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		HOME Introduction Visibility and Fine Particulates in the Air Plume Blight Contribution of Various Particulates to Haze Human Health Effects of Fine Particulates Pollutant Deposition Acid Deposition Nitrogen Effects Toxic Methylmercury and Atmospheric Deposition Addressing Deposition Impacts Ozone Natural Resource Effects of Ozone Human Health Effects of Ozone

Introduction

Air pollutants affecting our National Forests can be divided into two groups: primary and secondary pollutants. Primary pollutants come directly from sources such as industrial facilities, automobiles and forest fires. These include sulfur and nitrogen compounds, particulate matter, volatile organic compounds (VOC’s) like paints and toxic metals such as mercury. Secondary pollutants, such as ozone, are formed when primary pollutants undergo chemical reactions in the atmosphere. Most pollutants can be transported great distances from their source to impact lands far away.

Visibility and Fine Particulates in the Air

Two photos of same landscape showing comparison between clear blue sky and haze

Haze is caused by fine particles in the air that scatter and absorb light. When the number of fine particles increases, more light is absorbed and scattered, resulting in a shorter visual range, less clarity and altered color.

Five types of fine particles contribute to haze: sulfates, nitrates, organic carbon, elemental carbon, and crustal (soil) material. The importance of each type of particle varies across the U.S. and from season to season.

Plume Blight

A photo showing plume blight - a dark line crossing the sky

Plume blight occurs when a point source such as a smoke stack emits particulate matter or nitrogen dioxide into a stable atmosphere. These pollutants can form a thin, dark, coherent plume obscuring the view. This picture captures a classic example of plume blight. Blight happens before the plume has been dispersed so widely that it is indistinct from the background. Both contrast and coloration may vary depending upon the plume constituents, the viewing background, the viewer angle, and the angle of the sun.

Contribution of Various Particulates to Haze

Sulfate particles form in the air from sulfur dioxide gas. Most of this gas is released from industrial sources such as coal-burning power plants, smelters, and oil refineries. Sulfates are the largest contributor to haze in the eastern U.S. In humid environments, sulfate particles grow rapidly to a size that is very efficient at scattering light.

Organic carbon particles are emitted into the air and also form there as a reaction of various gaseous hydrocarbons. Major sources of organic carbon particles include vehicle exhaust, vehicle refueling and solvent evaporation. Hydrocarbon emissions from forests and wildland fire smoke are additional sources. Fire emissions also include primary organic particles in the form of uncombusted material.

Nitrate particles form in the air from nitrogen oxide gas. This gas is released from virtually all combustion activities, especially those involving cars, trucks, and motors like those in lawn mowers, and boats, power plants, oil and gas production, and other industrial sources. Like sulfates, nitrates scatter more light in humid environments.

Elemental carbon particles like soot are smaller than most other particles and absorb rather than scatter light. The brown clouds often seen in winter over urban areas and some mountain valley towns can be largely attributed to elemental carbon. These particles are emitted directly into the air from virtually all combustion activities. They are especially prevalent in diesel exhaust and smoke from the burning of wood and wastes.

Crustal material, like dust, enters the air from dirt roads, fields, and other open spaces as a result of wind, traffic, and other surface disturbing activities. For more information about visibility and cause of impacts to it, visit http://www.fsvisimages.com/viscause.html.

To reduce haze we must reduce emissions of haze-forming pollutants across broad areas of the country. Cars, trucks, and industries are much cleaner than they were in the past, and programs are in place to maintain this progress over the next several years. However, these programs are probably insufficient to restore visibility to its natural conditions in many protected areas.

The Forest Service is working closely with each of the 50 states as they develop plans to reduce regional haze impacting large wilderness areas and national parks which were in existence prior to the Clean Air Act. For more information, click here.

Human Health Effects of Fine Particulates

Smoke from a forest fire

Many of the same pollutants that impact visibility can also have serious human health effects. Exposure to fine particles in the air increases the chances of respiratory and cardiovascular illness. Even relatively brief exposures to particulate matter may aggravate asthma and bronchitis and cause heartbeat irregularities and heart attacks. Some particulate pollution also has carcinogenic effects to humans. For more information on health effects of particulates visit www.epa.gov/airnow/health/particle/pm-color.pdf.

