Mobile sources account for more than half of all the air pollution in the United States and the primary mobile source of air pollution is the automobile, according to the Environmental Protection Agency. Stationary sources, like power plants, emit large amounts of pollution from a single location, these are also known as point sources of pollution. Area sources are made up of lots of smaller pollution sources that aren't a big deal by themselves but when considered as a group can be. Natural sources can sometimes be significant but do not usually create ongoing air pollution problems like the other source types can.
Pollution On The Move
Pollution from human-generated and natural sources is often created in one place and transported through the air. Sometimes chemical reactions in the atmosphere change pollutants before they are deposited. Pollutants in the air can create haze, making it harder to see, and pollutant deposition can have biological effects. NPS areas experience these effects just like other places. Location and even the time of year can determine which pollution sources are most important to each park.