Making Visibility Degradation Visible

Eric G. Walther

Many decades ago the National Park Service (NPS) was formed to protect the special natural resources in Yellowstone, Grand Canyon, Yosemite and many other national parks and monuments. The air, especially its cleanness and clar- ity, has always been an important re- source within the parks, but in recent years it has suffered significantly be- cause of the growth of cities, power- plants, smelters, and other sources of air pollution.

The air pollution in cities be- came bad enough to result in several pieces of legislation since World War II, culminating in the 1970 Clean Air Act and its 1977 Amendments. The 1970 Act discussed the prevention of signifi: cant deterioration (PSD) of air cleaner than national ambient air quality stan- dards (NAAQS) and the protection of visibility,.but it did not provide a quan- titative measure of the required protec- tion. Since 1974, the nation has been divided into three classes for different amounts of protection against increases in sulfur dioxide (SO2) and total sus- pended particulates (TSP). Class I provides the most protection for areas intended to stay very clean, including 156 national parks and wilderness areas. Current law specifies small increments of SO2 and TSP concentrations that can legally be added by air pollution sources to the background concentrations in these areas. Class II areas are intended to provide moderate p r o t e c t i o n ' o f air cleaner than NAAQS, while Class III areas allow degradation of clean air to the NAAQS. Except for the federal mandatory Class I areas, the entire na- tion is Class II unless a change to Class I or Class III js pursued by a complex pro- cess of government approval.

The 1977 Clean Air Act Amendments require regulations to protect visibility to be promulgated by the Environmental Protection Agency (EPA). These regulations will be an- nounced in 1980.

Since the summer of 1978, the Visibility Research Center has installed and operated a network of fourteen visi- bility measuring sites for the NPS and the EPA. A manually operated multi- wavelength telephotometer is the instru- ment used to measure the apparent con- trast of distant mountains against the adjacent sky, very much like the eye views objects. These instruments allow the computat ion of visibility-related variables, including the extinction coef- ficient of the air along the sight path to the target and the visual range. Visual range is tile maximum distance one can see a black object viewed horizon- tally through a uniform layer of the atmosphere.

These measured and computed variables cannot communicate the actual view of a vista under different air qual- ity, but a picture can.

Methodology Tile technique of making visibility degradation visible is called visibility imaging and was pioneered at Los Alamos Scientific Laboratory in col- laboration with the John Muir Institute for Environmental Studies, Inc. It is based on common slide photography and computer processing of images. A 35 mm slide picture is taken of a vista at a time of good air quality. The type of camera is optional, but a single lens

�9 reflex with a 50 "mm lens seems to be the best choice for ease of use; it pro- duces an image with a field of view simi- lar to that of our eyes. Of the easily available slide films, Kodachrome 25 produces the best color fidelity. Good air quality for the "base" slide is often available just after the passage of cold front, when relatively clean continental air comes down from Canada and the Northern United States.

78 EIA REVIEW 1/1 Environmental Impact Assessment Review, Vol. 1, No. 1 0195-925518010300-0078503.0010

D k ! L g . �9 . . = - . _ . . . . . . . . = - -

This slide is then scanned with a light shining through a red, green, or blue filter and a small aperture about 25-50 microns square. The light trans- mitted through a spot on the slide is measured with a photomultiplier and stored on magnetic tape as a transmis- sion or optical density value. A normal slide measures 24 by 36 mm and a 25 micron aperture can scan most of tile slide in 700 lines with 900 pixels per line. These numbers are arbitrary be- cause the scanning instrument, called a microdensitometer, can be computer controlled for any choice of line and pixel per line numbers.

The test of the fidelity of tlle digitized image is to make a positive transparency from tlae computer stored optical densities, using a computer con- trolled film recorder. Experience has de- monstrated excellent fidelity. If a 25 micron aperture is used in the microden- sitometry, the final picture matches the eye-perceived resolution of the original slide.

There exist many mathematical models of air pollution simulating the rise of a plume from a stack, its disper- sion in an atmosphere of specified stabi- lity (turbulence structure), its chemical reactions, aerosol dynamics, and deposi- tion to the ground. These models have been used for years to compute the air- borne concentrations of pollutant par- ticles and gases. The optical properties of these pollutants can be computed with radiation transfer models simulat- ing the sunlight transferred down through the atmosphere and the trans- fer of light from a target along the sight path to an observer's eye or other re- ceiver.

These models are combined with the geometry of the original slide and the characteristics of a hypothetical air pollution source. For example, we specify a coal-fired powerplant 30 km to the left of the vista seen in the slide. We specify tile emissions from the source, the wind speed and direction, the atmospheric stability and elevations

EIA REVIEW 1/1 79

of the source and vista. A computer run of the models simulates the plume rising from the source and being transported down wind across the vista on the slide. The models compute the new optical densities of the view influenced by the plume. The new optical densities are transformed into a positive transparency on a film recorder, and we get pictures of the vista with the plume predicted by the models.

Conclusion All sorts of conditions can be specified to illustrate the visibility impacts of many kinds of sources, at any location, emitting any mix of pollutants into an atmosphere of specified meteorology. Any vista that can be photographed can be subjected to such hypothetical visi- bility impacts.

This technology should be- come important to new source siting, preconstruction government approval, and analysis of the visibility benefits o f improved air pollution control.

For more information contact:

Eric G. Walther Director Visibility Research Center Physics Department University of Nevada Los Vegas, Nevada 89154 Phone: (702) 739-3844

80 EIA REVIEW 1]1