Haze-forming pollution comes from both human and natural
sources. Windblown dust and soot from wildfires contribute to haze,
as do motor vehicles, electric generating facilities, industrial
fuel burning, and manufacturing operations. Pollution from
particulate matter (PM) is the major cause of reduced visibility
(haze) in the United States, including many of our national parks.
While some haze-causing particles are emitted directly to the air,
secondary particles can be formed when emitted gases form particles
well downwind of the emission sources. Nitrates and sulfates are
examples of secondary particles that contribute to haze. Some of
the pollutants that form haze have also been linked to health
problems and environmental damage.
In many scenic areas, the visual range can be substantially
reduced by air pollution. In eastern parks, average visual range
has decreased from 90 miles to 15–25 miles. In the West,
visual range has decreased from 140 miles to 35–90 miles. Haze
is measured in deciviews. A deciview (dv) is a unit of visibility
proportional to the logarithm of the atmospheric extinction (Figure
1).
Figure 1. Comparison of Extinction, Deciviews and Visual
Range.
Source: William Malm,
Introduction to Visibility, 1999.
The Regional
Haze Rule (PDF file), 
issued by the U.S. Environmental Protection
Agency in 1999, calls for state, tribal, and federal agencies to
work together to improve visibility in 156 national parks and
wilderness areas known as Class I areas (Figure 2). The rule
requires the state to consult with the EPA, the National Park
Service, the U.S. Fish and Wildlife Service, the U.S. Forest
Service, other states, and tribes. All interested parties will be
able to comment on the proposed Regional Haze revision to the SIP.
The national
map of all 156 Class I areas is available from the National Park
Service.
(Help with PDF.)
Figure 2. Map of mandatory Class I federal areas in
Texas and surrounding states
Source: National Park Service Air Resources Division (adapted)
Texas has two Class I areas located in the western part of the
state: Big Bend National Park and Guadalupe Mountains National
Park. On a clear day at Big Bend, visitors can see the detail of
large objects over 100 miles away. On a hazy day, the view can be
reduced to 30 miles.
Big Bend National Park Data
Use this web page to retrieve monthly summaries of hourly
average data collected at Continuous Air Monitoring System (CAMS)
691 and to see photos. View monthly summaries of the hourly
averages for all the pollutants and meteorological conditions
measured by the TCEQ at a selected monitoring site.
Guadalupe Mountains National Park Data
Use this web page to retrieve monthly summaries of hourly
average data collected at CAMS 5018; no photos are available at
this time. Limited summaries are available of the hourly averages
for all the pollutants and meteorological conditions measured by
the TCEQ.
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