IMAGE
 Atmospheric Deposition

INTRODUCTION
POLLUTANTS AND SOURCES
AFFECTED WATER RESOURCES
REGULATORY AVENUES
REFERENCES



INTRODUCTION

The control of atmospheric deposition of pollutants is the control of the anthropogenic sources that release those pollutants into the atmosphere. Few best management practices (BMPs) have been designed specifically to control atmospheric deposition at the point of deposition. Of course, any management practices that are installed to mitigate pollutants in stormwater runoff from watersheds should also target the nutrients and metals that are deposited from the atmosphere. Stormwater runoff BMPs are available for both industrial sites and urban areas. The following is a discussion of natural and anthropogenic sources of atmospheric pollutant deposition and the types of water resources affected by it, followed by a brief outline of the regulatory programs that target the control of atmospheric pollutant sources. This information should provide a starting point for further efforts to control this increasingly important pollutant source.

POLLUTANTS AND SOURCES

The deposition of atmospheric nitrogen and metals may impact surface waters. Both metals and nitrogen in the atmosphere are derived from natural and anthropogenic sources. Natural sources of metals include volcanic activity, forest fires, windblown dust, vegetation, and sea spray. The primary anthropogenic metals source is the smelting of ores (Salomons and Forstner, 1984). Other anthropogenic sources include stack and fugitive dust (dust that escapes emission controls). Historically, the deposition of lead (Pb) caused the greatest concern for human health. The introduction of unleaded gasoline has reduced the lead levels in the atmosphere to well below the standards outlined in the Clean Air Act. Mercury and other hazardous metals that are produced during industrial processes are strictly controlled at the source under provisions of the Clean Air Act. Thus, current and future metal deposition should not cause a significant problem in watersheds of the United States. However, metals deposited in the past are still causing water quality problems.

Atmospheric nitrogen, on the other hand, is derived from many elusive sources, many of which are not regulated under the Clean Air Act. Moreover, nitrogen levels appear to be increasing in the atmosphere. Studies indicate that atmospheric deposition of nitrogen poses great risk for the eutrophication of surface waters. Thus, the following discussion will focus primarily on the formation and survival of nitrogen in the atmosphere.

The predominant natural source of nitrogen is the microbial decomposition of organic matter in soil and water. Microorganisms release ammonia (NH3) to the atmosphere during the breakdown of amino acids (Oke, 1978; Smith, 1990). Less-pronounced natural sources include the release of organic nitrogen, in the form of amino acids and urea, from the activity of organisms (Paerl, 1993), and nitrogen fixation by lightning (Smith, 1990). Predominant anthropogenic atmospheric nitrogen sources include 1) emissions of nitrogen oxides (NOx) from the combustion of fossil fuels, 2) ammonia (NH3) and ammonium (NH4+) emissions from fertilizer and explosive factories, and 3) volatilization of ammonia-based fertilizer from agricultural fields (Oke, 1978; Lippman, 1989; Paerl, 1993).

Most anthropogenic nitrogen is emitted during the combustion of fossil fuels. Approximately 220 million tons of nitrogen is emitted per year from fossil fuel combustion (Schlesinger, 1991). Fossil fuel-burning power plants and large industries emit 53% of the yearly nitrogen emissions in the United States. Mobile sources, such as cars, trucks, and buses, account for 38% of the total emissions (Puckett, 1994). Under high temperatures and pressure, nitrogen and oxygen in the fuel and air combine to form the relatively harmless nitric oxide (NO) gas. Once in the atmosphere, nitric oxide is oxidized to nitrogen dioxide (NO2), an irritating gas. Nitric oxide and NO2 may also be converted to a series of other oxidized species, including HNO3, HNO2, HO2NO2, NO3, N2O5, and organic nitrates (Oke, 1978; Lippman, 1989).

The production and application of fertilizers comprise a much smaller, albeit significant, pool of anthropogenic nitrogen emissions. Of approximately 88 million tons of nitrogen fertilizer applied to terrestrial global ecosystems each year, 8 million tons escape to the atmosphere as NH3, NH4+, or NOx (NO + NO2) (Hinrichsen, 1986; Schlesinger, 1991).

AFFECTED WATER RESOURCES

Once emitted into the atmosphere, nitrogen may be deposited locally or may travel great distances before deposition. Many industrial and urban centers of the central U.S. emit nitrogen that is not only deposited locally downwind, but also as far away as the east coast of the U.S. (Paerl, 1993). More than 3.2 million tons of atmospheric nitrogen is deposited on the United States' watersheds each year. In addition, a sizable amount of atmospheric nitrogen is deposited in the Atlantic ocean. Galloway (1990) suggests that 18% to 27% of the total NOx emitted over the eastern U.S. is advected over the Atlantic Ocean and deposited.

