Acid rain's most visible symptom.

Acid rain's most visible symptom

On a clear day, you can see forever -- or so the song goes. In reality, various light-scattering and light-absorbing factors in the air, such as gas molecules, limit the distance at which our eyes can distinguish landscape features. In dry, pristine environments, that "visual range" usually extends for about 230 kilometers; in the humid eastern and midwestern United States, the farthest one can see is about 150 km. Haze, however, currently limits the median visual range to about 150 km in the western states and to about 25 km east of the Mississippi River, says John Trijonis of Santa Fe Research Corp. in Bloomington, Minn.

About 85 percent of the current eastern haze and 50 percent of the western haze results from air-pollutant particles smaller than 2.5 microns in diameter, especially those contributing to acid rain, Trijonis and five co-workers report.

Sulfates--the leading constituents of acid rain--rob about 60 percent of the visual detail from eastern haze regions and up to 30 percent from western ones. Soot, organic particles and the nitrates formed by nitrogen oxide emissions (principally from cars) roughly match sulfates' haze contribution in western states, which burn much less coal.

The researchers took photographs in normally hazy regions on unusually clear days, then added filters to duplicate the light-robbing air pollutants now typically in those areas. A set depicting Chicago's skyline, shown here, demonstrates the visual range under hazeless conditions (top) and under today's typical conditions. Trijonis speculates that the eastern visibility improvements from the sulfur-emission controls in President Bush's clean-air legislation (SN:6/17/89, p.375) would be "very minor," probably 15 to 20 percent.