Pollutant Deposition

Deposition occurs when compounds of various types of air pollution are deposited on the earth's surface through rain, clouds, snow, fog, or as dry particles. The amount of deposition received in a given area is affected both by the concentration of pollution in the atmosphere and the way in which it is deposited. This is because general factors such as climate, predominant meteorology and topography of a region can influence how much pollution reaches the area from both local and distant sources, as well as how much of that pollution actually impacts the earth's surface via the various wet and dry forms.

There are several types of ecosystem effects associated with deposition which tie to the form of pollutant being deposited. These include acidic deposition (a.k.a. "acid rain"), heavy metals (including mercury) and excess amounts of nitrogen.

Acid Deposition

Acidic inputs from the atmosphere, mainly sulfate (SO4) and nitrate (NO3), can negatively impact aquatic and terrestrial ecosystems. Their acidifying effects contribute to degradation of stream and lake water quality by lowering the ANC (acid neutralizing capacity) which can be thought of as the water's natural acid buffering system. As the ANC decreases, the pH will eventually decrease and thus the acid levels will increase. In areas such as the central and southern Appalachians, forest streams have acidified to the point where they are no longer capable of sustaining aquatic life. The sensitivity of lakes and streams to the negative effects of acid deposition are often linked to natural watershed characteristics, most notably the bedrock geology/lithology types. Watersheds containing naturally occurring geologies/lithologies that weather (break down easily) and that are made up of minerals containing high levels of base cations (nutrients that plants need) are less susceptible to negative impacts of acid deposition. Likewise, watersheds where the soils are derived from geologies/lithologies that are resistant to weathering or that contain thin shallow soils are very susceptible to acid deposition. Correspondingly, these same susceptible areas may not only exhibit lake and stream water chemistry changes, but they may also show effects in soil chemistry such as nutrient leaching. Nutrient leaching can eventually lead to deficiencies in macro nutrients important for plant growth.

These areas that receive high levels of acidic deposition and have bedrock geology with a naturally low buffering capacity may exhibit nutrient depletion and stream acidification.

Sulfate is the primary component of acidic rain in the eastern U.S. with the highest levels of emissions coming from the heavily industrialized Ohio River Valley. In spite of recent reductions across the eastern U.S., sulfate deposition is still higher than the ecosystems of the Appalachian states can tolerate. Nitrogen deposition is more of a factor in acidic rain in the mid and western United States.

Nitrogen Effects

In addition to contributing to acidic rain, nitrogen can cause other ecosystem impacts by unnaturally fertilizing land and water. These excess inputs of nitrogen termed nutrient enrichment and eutrophication can disrupt the natural flora and fauna by allowing certain species that would not naturally occur in abundance to out compete those that thrive in pristine nitrogen limited systems. The end result is an unnatural shift in species composition for sensitive species, which may have a subsequent impact on other components of the ecosystem.

Toxic Methylmercury and Atmospheric Deposition

Walleye can bioaccumulate mercury

Toxic air contaminants like mercury, are emitted primarily by coal-fired utilities, and may be carried thousands of miles before entering lakes and streams as mercury deposition. Mercury can bioaccumulate and greatly biomagnify through the food chain in fish, humans and other animals, initiated via the conversion of non-organic forms of mercury to methylmercury by sulfur reducing bacteria in aquatic sediments. Methylmercury is a potent neurotoxin, and has been shown to have detrimental health effects in human populations as well as behavioral and reproductive impacts to wildlife. As of 2006, 46 states have consumption advisories for certain lakes and streams, warning of mercury-contaminated fish and shellfish. High concentrations of mercury are measured in sediments and fish tissue, even in remote areas of the Arctic. Recently, elevated methylmercury loads have been monitored in upland bird species, calling into question the traditional wisdom that methylmercury contamination be directly linked to only aquatic systems. The link between sulfur reducing bacteria and biotic mercury concentrations has led researchers to establish that reductions in sulfur dioxide emissions and a resulting reduction in sulfate deposition will abate mercury concentrations in wildlife. Consequently, as the level of sulfates is reduced in aquatic systems, sulfur reducing bacteria will reduce less sulfur, and this will in turn lead to less inorganic mercury being methylated. Therefore hard won reductions in sulfur dioxide emissions will result in more than improved visibility and abated acid deposition. For more information, download the Mercury deposition briefing (.pdf 247kb).

