From information gathered in the 1930's and 1950's, it appears that the acidity of rainfall in the eastern United States has increased
significantly (in Great Smoky Mountains National Park: 1956 - 5.6; 1980 - 4.27). Some scientists dispute this theory, however.
The gases sulfur dioxide (SO2) and various nitrogen oxides (NOx) are mainly responsible for the increase in acidity.
These gases are primarily created by industrial factories, coal-fired power plants, and car emissions.
It has been estimated that approximately 26 million tons of SO2 and 22 million tons of NOx were put into the atmosphere in the United States in 1980. (If 1 ton = 20 students each weighing 100 lbs., how many students = 48 million tons of pollutants? Answer: 960 million or approximately 1 billion students).
Some scientists believe that natural sources of SO2 and NOx (forest fires, lightning, volcanoes, etc.) have an equally important effect on acid rain (pH levels) as manmade sources.
Areas where the soil has been disturbed (mines, forest fires, landslides, construction sites) may be making water in streams and lakes more acidic. Many soils are already acidic or contain minerals which react with rainwater, snow melt, etc., to form acids. These acidic waters are carried into stream and lakes.
The gases SO2 and NOx are thought to be carried long distances by air currents. Storm systems also move long distances, carrying with them any pollutants they may pick up. The interaction of airborne pollutants and clouds is not well understood.
Understanding the movement of prevailing air currents and storm systems can greatly aid scientists in discovering where acid rain is coming from and, just as importantly, where the rain is returning to earth.
Scientists hope to learn more about the movements of acid rain by studying storm events, high and low pressure movements (high pressure usually is an indication of good, clear weather, whereas low pressure fronts are associated with storms), and occluded fronts. Occluded fronts normally precede storm events, thus making them a good indicator for tracking storm events.
Researchers are attempting to track where the pollution (gases, fly-ash, soot, and dust particles) goes when it leaves the heavy industrial and populated areas by making projections of storm event and air current movements.