Explore Air

Visibility Monitoring


One of the clearest days at
Yosemite National Park, California

Our national parks and wilderness areas offer stunning mountain vistas and scenery full of unique landscapes and geologic features. The enjoyment and appreciation of these are linked to one's ability to see clearly through the atmosphere. Unfortunately, air pollution affects our ability to see clearly. Small particles suspended in the atmosphere, mostly as a result of human-caused air pollution, often create haze -- a grey or white veil over the scene that scrubs it of its colors, forms, and textures and the lessens the visitor's experience of seeing "forever".

One of the haziest days at
Yosemite National Park, California

The National Park Service and the U.S. Environmental Protection Agency (EPA) first began long-term visibility monitoring at selected national parks in 1979. In 1985, a national visibility monitoring program was established called Interagency Monitoring of Protected Visual Environments, or IMPROVE. IMPROVE is a cooperative effort led by a Steering Committee of representatives from the EPA, U.S. Forest Service, National Park Service, Fish and Wildlife Service, Bureau of Land Management, National Oceanic and Atmospheric Administration, and several interstate air quality management organizations.

The goals of the IMPROVE program are to

  • measure current visibility and aerosol conditions in mandatory Class I areas
  • identify chemical species and emission sources responsible for existing human-caused visibility impairment
  • document long-term visibility trends
  • and with the enactment of the regional haze regulations, to provide visibility monitoring representative of all visibility-protected Class I areas.

More information about IMPROVE is available on the IMPROVE web site. The publications, Introduction to Visibility and Air Quality in the National Parks, 2nd edition, also provide good general discussions of visibility and how it is affected by air pollution.

Three types of visibility measurements are undertaken in the IMPROVE network: particle, scene, and optical. The mass and chemical composition of the suspended fine particulate matter (PM-2.5) and the mass of coarse particulate matter (PM-10) is measured by the particle monitor. Every IMPROVE site has such a monitor.

The appearance of a scene viewed through the atmosphere changes under different particle concentrations and lighting conditions. The visual appearance of a scene is qualitatively documented with 35mm color slides, digital images, or video images.

Optical monitoring measures the ability of the atmosphere to scatter and/or absorb light as it passes through it. Optical monitoring instruments used in IMPROVE include transmissometers and nephelometers.


NPS Visibility Monitoring Program Manager Bret Schichtel (970) 491-8581
updated on 07/12/2007  I   http://www2.nature.nps.gov/air/monitoring/vismon.cfm   I  Email: Webmaster