Acid Rain in New England

Causes of Acid Rain

Two elements, sulfur and nitrogen, are primarily responsible for the harmful effects of acid rain.

Sulfur is found as a trace element in coal and oil. When these are burned in power plants and industrial boilers, the sulfur combines with oxygen to form sulfur dioxide (SO₂). Because SO₂ does not react with most chemicals found in the atmosphere, it can travel long distances. Eventually, if it comes in contact with ozone or hydrogen peroxide, it can be converted to sulfur trioxide. Sulfur trioxide can dissolve in water, forming a dilute solution of sulfuric acid.

Nitrogen makes up about 78% of the atmosphere. When heated to the temperatures found in steam boilers and internal combustion engines, it can combine with oxygen from the atmosphere to form nitrogen oxide and nitrogen dioxide (NO_x). NO_x is the sum of nitrogen oxide and nitrogen dioxide in a given parcel of air. These can dissolve in water, forming weak solutions of nitric and nitrous acids.

NO_x and SO₂ can come from natural or human made (anthropogenic) sources.

Volcanoes and sea spray are typical natural sources of SO₂. Lightning is the most common natural source of NO_x. Contributions from natural sources are generally small compared to those from anthropogenic sources.

EPA classifies the sources of anthropogenic emissions of pollutants into 3 groups: point (or stationary) sources, area sources, and mobile sources. Point sources include factories, power plants, and any other large "smoke stack" facilities. Area sources consist of smaller facilities which occur in greater numbers. These include residential heating equipment, small industry, and other categories in which it is impractical to analyze each individual emission source. Mobile sources include anything that can move. They can be divided into on-road sources (including cars, trucks, buses, motorcycles, etc.) and non-road (tractors, snowmobiles, boats, airplanes, lawnmowers, etc.).

Point sources emit the largest amount of SO₂. Of these, coal fired power plants are the highest emitters. The Brayton Point Station in southeastern Massachusetts is the largest point source for SO₂ in New England. In 2006, 16 units at 8 facilities emitted a total of 82,129 tons of sulfur dioxide.

The following charts show how much each group contributed to emissions of NO_x and SO_x (SO₂) in New England in 2002.

Once SO₂ and NO_x have been released into the air, they can be transported by the wind. The prevailing winds above most of the United States flow from west to east. Storm systems and other meteorological events may alter this flow. The final effects of these pollutants may occur as much as 1000 miles from where they were released. Eventually, these elements will dissolve in water droplets, and be converted into nitrate and sulfate ions. In this form, they may return to earth through acid deposition. Acid deposition occurs when these ions are deposited to the ground. It may be in the form of wet deposition, either indirect (acid rain, acid snow) or direct (acid fog), or dry deposition.

Prior to the mid 1990s, most scientists felt that the most common for method of acidic deposition was by rain and other forms of wet deposition. This is the most visible, and best understood means of deposition. It is easy to measure, and it effects are most obvious. Acid snow tends to carry less acidity per unit of water, since the chemistry is slower in cold weather, but since snow accumulates over the course of a season, when it melts, it releases a surge of acidity. At higher altitudes, direct deposition can occur when clouds descend to the surface. This can cause a severe problem because it may last for hours.

Dry deposition occurs when sulfate or nitrate ions do not dissolve in water, but rather fall to the surface as small particles or go directly from gaseous form in the atmosphere to soil or water. Unlike wet deposition, dry deposition is not easily measured. Very little falls at one time or at one location, but since dust is constantly settling to the Earth's surface, and the atmosphere is constantly in contact with the Earth, it can potentially have a large impact. As we have learned more about it, we realize it is an important part of acid deposition, and as the amount of acidity in rain decreases, dry deposition has become a more prominent route for deposition. Dry deposition now accounts 20-60% of the total deposition.