“Acid rain” is one of those terms that we occasionally hear about, but don’t quite understand.
Mark Nilles is a hydrologist with the U.S. Geological Survey. He says pure water in a clean atmosphere has a pH of 5.6. Anything below 5.6 is considered to be acid rain. This happens when moisture is mixed with acidic pollutants in the air. There are two primary precursors - sulfur dioxide, which is primarily from coal combustion, and nitrogen oxides, which have a different process.
"That’s actually very high temperature combustion of anything, and the nitrogen oxides are primarily produced by breaking the bonds of inert nitrogen-two," explains Nilles. "The atmosphere is predominantly stable N2 gas and high temperature combustion will break that, and create a reactive form of nitrogen which is also a precursor to acid rain."
Acid rain isn’t always wet. In a dry climate, Nilles says acidic compounds in the air will deposit in the form of a dry, dusty material.
A classic example of the effects of acid rain is in the Adirondacks in the Northeast.
"You had lakes that were “dead”, and the acids mobilized toxins in the soils, which moved to the lakes, which resulted in fish kills," he says. "And forest effects. Acids in forest soils can affect forest productivity, and favor certain species over other species that are more acid tolerant."
But there is good news. Nilles says the acidity of rainfall and acid rain itself in the United States has declined dramatically since its peak in the late 1970s.