Extension Notebook: Rain and acid

By Duane Friend

Looking at the title of this article sounds almost like a ′60s rock band, doesn’t it?

Actually, I want to talk about precipitation. You’ve probably heard about acid rain and the environmental concerns associated with it. What it really should be called is “precipitation that is more acidic than normal,” because precipitation is naturally slightly acidic. First let’s give some background on what acidity is.

In simple terms, a substance is acidic when it has the ability to donate a proton (also called a hydrogen ion). The more of these available, the more acidic a substance is. This is measured using a pH scale, which stands for the potential of hydrogen ion activity. The pH scale runs from 0 to 14. Neutral is 7, below 7 is acidic, above 7 is alkaline or basic. For acids, a whole number decrease equates to 10 times more acidity. Two numbers down, 100 times more acidity. For that reason, most pH numbers include a decimal to be more precise.

Precipitation forms thousands of feet above ground in clouds. As it falls through the atmosphere, it interacts with gases like carbon dioxide, creating carbonic acid. This lowers the acidity of “natural” precipitation to the 5.3 to 5.5 range. Natural processes in the environment are adapted and work well with this slight acidity in precipitation. And, yes, I keep saying precipitation instead of just rain, because all precipitation has this acidity.

Human activity has increased the acidity of precipitation. Burning coal releases huge amounts of sulfur dioxide into the atmosphere because most coal contains sulfur. When precipitation interacts with sulfur dioxide, it creates sulfuric acid, lowering the pH of rain and snow to 4.0 or less. Some of the lowest recorded pH was around 2.1 . This is 1,000 times more acidic than natural precipitation! The term “acid rain” was coined way back in the 1870s by Robert Angus Smith, a Scottish chemist. Much like greenhouse gases and atmospheric warming, these processes have been known as scientific fact long before they became environmental problems.

Nature is not adapted to this much acidity. Wetlands, wildlife, forests, roads, bridges, buildings and soils all degrade (or die out) quickly under highly acidic conditions. This was a major concern in Europe and the eastern United States in the 1970′s to the mid 2010s. While not quite back to normal, the acidity of precipitation has improved significantly, according to the National Atmospheric Deposition program. The University of Illinois and the Illinois State Water Survey is part of the NADP network.

Why have things gotten better? Because the amount of sulfur-laden coal being burned has dropped dramatically in the U.S.

In addition, something called “scrubbers” can be used to remove most of the sulfur from the combustion process.

Other areas of the world are still experiencing acid precipitation. Wherever coal use is still a main form of energy use, this will continue to be an environmental issue. The fact that things are trending for the better in some places shows that the environment can improve — it’s not just a downward spiral.

Duane Friend is the State Master Naturalist and Climate Change Specialist with University of Illinois Extension.