Atmospheric Deposition

Atmospheric deposition can be a major source of air pollution. Contaminants can travel large distances from their origins to become pollution problems in other states and nations. As sampling and analysis techniques advance more information is acquired about the nature and sources of atmospheric deposition.

As a result of combustion of tremendous quantities of fossil fuels such as coal and oil, the United States annually discharges nearly 50 million tons of sulfur and nitrogen oxides into the atmosphere. These pollutants are then transported hundreds of miles from their origin. Through a series of chemical reactions with other substances in the atmosphere these pollutants can be converted into weak acids, which may return to earth as components of rain, fog, snow or dry particles. This atmospheric deposition, more commonly known as acid rain, may have severe ecological impacts in widespread areas of the environment. Acid rain has caused lakes and streams to become acidic and unsuitable for many fish, damaged forests and caused deterioration of many man-made structures. Power plants burning coal, oil and natural gas account for about 70 percent of the sulfur dioxide emissions in the United States . Cars and trucks, coal-burning power plants and industrial boilers and heaters account for most of the nitrogen oxide emissions.

Monitoring suggests that atmospheric deposition is also a significant pathway of mercury pollution to Maine lands and waters. Emissions of mercury to the atmosphere come from human-made sources, natural emissions, and re-emission from historic human sources. Fossil-fuel power plants, especially coal-fired plants, generate the greatest mercury emissions. Municipal waste incinerators are also a large source of mercury emissions. Mercury is emitted in elemental or inorganic forms. Biological processes change the chemical forms of mercury to methylmercury, an organic form of mercury. Methylmercury accumulates in the food chain and can lead to high concentrations in fish consumed by humans. Consumption of contaminated fish is a major source of human exposure to mercury in the United States . By December 2000 there were 41 states where fish advisories had been issued due to mercury contamination. Maine is currently under a statewide advisory that warns against the consumption of contaminated fish by pregnant women and children.

Polycyclic aromatic hydrocarbons or PAHs are also deposited in Maine as a result of atmospheric deposition. PAHs result from the incomplete combustion of wood, fossil fuels and other organic substances. Significant emissions occur from home wood combustion and mobile sources such as cars, trucks and planes. There are more than 100 different PAH's. They are found throughout the environment in the air, water and soil.

There are direct and indirect health effects associated with the various pollutants in atmospheric deposition. Inhalation exposures to acid deposition precursors may exacerbate existing respiratory conditions. Research has also indicated decreased lung function and increased cardiopulmonary mortality associated with long-term exposure to the fine sulfate and nitrate aerosols. Atmospheric deposition is known to leach heavy metals such as mercury from rocks causing possible contamination of water supplies and increased exposure to humans through the consumption of fish. Mercury exposure effects include damage to the senses and brain. Exposure to other heavy metals can damage the kidneys and nervous system. Hundreds of lakes in North America and Scandinavia have become so acidic that they can no longer support fish life. The rain falling on forests and other non-farmlands could, in time, cause extensive changes in the soil chemistry. There is not enough information yet to make it possible to say exactly what the results might be, but there is no reason to think the changes will be beneficial.

The health effects of individual PAH's are not alike. Many factors may determine whether harmful effects will occur and what the type and severity of those health effects will be. Those factors can include dose, duration, the pathway of exposure and your individual characteristics such as age, sex, family traits, etc. The primary health concern is that several of the PAH's are classified as probable carcinogens.

There are no standards in effect or proposed for atmospheric deposition. Standards or controls are applied to emission sources to control the amount of emissions and reduce the amount of pollution due to atmospheric deposition.

Historical information on controls to reduce deposition(pdf file)

Currently there are seven sites collecting data on atmospheric deposition. Those seven sites included Bureau maintained sites in Greenville , Bridgton and Freeport , a Text Box: Atmospheric Deposition sampling site in Bridgton National Park Service maintained site at Acadia National Park , a University of Maine maintained site in Greenville , a National Weather Service/DEP maintained site in Caribou and a U. S. Geological Survey maintained site in Gilead . Weekly samples from these sites are normally retrieved every Tuesday morning at 9:00 a.m. Consequently, the samples are not necessarily a single storm event but are more likely to be a composite of all storm events during the previous week. The samples, if there was a significant storm, are used for field measurements of pH and conductivity and are then packaged up for shipment to the National Atmospheric Deposition Program (NADP) central laboratory in Illinois . In the central laboratory they are also tested for pH and conductivity as well as additional components. The final validated data from this program is usually not available for several months following the week of collection. Field measurement data taken from most of the sites are available on the air quality web site. Historical data, including some long term trends analysis is available in the latest annual air quality report .

The acidity of water is measured in the pH scale, which ranges from 0 to 14. A pH of 7.0 is defined as neutral, and pH above and below 7.0 are considered basic and acidic, respectively. The pH is a logarithmic inverse of the hydrogen ion concentration, thus, a unit decrease in pH represents a tenfold increase in acidity. Natural, unpolluted rain contains carbonic acid, which contributes to rainwater being slightly acidic (typically being around 5.6). The pH levels of several common substances are shown below.

pH Levels of Common Substances

MDN is the mercury deposition monitoring section of NADP. Currently there are four sites collecting mercury deposition samples: Acadia National Park , Greenville , Bridgton and Freeport . Like the NADP sample, the MDN sample is a compilation of a week's worth of precipitation events. Weekly samples are retrieved at 9:00 AM every Tuesday and sent to the MDN laboratory (Frontier Geosciences, Inc., Seattle Washington ) for analysis. Data from the MDN network can be found at the NADP website. The latest annual air quality report contains a summary of the Maine data from the MDN network.