Visibility

Every year there are over 280 million visitors to our nation’s wilderness areas and national parks. Congress has set these special places aside for the enjoyment of all that seek spectacular and inspiring vistas. Unfortunately, many visitors are not able to see the beautiful scenery they expect. During much of the year, a veil of haze often blurs their view. The haze is caused by many sources of both natural and manmade air pollution sources, including wildland fire.

This section describes measures of scenic visibility, the properties of the atmosphere and how these properties are affected by smoke from wildland fires, natural and current visibility conditions, as well as sources that contribute to visibility degradation. This is an important issue to wildland fire practitioners because smoke is of increasing interest to air regulators responsible for solving regional haze problems.

Measures of Visibility Impairment

Visibility is most often thought of in terms of visual range or the furthest distance a person can see a landscape feature. However, visibility is more than how far one can see; it also encompasses how well scenic landscape features can be seen and appreciated. Changes in visual range are not proportional to human perception. For example, a five-mile change in visual range can result in a scene change that is either imperceptible or very obvious depending on the baseline visibility conditions. Therefore, a more meaningful visibility index has been adopted. The scale of this index, expressed in deciviews (dv) is linear with respect to perceived visual changes over its entire range, analogous to the decibel scale for sound. A one-deciview change represents a change in scenic quality that would be noticeable to most people regardless of the initial visibility conditions. A deciview of zero is equivalent to clear air while deciviews greater than zero depict proportionally increased visibility impairment (IMPROVE 1994). The more deciviews measured, the greater the impairment, which limits the distance you can see. Finally, extinction in inverse megameters (Mm-1) is proportional to the amount of light lost as it travels through a million meters of atmosphere and is most useful for relating visibility directly to particulate concentrations. Table 3.2.1 compares each of these three forms of measurement (Malm 2000).

Properties of the Atmosphere & Wildland Fire Smoke

An observer sees an image of a distant object because light is reflected from the object along the sight path to the observer’s eye. Any of this image-forming light that is removed from the sight path by scattering or light absorption reduces the image-forming information and thereby diminishes the clarity of the landscape feature. Ambient light is also scattered into the sight path, competing with the image-forming light to reduce the clarity of the object of interest. This “competition” between image-forming light and scattered light is commonly experienced while driving in a snowstorm at night with the car headlights on.

In addition, relative humidity also indirectly affects visibility. Although relative humidity does not by itself cause visibility to be degraded, some particles, especially sulfates, accumulate water from the atmosphere and grow to a size where they are particularly efficient at scattering light. Poor visibility in the eastern states during the summer months is a result of the combination of high sulfate concentrations and high relative humidity.

The sum of scattering and absorption is referred to as atmospheric light extinction. Particles that are responsible for scattering are categorized as primary and secondary where primary sources include smoke from wildland fires and windblown dust. Other sources of secondary particles include sulfate and nitrate particles formed in the atmosphere. The closer the particle size is to the wavelength of light, the more effective the particle is in scattering light. As a result, relatively large particles of windblown dust are far less efficient in scattering light per unit mass than are the fine particles found in smoke from wildland fires. Finally, an important component of smoke from wildland fires is elemental carbon (also known as soot), which is highly effective in absorbing light within the sight path. This combination of light absorption by elemental carbon and light scattering caused by the very small particles that make up wildland fire smoke explains why emissions from wildland fire play such an important role in visibility impairment.

Natural Visibility Conditions

Some light extinction occurs naturally due to scattering caused by the molecules that make up the atmosphere. This is called Rayleigh scattering and is the reason why the sky appears blue. But even without the influence of human-caused air pollution, visibility would not always reach the approximately 240-mile limit defined by Rayleigh scattering. Naturally occurring particles, such as windblown dust, smoke from natural fires, volcanic activity, and biogenic emissions (e.g. pollen and gaseous hydrocarbon) also contribute to visibility impairment although the concentrations and sources of some of these particles remain a point of investigation.

Average natural visibility in the eastern U.S. is estimated to be about 60-80 miles (8-11 dv), whereas in the western U.S. it is about 110-115 miles (4.5-5 dv) (Malm 2000). Lower natural visibility in the eastern U.S. is due to higher average humidity. Humidity causes fine particles to stick together, grow in size, and become more efficient at scattering light. Under natural conditions, carbon-based particles are responsible for most of the non-Rayleigh particle associated visibility reduction, with all other particle species contributing significantly less. Scattering from naturally occurring sulfate particles from volcanic sulfur dioxide emissions and oceanic sources of primary sulfate particles are estimated to account for 9-12% of the impairment in the East and 5% in the West (NPS 1997). It is expected that coastlines and highly vegetated areas may be lower than these averages, while some elevated areas (mountains) could exceed these background estimates.

Current Visibility Conditions

Currently, average visual range in the eastern U.S. is about 15-30 miles, or about one-third of the estimated natural background for the East. In the West, visual range currently averages about 60-90 miles, or about one-half of the estimated natural background for the West. Current annual visual range conditions expressed in miles are shown in figure 3.2.2. Notice how much more impaired visibility is in the East versus the West.

In the East, 60-70% of the visibility impairment is attributed to sulfates. Sulfate particles form from sulfur dioxide gas, most of which is released from coal-burning power plants and other industrial sources such as smelters, industrial boilers, and oil refineries. Carbon-based particles contribute about 20% of the impairment in the East. Sources of organic carbon particles include vehicle exhaust, vehicle refueling, solvent evaporation, food cooking, and fires.

Elemental carbon particles (or light absorbing carbon) are emitted by virtually all combustion activities, but are especially prevalent in diesel exhaust and smoke from wood burning.

In the West, sulfates contribute less than 30% (Oregon, Idaho and Nevada) to 40-50% (Arizona, New Mexico and Southwest Texas) of light extinction. Carbon particles in the West are a greater percentage of the extinction budget ranging from 50% or greater in the Northwest to 30-40% in the other western regions. The higher percentages of the extinction budget associated with carbon particles in the West appear to be from smoke emitted by wildland and agricultural fires (NPS 1994).

In summary, the physics of light extinction in the atmosphere coupled with the chemical composition and physical size distribution of particles in wildland fire smoke combine to make fire (especially in the West) an important contributor to visibility impairment. Wildland fire managers responsible for the protection of the scenic vistas of this nation’s wilderness areas and national parks have a difficult challenge in balancing the need to protect visibility with the need to use fire for other resource management goals.