Pollutants

Purpose:

Air Pollution

Resource managers are concerned about air pollution because of its effects on human health and ecosystems; however, the problem is less serious in the northern Lake States than in many parts of the US. Air pollutants are generated primarily as by-products of fossil fuel combustion in industrial facilities, automobiles, and residences. Because population density is relatively low and there are few large industrial complexes in the northern Lake States, the area generates comparatively little air pollution. Also, prevailing southwesterly winds tend to carry airborne pollutants produced in the large urban-industrial complexes in the midwestern and northeastern US away from, rather than toward, the forests of Michigan, Minnesota, and Wisconsin.

Acidic deposition, whether as a constituent of rainfall, fog, or as dry particles that contain nitrogen and sulfur compounds, is a forestry concern in some parts of the US. In the northeast, for example, high-elevation spruce-fir forests have experienced dieback associated with acid deposition.  Levels of acidic deposition in the Lake States are considerably lower (click to view nitrate and sulfate deposition maps) and do not appear to be affecting forests at this time.  Studies have shown that nutrients can be displaced from soil particles and leached downward by acidic rainfall. Over time, it is possible that nutrient removal could lead to reductions in forest growth, affecting shallow or nutrient-poor sites first. Such impacts have recently been postulated in northeast forests, but growth reductions have not been proven. Acidic substances are also of concern in aquatic systems because of their impacts on organisms.

Effects of increased nitrogen in the ecosystem have been studied during the past decade. Nitrogen is a plant fertilizer which can increase growth up to a point, but systems can become "nitrogen saturated" and unable to utilize further additions. Then, adverse effects can occur. Nitrogen can leach into surface and groundwater and affect aquatic organisms or human health. Trees may suffer nutritional imbalances associated with forest decline, and microbial communities may be affected (Aber et al. 1998). While it appears likely that few forested ecosystems in the U.S. are near the point of nitrogen saturation at this time, continued accumulations could, over time, contribute to forest decline. Additional studies are needed to assess and predict impacts of increased nitrogen in forest ecosystems.

The air pollutant of greatest concern to forest scientists in the Lake States is ground-level ozone. While upper atmospheric ozone layers are beneficial in limiting ultraviolet (UV) radiation, high levels of ozone at ground level can damage plant tissues. Ground-level ozone is the major constituent of smog, produced by chemical reactions of nitrogen oxides (NOx) and volatile organic compounds (VOC’s) in the presence of heat and sunlight; thus, greater amounts of ozone are formed on warm, clear days.

Urban and industrial areas produce the most NOx and VOC’s and so are exposed to the highest amounts of ground-level ozone, but wind-transported ozone can affect forest vegetation several hundred miles away. Peak summertime ozone concentrations, or even moderate concentrations of about 70-80 parts per billion (ppb) over an extended time period, can damage the tissues of sensitive plants. Ozone enters plants through the stomata, so less injury is likely to occur when stomata tend to remain closed, as during droughts.

The US Environmental Protection Agency (EPA) has set standards for the maximum amount of ground-level ozone. The ozone standard was set based on human health risks, but is thought to also protect vegetation. The standard is attained when the three-year average of the annual 4^th highest daily average peak 8-hour concentration is below 80 ppb. The one hour standard of 120 ppb with one allowed annual exceedance, established in 1979, is also still in effect. It will be phased out as areas come into compliance.

Lake States summary data for an ozone exposure index (W126) were mapped by the Wisconsin Department of Natural Resources (click to view ozone map). The W126 index is believed to provide a better representation of plant response throughout the growing season than the 8-hour standard, which is related to human health effects. The W126 values indicate that the sum of ozone concentrations during the growing season of 1997 reached 14-17 ppm-hours in some Michigan forests, and 6-10 ppm-hours in parts of northern Wisconsin and Minnesota. These ozone levels have been reported to affect highly sensitive forest species such as black cherry (Southern Appalachian Man and the Biosphere, 1996). The high levels of ozone in Michigan come primarily from the Chicago metropolitan area and are carried by wind across Lake Michigan, even sometimes affecting forests as far away as eastern Upper Michigan. Daily time-lapse ozone maps available from the EPA show ozone concentrations spreading from the Chicago area.

