Visibility and Air Pollution
What is Visibility?
Particulate matter and gaseous air pollution affects visibility to some degree in every national park. Air pollution can create a white or brown haze that affects not only how far we can see but also how well we are able to see the colors, forms, and textures of a scenic vista. Haze results from air pollutants, sucha as fine particles that absorb and scatter sunlight. Both natural and manmade sources contribute to haze-causing particles and gases in the atmosphere. Natural sources include windblown dust and soot from wildfires. Manmade sources include motor vehicles, power plants, and industrial operations. Such air pollutants are often carried by wind hundreds of miles from where they originated.
Visual range is one measure of visibility and is defined as the greatest distance at which a large black object can be seen and recognized against the background sky. The larger the visual range the better the visibility. It is not directly measured but rather calculated from a measurement of light extinction which includes the scattering and absorption of light by particles and gases. Extinction depends on the mass and chemical composition of the particles and gases and is a quantitative measure of how the passage of light from a scenic feature to an observer is affected by air pollutants. Extinction is monitored with transmissometers, nephelometers, or reconstructed from measurements of particle mass and chemical composition.