Atmospheric nitrogen may be deposited in dry or wet form. Dry deposition involves the settling of particulates over time with gravity. Wet deposition occurs when particulates and aerosols are removed from the atmosphere by a precipitation event (Paerl, 1993). Wet deposition accounts for the majority of nitrogen removed from the atmosphere (Paerl et al., 1990).

Deposition of nitrogen (wet and dry) occurs over land and water. The terrestrial ecosystem will incorporate the wet and dry-deposited nitrogen as a nutrient source whenever possible. Between 30% and 60% of the nitrogen deposited on land is thought to be absorbed by the ecosystem. The degree to which a watershed can retain nitrogen is a function of the soil characteristics, topography, underlying geology, the amount and type of surface vegetation, and the degree of impervious cover (Paerl, 1993). Inevitably, a significant amount of deposited nitrogen will be transported during a precipitation event, via overland or subsurface flow, into a freshwater system. Usually freshwater systems are phosphorus-limited and will not use the excess nitrogen. Thus, most of the nitrogen will be delivered to estuarine systems.

Recent studies indicate that atmospheric nitrogen accounts for a large portion of the allochthonous (derived from outside the water body) nitrogen in estuaries and coastal oceans. A study by Paerl (1993) indicates that North Carolina estuaries may receive between 30% and 40% of the outside nitrogen from the atmosphere, while coastal oceans may receive up to 50% from the atmosphere. Estimates from other areas of the eastern seaboard are strikingly similar. Actual percentages in each area vary depending on the location, hydrologic regimes, and human activities.

REGULATORY AVENUES

Regulatory control of atmospheric nitrogen and metals deposition is the responsibility of local air quality officials and facility managers. Lead and nitrogen are classified as criteria air pollutants, and are governed by National Ambient Air Quality Standards (NAAQS). States must create and implement plans that will permit "air quality areas" to meet the standards for the criteria air pollutants. Areas not meeting the standards are classified as "non-attainment areas" and are subject to further regulation and potential grant withholding (Vandenberg, 1994). Hazardous metals (other than lead) are governed under the National Emission Standards for Hazardous Air Pollutants (NESHAP). NESHAPs are set for individual source types. Every facility governed by NESHAPs is monitored and regulated individually (Vandenberg, 1994). Local air quality officials should be contacted with any questions concerning the emissions from facilities in your vicinity.

LINKS

  • US EPA, Office of Water, Air Pollution and Water Quality
  • US EPA, Office of Air and Radiation
  • National Atmospheric Deposition Program
  • Atmospheric Deposition Measurement and Analysis Information Resource
  • Atmospheric Deposition in Maryland
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    • REFERENCES

    Galloway, J.N., 1990. The intercontinental transport of sulfur and nitrogen. In A.H. Knap (ed.), The Long-Range Atmospheric Transport of Natural and Contaminant Substances. Kluwer Academic Publishers, Netherlands.

    Hinrichsen, D., 1986. Multiple pollutants and forest decline. Ambio, 15:258-265.

    Lippman, M., 1989. Health benefits of air pollution control: a discussion. Congressional Research Service (CRS) Report for Congress. The Library of Congress, Washington D.C.

    Oke, T.R., 1978. Boundary Layer Climates. Methuena Co. Ltd., London.

    Paerl, H.W., 1993. Emerging role of atmospheric deposition in coastal eutrophication: biogeochemical and trophic perspectives. Can. J. Fish. Aquat. Sci., 50:2254-2269.

    Paerl, H.W., J. Rudek, and M.A. Mallin, 1990. Stimulation of phytoplankton production in coastal waters by natural rainfall inputs: nutritional and trophic implications. Marine Biol., 107:247-254.

    Puckett, L.J., 1994. Nonpoint and point sources of nitrogen in major watersheds of the United States. U.S.G.S. Water Investigations Report 94-4001. U.S. Geological Survey, Reston, Virginia.

    Salomons, W. and U. Forstner, 1984. Metals in the Hydrocycle. Springer-Verlag, Heidelberg, Germany.

    Schlesinger, W.H., 1991. Biogeochemistry: An Analysis of Global Change. Academic Press, Inc., San Diego, CA.

    Smith, R.L., 1990. Ecology and Field Biology. Harper Collins Publishers, NY.

    Vandenberg, J., 1994. Air Quality Management Class Notes. Duke University School of the Environment. Fall 1994.