VISIBILITY OVERVIEW
This page provides a
brief overview of visibility regulations, monitoring programs and the causes of
visibility impairment. Included are
discussions of the following:
Visibility Monitoring Approaches
IMPROVE WEB SITE - See the IMPROVE website (http://vista.cira.colostate.edu/IMPROVE) 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,
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.
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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.
(http://www.hazecam.net/poor-vis.html), EPA 2000
Prepared by Air
Resource Specialists, Inc.
(Contributions from EPA Air Program Web
Site)
Under NPS
Contract CX-1270-96-006
2001