Hubbard Brook Research Foundation’s 2001 report: ‘Acid Rain Revisited’

Assessment of Forest Sensitivity to Nitrogen and Sulfur Deposition in New Hampshire and Vermont.

While most people have heard of acid rain, most don’t really understand what it is or why it’s of concern to us. The notion of acid falling from the sky and burning our skin is a good story-line for science fiction, but is a good stretch from reality. In fact, the acidity of precipitation cannot be sensed by people, but it does affect our water and forest quality, the sustainability of aquatic organisms, and animals that feed on aquatic animals. Acid rain also contributes to the deterioration of limestone and marble statues and monuments.

Acid rain is more correctly referred to as acid deposition because the acid can also be removed from the air in forms other than rain, such as snow, drizzle, dew, fog, and as dry dust. Within New Hampshire, the high elevation mountain-tops receive the highest acid deposition on an annual basis. While these high elevations receive wet and dry deposition at rates similar to the lower elevations within the state, they also get a good dose of acid fog from the frequent low-level clouds that "sock-in" the mountain-tops.

Complex chemical reactions in the atmosphere act to acidify several air pollutants. Most commonly, sulfur dioxide (SO₂) and nitrogen oxide (NOx) emission are oxidized and when the process involves the moisture in clouds, acids form. SO₂ becomes sulfuric acid and NOx becomes nitric acid. Chlorine emissions can also lead to hydrochloric acid, but this usually only accounts for a small fraction of acid rain. Once the acids are formed in the atmosphere, they can travel long distances before being deposited.

Acidity is measured in the units of pH (based on the number of free hydrogen ions available in solution). The scale is logarithmic and ranges from 1 to 14. The lowest values are the most acidic and the highest values are the most alkaline. Since the scale is logarithmic, a pH of 6 is 10 times more acidic than a pH of 7 and a pH of 5 is 100 times more acidic than a pH of 7. While a pH of 7 is considered neutral (not acidic or basic), "clean" precipitation is usually about 5.6 because the carbon dioxide in the air acidifies it slightly by converting to carbonic acid. Precipitation with a pH below 5 can usually be assumed to have been influenced by man-made sources. The pH of the precipitation in New Hampshire is typically between 4.0 and 4.8, but clouds have been measured to be as acidic as 2.1 (similar to lemon juice). Some lakes and ponds in the Northeast have a low ability to neutralize incoming acids and have been found to have pHs as low as 4.1.

Airborne acids not only acidify water, causing some species of fish to die-off, it also strips essential nutrients for vegetation from soils, causing many plants/trees to become more susceptible to damage from insects, freezes, and air pollution (ozone). It takes a long time to recover from acid damage. TheHubbard Brook Foundation estimates that if all acid producing sources were shut-off today, it would take about 20 years for the ecosystem to fully recover. Ecosystems with such issues include the Green and White Mountains in New England, the Adirondack and Catskill Mountains in New York, and the Appalachian and Blue Ridge Mountains in North Carolina and Virginia. The Hubbard Brook Experimental Forest, located in the White Mountains, was one of several organizations that documented acid rain over 30 years ago.

The vast majority of the acid deposition experienced in New Hampshire is believed to originate in the industrial Midwest where many large coal-fired power plants produce large quantities of SO₂ and NOx. When these power plants were constructed, their stack were designed to be very tall to enhance air pollution dispersion to minimize local impact. Unfortunately, these tall stacks also encourage efficient transport of pollution with the prevailing winds into the Northeast. n 1990, Congress implemented amendments to the federal Clean Air Act , which included requiring the largest sources of acid precursors reduce their SO2 and NOx emissions. EPA tracks this progress through the National Acid Deposition Program (NADP). Locally, the Hubbard Brook Foundation tracks atmospheric deposition and how the ecosystem is reacting to it.