Context: 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-20th 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. Reducing air pollution from vehicles can be done in a number of ways. Breaking down pollutants before they are released into the atmosphere. Motor vehicles emit less pollution than they once did because of catalytic converters (Figure 1.1). Catalytic converters contain a catalyst that speeds up chemical reactions and breaks down nitrous oxides, carbon monoxide, and VOCs. Catalytic converters only work when they are hot, so a lot of exhaust escapes as the car is warming up. Catalytic converters are placed on mod- ern cars in the United States. Making a vehicle more fuel efficient. Lighter, more streamlined vehicles need less energy. Hybrid vehicles have an electric motor and a rechargeable battery. The energy that would be lost during braking is funneled into charging the battery, which then can power the car. The internal combustion engine only takes over when power in the battery has run out. Hybrids can reduce auto emissions by 90% or more, but many models do not maximize the possible fuel efficiency of the vehicle. A plug-in hybrid is plugged into an electricity source when it is not in use, perhaps in a garage, to make sure that the battery is charged. Plug-in hybrids run for a longer time on electricity and so are less polluting than regular hybrids. Plug-in hybrids began to become available in 2010. Developing new technologies that do not use fossil fuels. Fueling a car with something other than a liquid organic-based fuel is difficult. A fuel cell converts chemical energy into electrical energy. Hydrogen fuel cells harness the energy released when hydrogen and oxygen come together to create water (Figure 1.2). Fuel cells are extremely efficient and they produce no pollutants. But developing fuel-cell technology has had many problems and no one knows when or if they will become practical. Pollutants are removed from the exhaust streams of power plants and industrial plants before they enter the atmo- sphere. Particulates can be filtered out, and sulfur and nitric oxides can be broken down by catalysts. Removing these oxides reduces the pollutants that cause acid rain. Particles are relatively easy to remove from emissions by using motion or electricity to separate particles from the gases. Scrubbers remove particles and waste gases from exhaust using liquids or neutralizing materials (Figure 1.3). Gases, such as nitrogen oxides, can be broken down at very high temperatures. A hydrogen fuel-cell car looks like a gasoline-powered car. Scrubbers remove particles and waste gases from exhaust. Gasification is a developing technology. In gasification, coal (rarely is another organic material used) is heated to extremely high temperatures to create syngas, which is then filtered. The energy goes on to drive a generator. Syngas releases about 80% less pollution than 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. Also, heating the coal to high enough temperatures uses a great deal of energy, so the technology is not energy efficient. In addition, large amounts of the greenhouse gas CO2 are still released with clean coal technology. Nonetheless, a few of these plants are operating in the United States and around the world. How can air pollution be reduced? Using less fossil fuel is one way to lessen pollution. Some examples of ways to conserve fossil fuels are: Riding a bike or walking instead of driving 
 Question: these help to remove particles and waste gases from exhaust using liquids or neutralizing materials.
Output: scrubbers