Addressing Deposition Impacts in the National Forests and Class I Areas

It is important for Federal Land Managers to understand not only how much deposition is occurring on the Forest, but how these current levels of deposition are affecting Forest resources. Long-term air quality and resource monitoring on and near the National Forests and Class I areas has helped establish air pollution trends and existing condition of the resources. For more information on monitoring, click here. Based on these existing conditions, and documented cause and effect relationships, the air resource specialists in the Forest Service have identified pollutant exposures and concern thresholds for evaluating potential impacts of new sources of air pollution for each Class I area.

Sampling rain in wilderness

Forest Service air resource specialists are also trying to establish critical and target loads for class I areas. Knowing critical and target loads would provide an additional tool to help assess adverse effects from air pollution. A critical load can be defined as "a quantitative estimate of the exposure to one or more pollutants below which significant harmful effects on specific sensitive elements of the environment do not occur according to present knowledge". A target load is set based on policy and management direction, and depending on whether or not current critical loads values have been exceeded, a target load can be above or below the critical load. In general, the critical load is based on modeled or measured dose-response data, while target load can be based on political, economic, spatial, or temporal considerations in addition to scientific information (Federal Land Manager Critical Loads for Sulfur and Nitrogen Deposition Workshop). Defining the critical and target loads for areas on the Forest would help resource managers communicate the effects of air pollution on resources to Forest decision makers as well as air regulatory agencies. This information could also be used to assess how some management activities may exacerbate air pollution related problems or identify areas where mitigations may be an option for resources that have already been negatively affected. This information can also be used in a regulatory context when consulting with and advising air regulatory agencies on effects to Forest resources resulting from new and existing sources of air pollution. Data are currently being collected to determine critical loads at various locations around the country. For more information on critical loads and the related science, please see: http://www.fs.fed.us/ne/durham/4352/critical_loads/Critical_loads_webs/home.htm

For more on acid rain, visit www.epa.gov/acidrain or download the Ecological effects briefing (.pdf 408kb).

Ozone

Ozone-damaged ash leaf

Ozone is a colorless gas that exists naturally in the upper atmosphere where it shields the earth from the sun’s harmful ultraviolet rays. Ozone close to the earth’s surface is an air pollutant. It is formed by chemical reactions between volatile organic compounds (VOCs) and oxides of nitrogen in the presence of sunlight and elevated temperatures. The primary human sources of VOCs and nitrogen oxides are industrial and automobile emissions. Ozone can be transported hundreds of miles to remote areas of the country.

Natural Resource Effects of Ozone

Ozone is one of the most toxic air pollutants to plants. It causes considerable damage to vegetation throughout the world. Plants are generally more sensitive to ozone than humans. Many native plants in natural ecosystems are sensitive to ozone. The effects of ozone range from visible injury to the leaves and needles of deciduous trees and conifers to premature leaf loss, reduced photosynthesis, and reduced growth in sensitive plant species. Other factors, such as soil moisture, presence of other air pollutants, insects or diseases, genetics, or topographical locations can lessen or magnify the extent of ozone injury. For example, higher ozone exposure levels occur at higher elevations so high elevation vegetation is more at risk. For more information on ozone effects and monitoring, visit http://webcam.srs.fs.fed.us/calculator/ozone.htm.

Human Health Effects of Ozone

High concentrations of ozone can cause inflammation and irritation of the respiratory tract, particularly during physical activity and can aggravate asthma attacks. The resulting symptoms may include pain when taking a deep breath, coughing, throat irritation, and breathing difficulties. Exposure to ozone can damage lung tissue and increase the susceptibility of the lungs to infections, allergens, and other air pollutants. Medical studies have shown that health problems caused by ozone may continue long after exposure.

In some National Forests in the Southeast, Northeast and California, ozone concentrations have exceeded standards set by EPA to protect human health. While the Pacific Northwest and Intermountain West experience lower levels of ozone pollution than other regions of the country, levels are increasing in the Colorado Plateau and Rocky Mountains regions. For more information on health effects of ozone visit www.epa.gov/airnow/health/smog.pdf.