Course: BIS 105 Environmental Biology

Unit 12 Learning Module

Atmospheric Science (7/7)

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.

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 stuff. 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. A future unit will look at our energy sources in depth.

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. 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 (N_2) and the oxygen forms O_2. VOCs and CO are similarly broken apart into the greenhouse gases H_2O and CO_2. Catalytic converters only work when they are hot, so greater concentrations of pollutants escape as the car is warming up.

A large catalytic converter on a van.

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.

Diagram of one type of scrubber.

Scrubbers remove particles and waste gases from exhaust. 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 CO_2 are still released even with clean coal technology.