Explained: Acid Rain

What is Acid Rain?

Acid rain, also known as acid deposition, is a general phrase that refers to any type of precipitation that contains acidic components, such as sulfuric or nitric acid, and falls to the ground in wet or dry form from the atmosphere. This can include acidic rain, snow, fog, hail, or even dust.

What Causes Acid Rain?

Sulfur dioxide (SO₂) and nitrogen oxides (NOx) are released into the atmosphere and carried by wind and air currents, resulting in acid rain. Sulfuric and nitric acids are formed when SO₂ and NOx combine with water, oxygen, and other molecules. After mixing with water and other things, they fall to the earth.

While some of the SO₂ and NOx that create acid rain come from natural sources like volcanoes, the majority of it comes from the combustion of fossil fuels. The following are the major sources of SO₂ and NOx in the atmosphere:

  • Electricity is generated by burning fossil fuels. Electric power generators account for two-thirds of SO₂ and a fourth of NOx in the environment.

  • Heavy machinery and vehicles

  • Other industries include manufacturing, oil refineries, and others.

Acid rain is an issue for everyone, not just those who live near these sources because SO₂ and NOx can be carried great distances and over borders by the wind.

This illustration depicts the path that acid rain takes in our environment: (1) SO₂ and NOx emissions are discharged into the atmosphere, where (2) the pollutants are converted into acid particles that can travel great distances. (3) These acid particles then fall to the ground as wet and dry deposition (dust, rain, snow, etc.) and may harm soil, forests, streams, and lakes.

Acid Deposition in Different Forms

Wet Deposition

Acid rain is most typically associated with wet deposition. Sulfuric and nitric acids generated in the sky mix with rain, snow, fog, or hail to fall to the ground.

Dry Deposition

In the absence of moisture, acidic particles and gases can form dry deposition from the environment. Acidic particles and gases can easily settle on surfaces (water bodies, vegetation, and structures) or react with larger particles in the atmosphere, posing a health risk. When the stored acids are washed away by the following rain, acidic water pours over and through the ground, harming plants and wildlife such as insects and fish.

The quantity of acidity in the atmosphere that falls to earth as a result of dry deposition is determined by the amount of rain that falls in a given area. Desert locations, for example, have a larger ratio of dry to wet deposition than areas that receive several inches of rain each year.

Acid Rain Measurement

The pH scale, on which 7.0 is neutral, is used to determine acidity and alkalinity. The lower the pH (less than 7) of a substance, the more acidic it is, and the higher the pH (greater than 7) of a substance, the more alkaline it is. Rain has a pH of about 5.6 and is somewhat acidic because CO₂ dissolves in it and forms weak carbonic acid. A pH of 4.2 to 4.4 is typical with acid rain.

The National Atmospheric Deposition Program's (NADP) National Trends Network (NTN) provides observations of wet deposition to policymakers, research scientists, ecologists, and modellers. The NADP/NTN collects acid rain data from over 250 monitoring stations in the United States, Canada, Alaska, Hawaii, and the United States Virgin Islands. Dry deposition is more difficult and costly to measure than wet deposition. The Clean Air Status and Trends Network provides estimates of dry deposition of nitrogen and sulphur pollutants (CASTNET). CASTNET measures air concentrations at over 90 different places.

Acid deposition can cause lakes and streams to become acidic. The Long-Term Monitoring (LTM) Network measures and monitors surface water chemistry at over 280 locations to give important information on aquatic ecosystem health and how water bodies adapt to changes in acid-causing emissions and deposits.


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