Visibility Effects of Air Pollution

What affects visibility?

Fine particles and gaseous air pollution affect visibility to some degree in every national park. Air pollution can create a white or brown haze that affects how far we can see. Air pollution also affects how well we are able to see the colors, forms, and textures of a scenic vista. Haze results from air pollutants, such as fine particles that absorb and scatter sunlight. Haze is mostly caused by air pollution from industry and motor vehicles. Some haze can also occur naturally due to dust, fog, and wildfire smoke.

How visibility is impacted by pollution?

Visibility is affected by the physical interaction of light with particles and gases in the atmosphere. However, visibility involves more than how light is absorbed and scattered by the atmosphere. Visibility is the process of perceiving the environment through the use of the eye-brain system.

Important factors involved in seeing a scenic vista are shown in the following figure. Image-forming information from an object is reduced (scattered and absorbed) as it passes through the atmosphere to the eye. Sunlight is also added to the sight path by scattering processes. Sunlight and light reflected from the ground are absorbed and scattered by particles located in the sight path. Some of this scattered light remains in the sight path, and at times can become so bright that the image essentially disappears.

What does haze look like?

Air pollution does not impact views on clear days but can be seen as a plume, layered haze, or uniform haze when air pollution is present. A plume of air pollution is a tight, vertically constrained layer of air pollution coming from a point source (such as a smoke stack). Layered haze is any confined layer of pollutants that creates as a visible contrast between that layer and the sky or landscape behind it. In an unstable atmosphere, plumes and layers mix with the surrounding atmosphere creating a uniform haze or overall reduction in air clarity.

Plumes and layered haze are more common during cold winter months when the atmosphere is more stagnant. Uniform haze occurs when warm turbulent air causes atmospheric pollutants to become well mixed.

How is visibility measured?

Visual range is a 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. Scattering is measured with nephelometers. 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. Light extinction is reconstructed from measurements of particle mass and chemical composition.

• Visibility monitoring at NPS »
• Understanding visibility results »
• IMPROVE – Interagency Monitoring of Protected Visual Environments »