This page provides a brief overview of visibility regulations, monitoring programs and the causes of visibility impairment.  Included are discussions of the following:




Clean Air Act Background


National Air Programs


Visibility Monitoring Approaches


Causes Of Poor Visibility


IMPROVE WEB SITE  - See the IMPROVE website ( for a full accounting of visibility in IMPROVE’s Class I areas where visibility is an important attribute. 






Congress recognized visibility, defined as "the appearance of scenic features when viewed from a distance," as a resource to be valued and preserved.  Specifically, Section 169A of the 1977 amendments to the Clean Air Act required the Environmental Protection Agency (EPA) to promulgate regulations to assure reasonable progress toward the congressionally declared National Goal of "the prevention of any future, and the remedying of any existing, impairment of visibility in mandatory Class I federal areas which impairment results from man-made air pollution."  The intent of Congress to protect visibility was further strengthened in Section 169B of the 1990 Clean Air Act Amendments.  The EPA has promulgated regulations to ensure that on-going efforts are made to protect visual air quality.


To effectively track progress toward meeting the National Goal, numerous federal, state, tribal, and local visibility monitoring sites and monitoring programs have been established.

EPA, Visibility Monitoring Guidance Document (Draft, January 1998)






In the mid-1980's, the National Park Service (NPS), United States Fish and Wildlife Service (USFWS), Bureau of Land Management (BLM), and United States Forest Service (USFS) entered into a formal cooperative relationship with the EPA to establish the IMPROVE Program (Interagency Monitoring of Protected Visual Environments).  In 1991, several additional organizations joined the effort: State and Territorial Air Pollution Program Administrators and the Association of Local Air Pollution Control Officials (STAPPA/ALAPCO), Western States Air Resources Council (WESTAR), and Northeast States for Coordinated Air Use Management (NESCAUM).  The IMPROVE Program continues to collect and analyze visibility data from Class I area monitoring sites throughout the United States.


The objectives of the IMPROVE Program are to:


·          Establish current visibility and aerosol conditions in mandatory Class I areas,


·          Identify chemical species and emission sources responsible for existing human-made visibility impairment,


·          Document long-term trends for assessing progress towards the national visibility goals,


·          With the enactment of the Regional Haze Rule, to provide regional haze monitoring representing all visibility-protected federal Class I areas where practical.


Data collected at these sites are used by land managers, industry planners, and air quality regulators to understand and protect the visual air quality resource in Class I areas.  Most importantly, the IMPROVE Program scientifically documents for American citizens, the visual air quality of their wilderness areas and national parks.







A variety of monitoring techniques exist to document visibility conditions and to make quantitative measurements of the atmospheric properties that effect visibility.  The IMPROVE Program (Interagency Monitoring of Protected Visual Environments) has partitioned visibility-related characteristics and measurements into three groups:


Aerosol:  the physical properties of the ambient atmospheric aerosols (chemical composition, size, shape, concentration, temporal and spatial distribution, and other physical properties) through which a scene is viewed.  Fine particle measurements are commonly made to quantify aerosol characteristics.


Optical:  the ability of the atmosphere to scatter or absorb light passing through it.  Extinction, scattering, and absorption coefficients, plus an angular dependence of the scattering, known as the scattering phase function, describe the physical properties of the atmosphere.  Optical characteristics integrate the effects of atmospheric aerosols and gases.  Commonly applied optical monitoring instruments include transmissometers and nephelometers.


Scene:  the appearance of a scene viewed through the atmosphere.  Scene characteristics include observer visual range, scene contrast, color, texture, clarity, and other descriptive terms.  Scene characteristics change with illumination and atmospheric composition.  Photographs, video images, and digital images are effective ways to document scene characteristics.






The United States is endowed with many extraordinary natural areas. Congress has taken steps to preserve and protect many of these areas, Text Box: Contribution of Various Particulates to Haze

Eastern U.S.

Western U.S.

such as federal Class I lands, wildernesses, and national parks.  Unfortunately, visitors to these areas are often prohibited from enjoying the spectacular views they expect.  Much of the time a white or brown haze hangs in the air and affects the view.  This haze is not natural, it is caused by human-made air pollution, often carried by the wind hundreds of miles from where it originated.


