Haze may confound effects of ozone loss.

A layer of stratospheric ozone acts as a shield that protects Earth's inhabitants from much of the sun's damaging ultraviolet radiation. With recent recognition that certain pollutants are eroding this natural radiation filter, researchers have raced against the clock to find substitutes for the chlorofluorocarbons and other compounds responsible. Adding to the urgency, the United Nations announced in October that summertime ozone levels above Earth's midlatitudes have decreased by about 3.3 percent per decade since 1979 (SN: 11/2/91, p.278).

Haze -- a suspension of sulfates and other largely combustion-generated pollutants in the lowr atmosphere--is best known for its ability to reduce daytime visibility. However, it can also filter out harmful ultraviolet rays. Indeed, a new theoretical study suggests that ambient haze levels in even rural areas of the United States and other developed, midlatitude nations currently filter out an amount of ultraviolet light equal to or greater than the excess now leaking through the stratosphere.

The bad news is that this balancing act should prove transitory. As industrial nations tighten controls on hazeforming pollutants and as stratospheric ozone continues to thin, both ultraviolet shields will erode further.

Using measurements collected at rural sites in the United States and Europe, Shaw C. Liu of the National Oceanic and Atmospheric Administration in Boulder, Colo., and his colleagues estimated the concentrations of the microscopic particles, or aerosols, responsible for haze. Since summertime solar heating mixes these fine, ultraviolet-scattering aerosols in the lowest kilometer or two of the atmosphere quite effectively, says Liu, "we assumed the aerosol concentration we derived from visibility measurements was constant with height throughout this mixed layer." And because aerosol blockage of ultraviolet light above this layer "is rather small," he says, "we pretty much assumed there were essentially no aerosols from the top of this boundary layer to the sun."

In the December GEOPHYSICAL RESEARCH LETTERS, Liu's team concludes that haze levels typical of northern-midlatitude rural sites have decreased ground-level exposures to damaging UV-B radiation (280- to 315-nanometer wavelengths) by 5 to 18 percent compared with preindustrial times. In urban areas such as New York City, where haze levels may be two to five times higher, "it's a safe bet that the ultraviolet -- from air pollution alone -- has been cut down something like 20 to 30 percent from preindustrial times," Liu told SCIENCE NEWS. Validation of these estimates must await publication of precise ultraviolet data now being collected by others, he says.

Because most ultraviolet monitoring occurs in haze-prone regions, Liu and his coauthors speculate that future reductions in both aerosols and stratospheric ozone "may result in a stronger [increase] in UV-B than expected from either constituent alone."

Atmospheric chemist F. Sherwood Rowland of the University of California, Irvine, calls the procedures used by Liu's group "quite defensible" and the team's conclusions "quite plausible."