The pH scale is a measure of acidity and alkalinity. Acid rain has a pH of 5.0 or less.

The pH of Acid Rain

Scientists express the acidity of acid rain using the pH scale. The scale defines a solution's acidity, neutrality or alkalinity based on its concentration of hydrogen ions. Acids have a high concentration of hydrogen ions and a low pH. The scale ranges from zero to 14, with pure water at a neutral 7.0. Most water, however, is not exactly pure. Even clean, normal rain has a pH of about 5.6. This is because it reacts with carbon dioxide in the atmosphere and forms mildly acidic carbonic acid before it becomes rain.

Acid rain has a pH of 5.0 or less. Most acid deposition ranges from pH 4.3 to 5.0 -- somewhere between the acidity of orange juice and black coffee. But comparing acid rain to safe, natural acids can be misleading. Even at its weakest, acid rain wrecks ecosystems by stunting sensitive plants and killing delicate aquatic eggs.

Programs that monitor acid rain analyze hydrogen content to determine pH. They also measure atmospheric concentrations of nitric acid, nitrate, sulfur dioxide, sulfate and ammonium. In the United States, the National Atmospheric Deposition Program (NADP) supervises wet deposition while the Clean Air Status and Trends Network (CASTNET) observes dry deposition. Monitoring acid deposition helps determine critical loads, or the amount of pollutants an ecosystem can support before damage. Accurate critical loads help set effective targets for SO2 and NOx reductions.

Now we'll learn about the harmful effects of acid rain on aquatic environments, forests, finishes, building materials and human health.

Surface Waters

Surface waters and their fragile ecosystems are perhaps the most famous victims of acid rain. Most of the precipitation that enters a lake, river, stream or marsh must first pass over and seep through soil. All soil has a buffering capacity, or ability to resist changes in acidity and alkalinity. The soil's buffering capacity determines a water body's acidity. If the capacity is low, or has reached its limit, acid rain can pass through un-neutralized.

Acid deposition weakens trees and pollutes surface waters.

Photographer: Detlev Voss | Agency: Dreamstime.com

Most life is comfortable at a near-neutral pH -- stray too far from pH 7.0, and delicate organisms begin to die. Plankton and invertebrates are sensitive to changes in acidity and die first. At pH 5.0, fish eggs degrade and young cannot develop. Adult fish and frogs can sometimes tolerate acidities as low as pH 4.0, but they starve as their weaker food sources die out. When acid rain disrupts the food chain, biodiversity decreases.

­Nitrogen deposition from acid rain also damages coastal waters and estuaries. Nitrogen-rich water supports ­massive algae growth­ and algal blooms. Bacteria decompose the dead algae, flourish themselves and soak up the water's available oxygen. Fish, shellfish, sea grass beds and coral reefs die in the algae-choked, oxygen-depleted waters. Scientists estimate that 10 percent to 45 percent of human-produced nitrogen that winds up in coastal waters comes from atmospheric deposition [Source: Environmental Protection Agency].

Most acidic bodies of water do not look polluted. As decaying organic matter settles, acidified water can appear clear and blue. Some species, like rushes and moss, even thrive in acidic conditions. But the greenery and clear waters belie an unwholesome environment. Diversity drops, and species left without predators often grow disturbingly large.

Acid rain also damages forests, as we'll see in the next section.

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