Typical visual range in the western U.S. is 60 to 90 miles, or about one-half of what it would be without human-made air pollution.  In the East, the typical visual range is 15 to 30 miles, or about one-third of the visual range under natural conditions.


Haze is caused by tiny particles that scatter and absorb light before it reaches an observer.  As the number of particles increases, more light is absorbed and scattered, resulting in less clarity, color, and visual range.


Five types of particles contribute to haze: sulfates, nitrates, organics, elemental carbon, and soil.  The importance of each type of particle varies across the U.S. and from season to season.  The typical importance of each particle type in the eastern and western U.S. is shown in the figure to the right.  Details on each particle type are provided below.


·         Sulfate particles form in the air from sulfur dioxide gas.  Most of this gas is released from coal-burning power plants and other industrial sources, such as smelters, industrial boilers, and oil refineries.  Sulfates are the largest contributor to haze in the eastern U.S., due to the region’s large number of coal-fired power plants.  In humid environments, sulfate particles grow rapidly to a size that is very efficient at scattering light, thereby exacerbating the problem in the East.


·         Nitrate particles form in the air from nitrogen oxide gas.  This gas is released from virtually all combustion activities, especially those involving cars, trucks, off-road engines (e.g., construction equipment, lawn mowers, and boats), power plants, and other industrial sources.  Like sulfates, nitrates scatter more light in humid environments.



·         Organic particles are emitted directly into the air and also form there as a reaction of various gaseous hydrocarbons.  Sources of direct and indirect organic particles include vehicle exhaust, vehicle refueling, solvent evaporation (e.g., paints), food cooking, and various commercial and industrial sources.  Gaseous hydrocarbons are also emitted naturally from trees and from fires, but these sources have only a small effect on overall visibility.

·         Elemental carbon particles are very similar to soot.  They are smaller than most other particles and tend to absorb rather than scatter light.  The “brown clouds” often seen in winter over urban areas and mountain valleys is largely attributable to elemental carbon.  These particles are emitted directly into the air from virtually all combustion activities, but are especially prevalent in diesel exhaust and smoke from the burning of wood and wastes.

·         Soil particles are very similar to dust.  They enter the air from dirt roads, fields, and other open spaces as a result of wind, traffic, and other surface activities.  Whereas other types of particles form from the condensation and growth of microscopic particles and gases, soil particles result from the crushing and grinding of larger, earth-born material.  Because they are difficult to reduce to microscopic sizes, soil particles tend to be larger than other particles and tend to fall from the air sooner, contributing less to the overall effect of haze.


There are generally three types of visibility impairment:  uniform haze, layered haze, and plumes.

  • A uniform haze degrades visibility evenly across the horizon and from the ground to a height well above the highest features of the landscape. Uniform haze often travels long distances and covers large geographic area, in which case it is called a regional haze.


  • In a layered haze, you can see the top edge of the pollution layer. This is often the case when pollution is trapped near the ground beneath a temperature inversion.


  • Plumes result from local sources. Plumes and plume-like layers of elevated pollution take their shape under certain meteorological conditions where the air is stable or constrained.



Some of the pollutants that form haze have been linked to serious health effects and environmental damage.  Exposure to particles in the air have been linked with increased respiratory illness, decreased lung function, and premature death.  In addition, sulfate and nitrate particles contribute to acid rain, which can damage forests, reduce fish populations, and erode buildings, historical monuments, and even car paint.


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 several programs are in place to maintain this progress over the next several years.  Nonetheless, these programs by themselves are unlikely to restore visibility to its natural conditions in many protected areas.


In April 1999 the U.S. Environmental Protection Agency (EPA) issued regulations to further reduce haze and protect visibility across the country.  The EPA and federal land managers from other agencies are working with state, local, and tribal authorities to promote steady improvements in visibility for decades to come.

(, EPA 2000

Prepared by Air Resource Specialists, Inc.
(Contributions from EPA Air Program Web Site)

Under NPS Contract CX-1270-96-006