UV hazard: ozone lost versus ozone gained.

Living amid polluted air can cause serious health problems, especially for the elderly and asthma sufferers. But bad air also has its good side. Measurements in New Zealand and Germany reveal that a polluted environment can provide significant protection against the harmful ultraviolet (UV) radiation streaming through Earth's damaged ozone layer.

"We found unexpected, big differences in the UV radiation at the two sites," New Zealand and Germany, says Gunther Seckmeyer, an atmospheric scientist with the Institute for Biochemical Plant Pathology in Neuherberg, Germany. Levels of UV light were nearly twice as high in the relatively clean air of New Zealand as in the more polluted air of Germany, report Seckmeyer and Richard L. McKenzie of the National Institute of Water and Atmospheric Research, Ltd., in Omakau, New Zealand.

UV light from the sun can cause skin cancers and cataracts and can lower immunity to diseases. The ozone layer in Earth's stratosphere filters out most of this harmful radiation, but scientists warn that UV levels will climb as human-made chemicals thin the ozone layer.

In the last several years, satellite and ground-based instruments have detected significant thinning of stratospheric ozone over much of the world. However, it is not clear how UV levels at Earth's surface have changed, because no worldwide measurement network exists.

Reports from limited regions differ on whether UV levels have risen. While measurements in the Alps show a strengthening in UV intensity, those in the United States show a weakening, causing some to wonder whether pollution masks the loss of stratospheric ozone. (SN: 2/20/88, p.119; 4/14/90, p.228).

In their study, Seckmeyer and McKenzie compared UV levels during cloud-free days in New Zealand at 45 degrees south latitude and in Germany at 48 degrees north latitude. By using the same spectroradiometer in both locations, the researchers eliminated errors caused by comparing data from two instruments.

Seckmeyer and McKenzie found that on a summer day at each site, DNA-damaging wavelengths of UV were 90 percent higher in New Zealand, while wavelengths capable of producing sunburn were 60 percent higher there. Although many factors can control the amount of UV light reaching the surface, analysis of the light's strength at various wavelengths indicates that ozone differences accounted for the big discrepancy between the two sites. Wavelengths of light not affected by ozone were virtually the same at the two sites.

To examine whether the UV levels on the days studied were typical of an entire summer, the researchers used a model to calculate average UV levels on the basis of such factors as ozone amount, latitude, and weather variables. The calculations suggest that DNA-damaging wavelengths of UV light were 81 percent stronger and sunburn-producing wavelengths were 44 percent stronger in New Zealand, they report in the Sept. 10 Nature. The researchers say they had not expected such a large hemispheric difference.

Ozone measurements for the two regions indicate that the air over the New Zealand station had roughly 15 percent less ozone than did air over the German station. Instruments carried by balloons reveal two separate causes underlying that difference. In the troposphere, or lower atmosphere, Germany has much higher levels of ozone pollution; this accounts for about half the total ozone difference between the two sites. The rest stems from inequities in the stratosphere, where Germany also has greater concentrations of ozone.

To explain the stratospheric discrepancy, Seckmeyer notes that New Zealand lies close to Antarctica, where vast amounts of ozone are destroyed each September during the formation of the ozone hole. Indeed, ozone levels in Antarctica are currently dropping at a record rate, suggesting that this year's ozone hole will be particularly severe. When the hole breaks up during October and November, pockets of ozone-depleted air drift northward and mix with air over New Zealand and other regions in the southern hemisphere.

The same chemicals that cause the ozone hole also eat away at the global ozone layer. But so far, increases in tropospheric ozone pollution at the German site have largely offset the stratospheric loss there, the researchers say.

UV levels in Germany and other industrialized countries might actually be lower today than they were in preindustrial times, says Sasha Madronich of the National Center for Atmospheric Research in Boulder, Colo. Aside from considerable tropospheric ozone pollution, these regions also suffer from sulfuric acid haze, another factor that filters out UV light, says Madronich.

In a recent study, Madronich and colleagues calculate that the haze factor alone should have reduced UV levels by 5 to 18 percent in rural parts of industrialized countries (SN: 1/4/92, p.5). As nations begin to reduce air pollution, the cleaner skies will combine with stratospheric ozone loss, permitting more ultraviolet radiation to reach the surface, Madronich says.