Natural causes of sulfate (sulfur oxides) in the atmosphere include ocean spray, volcanic emissions, and readily oxidized hydrogen sulfide released from the decomposition of organic matter found on land and in water. Natural sources of nitrogen or nitrates include nitrogen oxides produced by microorganisms in soils, by lightning during thunderstorms, and by forest fires. Scientists believe that one-third of the sulfur and nitrogen emissions in the United States comes from these natural sources.
The island of Hawaii provides a good example of the natural occurrence of acid rain. Sulfur dioxide gas and other pollutants emitted from the Kilauea volcano on the island of Hawaii combine and interact chemically in the atmosphere with water, oxygen, dust, and sunlight to produce "vog" (volcanic smog) and acid rain. Vog is a visible haze consisting of gas and aerosols (a suspended mix of very tiny liquid and solid particles) that can be a health hazard because it aggravates preexisting respiratory ailments. When rain falls in areas that have vog, the crops and local water supplies can be damaged by the resulting acid rain.
Many residents and visitors on the island of Hawaii report physical symptoms associated with vog. These include headache, breathing difficulties, greater susceptibility to respiratory ailments, general lack of energy, sore throat, watery eyes, and other flu-like symptoms. Although the amount of particulate material in the air does not routinely exceed the federal standards, sulfur dioxide concentrations do. Sulfur dioxide emission rates from Kilauea were first measured in 1975 and have been measured on a regular basis since 1979. Periodic reporting of these sulfuric dioxide emission rates is done by the U.S. Geological Survey.
The tiny sulfuric acid droplets in vog have the corrosive properties of diluted battery acid. When these droplets combine with moisture in the air to form acid rain, plant damage and acceleration of the rusting of metal objects such as vehicles and machinery occurs. Crop damage is another frequent occurrence, even in greenhouses, because the vog enters through vents and mixes with the moisture on plant leaves.
The combination of vog and acid rain created an unusual water supply problem on Hawaii. Many homes relied on rooftop rainwater catchment basins for drinking water. In 1988 the drinking water in more than 40% of the homes was found to contain elevated lead levels. Upon further study it was determined that the process of acid-induced leaching from lead roofing and plumbing materials was the cause of the elevated lead levels. Tests confirmed elevated lead levels in the blood of residents. This finding led to a major island-wide effort to remove lead materials from rainwater catchment systems.
Most human-made emissions of sulfur dioxide and nitrogen oxides are the result of burning fossil fuels (coal, oil, and gas) for energy. This includes fossil-fueled electric utilities and industrial plants, motor vehicles using gasoline or diesel fuel, and commercial or residential heating. Nonenergy sources of emission include metal smelters that emit sulfur compounds and nitrogen compounds from agricultural fertilizers that are carried by the wind to other areas.
Levels of pollutants are measured in two ways: emissions and concentrations. Emissions are those pollutants expelled into the air by a source, whereas concentrations are the total saturation of a contaminant over time. Table 9.1 shows sulfur dioxide emission sources from 1970 to 2002, while Table 9.2 shows nitrogen oxide emissions for that same period. Fuel combustion from fossil-fueled utilities accounts for most of the man-made sulfur dioxide emissions but less than half of the nitrogen oxides. The primary source of human-generated nitrogen oxides is transportation (car, truck, bus, and other vehicle emissions), which accounts for a little more than half of man-made nitrogen oxides emissions.
There has been great progress in reducing sulfur dioxide concentration. Between 1980 and 1999 the average annual mean concentration of sulfur dioxide dropped by half. According to EPA data, annual mean nitrogen oxides concentrations declined in the early 1980s, were relatively stable during the mid-to-late 1980s, and resumed their decline in the 1990s. Concentrations of nitrogen oxides declined 25% between 1980 and 1999.