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Measuring Acid Rain

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For Students

To learn more about measuring the pH of water and soil, visit the Science Experiments page. This page includes information on how to measure pH, how to make a natural pH indicator, and more.

Acid rain is measured using a scale called “pH.” The lower a substance's pH, the more acidic it is. See the pH page for more information.

Pure water has a pH of 7.0. However, normal rain is slightly acidic because carbon dioxide (CO2) dissolves into it forming weak carbonic acid, giving the resulting mixture a pH of approximately 5.6 at typical atmospheric concentrations of CO2. As of 2000, the most acidic rain falling in the U.S. has a pH of about 4.3.

Two networks, both supported by EPA, monitor acid rain’s pH and the chemicals that cause acid rain. The National Atmospheric Deposition Program Exit EPA Disclaimer measures wet deposition and developed maps of rainfall pH (follow the link to the isopleth maps) and other important precipitation chemistry measurements.

The Clean Air Status and Trends Network (CASTNET) measures dry deposition. This EPA Web site features information about the data collected, the measuring sites, and the types of equipment used.

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