestration is also untested and may not prove to be significant in fighting global warming.

Just as individuals can diminish other types of air pollution, people can fight global warming by conserving energy. Also, people can become involved in local, regional and national efforts to make sound choices on energy policy.

Lesson Summary

Air pollutants can be reduced in many ways. The best method is to not use the energy that produces the pollutants by conservation or increasing energy efficiency.
Alternative energy sources are another good way to reduce pollution. Most of these alternate energy technologies are still being refined (solar, wind) and some have other problems associated with them (nuclear, biofuels).
Pollutants can be removed from an exhaust stream by being filtered out or broken down. Some pollutants are best not released at all like CFCs.

Review Questions

Since the Clean Air Act was passed in 1970, why is the air still not clean?
What are some ways that can help conserve energy?
How does reducing air pollutants, as described in the Clean Air Act of 1970, affect greenhouse gas emissions?
What has to be done before alternative energy sources can replace fossil fuels?
What are catalytic converters?
Why are hybrid vehicles more energy efficient than regular vehicles powered by internal combustion engines?
Why aren't fuel cell vehicles widely available yet?
How does a cyclone reduce particulate pollution?
How can coal power be made so that it has nearly zero carbon contribution to the atmosphere?
Why is it that the ozone hole will not be healed for several decades?
Many people think that biofuels are the solution to a lot of the problem of climate change, but others disagree. What requirements would biofuels have to meet if they were to be really effective at replacing gasoline in motor vehicles?

Vocabulary

biofuel: A fuel made from living materials, usually crop plants.
carbon sequestration: Removal of carbon dioxide from the atmosphere, so that it does not act as a greenhouse gas in the atmosphere.
catalyst: A substance that increases (or decreases) the rate of a chemical reaction but is not used up in the reaction.
catalytic converter: Found on modern motor vehicles, these devices use a catalyst to break apart pollutants.
fuel cell: An energy cell in which chemical energy is converted into electrical energy.
gasification: A technology that cleans coal before it is burned, which increases efficiency and reduces emissions.
hybrid vehicle: A very efficient vehicle that is powered by an internal combustion engine, an electric motor and a rechargeable battery.

Points to Consider

Why is it important to reduce air pollution?
What can you do in your own life to reduce your impact on the atmosphere?
Why is a worldwide effort needed to reduce the threat of global climate change?

← Effects of Air Pollution · Observing and Exploring Space →