EPA's Clean Air Markets Programs address several environmental issues using emissions caps and allowance trading. This section of the Web site provides information about the major environmental issues being addressed by the Clean Air Markets Programs, including links to scientific information and data.
- Acid Rain
Acid deposition, or acid rain as it is commonly known, occurs when emissions of sulfur dioxide (SO2) and oxides of nitrogen (NOx) react in the atmosphere with water, oxygen, and oxidants to form various acidic compounds. These compounds then fall to the earth in either dry form (such as gas and particles) or wet form (such as rain, snow, and fog). Prevailing winds transport the compounds hundreds of miles, across state lines and national borders.
- Smog/Regional Transport of Ozone
Smog is a brownish haze that usually appears over cities in the summer. Smog can make breathing difficult for some people, and it also can impair visibility. The main component of smog is ground-level ozone, a gas that is created when NOx reacts with other chemicals in the air, especially in the presence of strong sunlight. NOx can travel long distances before reacting to form ozone, creating regional problems instead of only affecting the local area where it is emitted.
Mercury is a naturally occurring element that can be found throughout the environment. Human activities, such as burning coal and using mercury to manufacture certain products, have increased the amount of mercury in many parts of the environment including the atmosphere, lakes, and streams.
- Regional Haze/Particulate Matter
Regional haze is the visibility impairment that occurs when particles and gases in the atmosphere, including sulfates and nitrates, scatter and absorb light. The pollutants that are associated with acid rain are the same ones that reduce visibility. Visibility tends to vary by season and geography because it is also affected by the angle of the sunlight and the level of humidity. High relative humidity heightens pollution's effects on visibility because particles, such as sulfates, accumulate water and grow to a size at which they scatter more light, thus creating haze.
- Climate Change
According to scientists, the Earth's surface has risen in temperature by about 1 degree Fahrenheit in the past century. There is increasing evidence that certain human activities are contributing to this change in temperature through activities that increase the levels of greenhouse gases, primarily carbon dioxide, methane, and nitrous oxide, in the atmosphere. Greenhouse gases trap heat that would normally escape back into the atmosphere, thus increasing the Earth's natural greenhouse effect and increasing temperature over time. To learn more about greenhouse gasses, view the Green house Gas Inventory