Acid Rain in New England
Acid rain is rain with a higher concentration of positively charged atomic particles (ions) than normal rain. Acid rain and its frozen equivalents, acid snow and acid sleet, are part of a larger problem called acid deposition. Acid deposition also includes direct deposition, in which acidic fog or cloud is in direct contact with the ground; and dry deposition, in which ions become attached to dust particles and fall to the ground.
On this website, unless otherwise indicated, the term "acid rain" will refer to all types of acid deposition.
Acid rain is one type of atmospheric deposition. Atmospheric deposition includes any precipitation, airborne particles or gases deposited from the atmosphere to the Earth’s surface. Other forms of atmospheric deposition may also be by wet or dry methods. Much of the material in atmospheric deposition may be a nuisance, but does not harm the environment. Some air pollutants, such as those in acid rain, can cause environmental problems. Over many decades, the combined input of contaminants to sensitive environments can lead to widespread environmental problems. Most smaller particles with a diameter of 10 microns (.004 inches) or less are too light to be deposited, and so remain in the atmosphere where they can cause health problems. They pose a different problem, and are regulated as particulates, or PM.
Acid rain occurs when sulfur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in clouds. The droplets then fall to earth as rain, snow, or sleet. This can increase the acidity of the soil, and affect the chemical balance of lakes and streams. Decades of enhanced acid input has increased the environmental stress on high elevation forests and aquatic organisms in sensitive ecosystems. In extreme cases, it has altered entire biological communities and eliminated some fish species from certain lakes and streams. In many other cases, the changes have been more subtle, leading to a reduction in the diversity of organisms in an ecosystem. This is particularly true in the northeastern United States, where the rain tends to be most acidic, and often the soil has less capacity to neutralize the acidity. Acid rain also can damage certain building materials and historical monuments. Some scientists have suggested links to human health, but none have been proven.
Acidity is measured on the per-hydrogen, or pH scale. This is a measure of the concentration of positively charged ions in a given sample. It ranges from 14 (alkaline or negatively charged ions) to 0 (acidic or positive ions). Pure water has a pH of 7 (neutral). Most rainwater is slightly acidic (pH about 6). A change in the pH scale of one unit reflects a tenfold (10X) change in the concentration of acidity. Generally rain with a pH value of less than about 5.3 is considered acid rain. As the map below shows, most of the rainwater which falls in the eastern United States has a pH between 4.0 and 5.0. This is generally lower (more acidic) than the national average.