Foliar symptoms of ozone damage have also been observed in the Lake States. Symptoms include stippling (dark purplish or black spotting) on upper leaf surfaces; as injury levels increase, leaves may turn brown and fall off. Ozone injury symptoms vary by species, and in some species like aspen, are difficult to distinguish from other kinds of injury. Species that are sensitive to ozone damage include aspen, white pine, black cherry, white and green ash, milkweed, blackberry, and largeleaf aster. Monitoring studies in Michigan have found foliar symptoms on some trees, while in northern Wisconsin and Minnesota symptoms have been found only on blackberry and milkweed thus far (Forest Health Monitoring Program). Ozone injury can cause growth reductions in forest species, and can change the genetic composition of a population by eliminating individuals that are particularly ozone-sensitive (Southern Appalachian Man and the Biosphere, 1996). Additional work is in currently in progress by the EPA and several universities to better determine species’ sensitivity to various levels of ozone concentration.

For more information on the EPA ozone standard, see http://ttnwww.rtpnc.epa.gov/naaqsfin/o3fact.htm. For information on the EPA’s analysis of health and environmental effects of ground-level ozone, see http://ttnwww.rtpnc.epa.gov/naaqsfin/o3health.htm. To view the results of some research work in progress at the North Central Research Station, see http://www.pnl.gov/atmos_sciences/greatlakes.html.

 Airborne fine particulate matter is another concern for land managers. Fine particles can be produced during prescribed burns, depending on weather, fire temperature, and fuel conditions, and may affect the health of fire crews and nearby residents. Airborne particles can also cause haze, which reduces visibility.

The EPA standard for airborne particles of 2.5 micrometers in diameter or smaller (PM2.5) was first set in July, 1997, and data are not yet available to determine whether actual levels exceed the standard. A standard for particles of 10 micrometers in diameter or smaller (PM10) has been in effect since 1987, though there are few data for forested parts of the Lake States because monitoring sites are concentrated in urban centers. Those data that do exist indicate that the PM10 standard is not being exceeded in the northern Lake States (click here to read more about Fine Particulate Matter).

For more information on the EPA particulate matter standard, see http://ttnwww.rtpnc.epa.gov/naaqsfin/pmfact.htm. For information on the EPA’s analysis of health and environmental effects of ground-level ozone, see http://ttnwww.rtpnc.epa.gov/naaqsfin/pmhealth.htm.

 For this assessment, ozone and particulate matter data were gathered from the Aerometric Information Retrieval System (AIRS) database, US-EPA, and summarized by the Wisconsin Department of Natural Resources, Bureau of Air Management. Additional information on ozone monitoring conducted in cooperation with States and the Forest Health Monitoring program is available at http://www.dnr.state.wi.us/org/aw/air/.

 References

Aber, J., W. McDowell, K. Nadelhoffer, A. Magill, G. Berntson, M. Kamakea, S. McNulty, W. Currie, L. Rustadt, and I. Fernandez. 1998. Nitrogen saturation in temperate forest ecosystems. BioScience 48(1):921-934.

Southern Appalachian Man and the Biosphere (SAAMB). 1996. The Southern Appalachian Assessment Atmospheric Technical Report. Report 3 of 5. Atlanta: U.S. Department of Agriculture, Forest Service, Southern Region. Viewable online at http://real.utk.edu/samab/.

Analysis Team:

USDA Forest Service--Southern Research Station, Rhinelander, WI
David Cleland, Ecologist

Available GIS Maps:

Nitrate Ion Depostion for 1993
Sulfate Ion Deposition for 1993
Nitrogen Dioxide Emissions for 1995
Sulfur Dioxide Emissions for 1995
Ozone Exposure Levels for Aspen and White Pine forest types (1997)
Lakes with Mercury Advisories near the Chequamagon National Forest
Lakes with Mercury Advisories near the Nicolet National Forest
Lakes with Mercury Advisories near the Chippewa National Forest
Lakes with Mercury Advisories near the Superior National Forest
Lakes with Mercury Advisories in Northern Minnesota
Lakes with Mercury Advisories in Northern Wisconsin