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Air Resource Management Program - Air Pollutants

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Ozone

A green leaf with dark patches after damage by ozone.
An example of ozone symptoms on a leaf. (USDA Forest Service photo by Sharon Grant)

Ozone is a colorless gas that naturally exists in the upper atmosphere, shielding 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 and oxides of nitrogen in the presence of sunlight and, often, elevated temperatures. Industrial and automobile emissions are the primary human sources of volatile organic compounds and nitrogen oxides. Ozone can be transported hundreds of miles to remote areas of the country.

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. The effects of ozone on native plants in natural ecosystems 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 elevations typically see higher ozone exposure levels leaving high-elevation vegetation more at risk. There are several factors that may contribute to higher ozone occurrence at higher elevations. For example, higher rural ozone concentrations can occur as a result of precursor pollutants being transported downwind from urban areas. Wildfires in remote areas may also contribute to higher ozone. Hot, sunny days are especially conducive to its formation. Ozone formation potential is lower in areas with cooler temperatures, decreased solar radiation intensity, and more cloudiness.

Health Impacts of Ozone

High ozone concentrations can also 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.

Many regions across the continental United States have exceeded ozone concentration standards set by the Environmental Protection Agency to protect human health, particularly in the national forests in both the south- and northeast. While ozone levels in the west typically experience lower ozone concentration levels, some regions, such as California and urban-adjacent forests in the Rocky Mountains, are experiencing increasing ozone or have also exceeded standards.

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Greenhouse Gases

Greenhouse gases include carbon dioxide, methane, nitrous oxide, fluorinated gases, and water vapor. Climate change is caused by increases of greenhouse gasses in the atmosphere. These increases come primarily from agriculture, deforestation, and our use of fossil fuels, which can include fossil fuel and electricity production for industrial processes, transportation, and buildings. Greenhouse gas emissions can persist for many years or decades; this is referred to as an atmospheric lifetime.

An atmospheric lifetime is the average time a gas molecule spends in the atmosphere, this varies for each greenhouse gas. For example, methane’s lifetime is about 11.8 years, while nitrous oxide’s lifetime is about 109 years, according to the Environmental Protection Agency. The lifetime of a greenhouse gas is not the only important factor; some gases are better at trapping heat than others. For example, methane is substantially more effective at trapping heat when compared with carbon dioxide. As these greenhouse gases have increased in the atmosphere, the amount of energy and heat in the atmosphere has increased, disrupting the global climate system.

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Methane

Methane is a greenhouse gas with a warming potential approximately 30 times that of carbon dioxide over a 100-year period and 80 times over a 20-year period. This is due to methane’s ability to absorb much more energy. Human activities that are sources of methane include the extraction, production, and transport of oil, natural gas, and coal. Other sources include raising livestock, agriculture, and landfills.

Orphaned wells are abandoned oil or gas wells without a responsible owner or operator. Unplugged or improperly plugged orphaned wells can:

  • Leak methane and other greenhouse gases into the atmosphere, contributing to climate change and leading to harmful ground-level ozone pollution.

  • Act as a source of pollutants that can degrade regional air quality and contaminate surroundings soil and waters.

  • Increase the risk of fires or explosions due to the flammability of natural gas.

  • Prevent lands from being used for recreation or other productive purposes.

The Forest Service participates in plugging orphaned wells to reduce methane emissions and prevent or decrease these significant environmental and social problems.

Visibility

A side-by-side image of a mountain range and valleys on a clear day (left) and a hazy day (right).
Photographs taken from a vista on a clear day (left) compared to a hazy day (right). (USDA Forest Service photo)

Through the Clean Air Act of 1977, Congress established a national goal of remedying existing and preventing future, human-caused visibility impairment in existing wilderness areas, national parks, and national wildlife refuges. However, air pollution impairs visibility to some degree on all federal lands. In 2000, the average visual range within the rural eastern United States was measured to be just 15 to 30 miles. As a result of significant reductions of haze-forming air pollution, by 2020, the average visual range in the rural eastern United States improved to 45-80 miles. This represents a significant improvement to visitors wanting to experience clean air in the eastern United States. In the western states, average visual range generally increased from 2000 to 2020, though to a lesser degree than in the east. In recent years many wilderness areas in the western United States have worse than average visibility due to smoke from large wildfires.

Haze

Fine particles in the air that scatter and absorb light cause haze. When the number of fine particles increases, more light is absorbed and/or 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 relative impact of each type of particle varies from season to season and region to region across the United States.

Plume Blight

Plume blight occurs when a point source, such as a smokestack, 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 occurs before the plume has been dispersed so widely that it is indistinct from the background. Both contrast and coloration may vary depending on the plume constituents, viewing background, viewer angle, and the angle of the sun.

Side-by-side photos showing a plume blight on the left, and a smoke plume rizing into the sky on the right.
LEFT: A photo showing plume blight effects on a vista. (USDA Forest Service photo) 
RIGHT: Smoke plume from a prescribed burn on Inyo National Forest. (USDA Forest Service photo by Lisa Cox)

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Smoke

A large plume of smoke rising from a raging forest fire at the peak of a tree and brush covered mountain.
Saint Mary’s Wilderness 2008 Wildfire in the George Washington and Jefferson National Forest. (USDA Forest Service photo)

Smoke from wildfires is a significant source of air pollution. It can pose potential risks to health, visibility, and safety. Forest managers, fire managers, and air resource specialists must address these issues to minimize smoke impacts on public health.

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