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From the Spring 2013 Conservationist for Kids

the acid rain cycle

Acid Rain

By Gina Jack

What's in rainwater?

Raindrops form when water vapor condenses on tiny particles of dust floating in the atmosphere. As more water vapor condenses, the tiny droplets continue to grow. Eventually, they become so large they fall to the ground. Besides water and dust, raindrops also contain chemicals mixed in from the atmosphere. When water and chemicals in the atmosphere combine and react with each other, the water becomes slightly acidic. Normal "clean" rainwater has a pH around 5.6. (See "What is pH?")

What's in acid rain?

We burn fossil fuels (coal, oil, natural gas) to generate electricity, run factories, and power our vehicles. When we burn these fuels, sulfur dioxide (SO2) and nitrogen oxides (NOX) are among the polluting gases released into the atmosphere. They mix and react with moisture in the air and form sulfuric acid and nitric acid. Extra acids in the air make rain more acidic. If rainwater has a pH lower than 5.6 (normal clean rain), it's considered to be acid rain. Any form of precipitation-rain, snow, sleet, hail, even fog-can become acidic from pollution.

The average pH of rainfall in New York State is between 4.5 and 5.0-acid rain.

It's not just the rain. Acids from the atmosphere also settle to the ground as dry particles and gases. They are deposited on forests, lawns, lakes, buildings, and more. We call this "dry deposition", and it causes damage, just like wet deposition (rain, snow, etc.).

Acid deposition can...

  • damage forests
  • harm plants
  • harm animals
  • damage soil
  • pollute water
  • damage buildings and other structures (stone, concrete, metal)
image of the pH scale
pH values

Acid rain won't burn your skin like very strong acids do, but it can affect your health. The same pollutants-gases and tiny particles-that wash out of the air as acid deposition can irritate our lungs when we inhale them. They're especially bad for people with respiratory illnesses such as asthma and bronchitis.

What is pH?

pH is a measurement of how acidic or basic (alkaline) a substance is. A reading of 0 is the most acidic on the pH scale, while 14 is the most basic, and 7 is neutral, neither acidic nor basic. Substances at the extreme ends of the scale-either very acidic or very basic when concentrated-can cause chemical burns. Just as mixing hot and cold water makes warm water, acids and bases can cancel out each other and result in a more neutral pH.

Photo: Wind carries air pollution far from its source. Most acid rain that falls in New York State began its journey hundreds of miles away from here.