Acid rain is the common name for acidic deposits that fall to Earth from the atmosphere.The term was coined in 1872 by English chemist Robert Angus Smith to describe the acidic precipitation in Manchester, England.Fuel combustion by fossil-fueled electric utilities historically has been by far the greatest source of these emissions, accounting for 85 percent of them in 2002.
NO emission sources are shown in Figure 5.6 in Chapter 5.
Transportation vehicles are the primary source, accounting for 56 percent of the total in 2002.
About one-third of the total sulfur compounds deposited over the eastern United States originates from sources in the Midwest more than 300 miles away.
In drier climates, such as those of the western United States, windblown alkaline dust moves more freely through the air and tends to neutralize atmospheric acidity.
Moist deposition occurs when the acid is trapped in cloud or fog droplets.
This is most common at high altitudes and in coastal areas.Values higher than seven are considered more alkaline or basic (the p H of baking soda is eight); values lower than seven are considered acidic (the p H of lemon juice is two). This means that every p H change of one is a ten-fold change in acid content.Therefore, a decrease from p H seven to p H six is a ten-fold increase in acidity; a drop from p H seven to p H five is a 100-fold increase in acidity; and a drop from p H seven to p H four is a 1,000-fold increase.When a basic and an acid chemical come into contact, they react chemically and neutralize each other.On the other hand, in more humid climates where there is less dust, such as along the eastern seaboard, precipitation is more acidic.(See Figure 7.2.) Pure, distilled water has a neutral p H of seven. It is slightly acidic because it accumulates naturally occurring sulfur oxides (SO) as it passes through the atmosphere. Figure 7.3 shows the average rainfall p H measured during 2002 at various field laboratories around the country by the National Atmospheric Deposition Program, a cooperative project between many state and federal government agencies and private entities.Rainfall was most acidic in the Mid-Atlantic states, particularly New York, Pennsylvania, Maryland, Ohio, West Virginia, and in portions of western Virginia, North Carolina, eastern Tennessee, and Kentucky.Natural sources of nitrogen or nitrates include NO produced by microorganisms in soils, by lightning during thunderstorms, and by forest fires.Scientists generally speculate that one-third of the sulfur and nitrogen emissions in the United States comes from these natural sources (this is a rough estimate as there is no way to measure natural emissions as opposed to those that are manmade.) The primary anthropogenic (human-caused) contributors to acid rain are SO emissions by source from 1983 to 2002.The movement of air masses transports emitted pollutants many miles, during which the pollutants are transformed into sulfuric and nitric acid by mixing with clouds of water.In the United States a typical transport pattern occurs from the Ohio River Valley to the northeastern United States and southeastern Canada, as prevailing winds tend to move from west to east and from south to north.