Northern hemisphere ozone hits record low.

The north has taken center stage in the fast-moving global ozone drama. Six months ago, world attention focused on the extreme south when atmospheric scientists reported that the yearly ozone hole over Antarctica had expanded over a record area. Now the spotlight has shifted to the opposite end of the Earth as researchers announce finding substantially less than normal ozone over much of the northern hemisphere during the last three months.

Rumen D. Bojkov, an atmospheric scientist with the United Nations' World Meteorological Organization in Geneva, reports that concentrations of stratospheric ozone measured 9 to 20 percent below average in the middle and high latitudes of the northern hemisphere during December, January, and February Ozone concentrations were even lower this year than last, when they also measured well below the average for the last 30 years, says Bojkov, who analyzed groundbased measurements as well as some data from US. satellites.

To have two years in a row that hit [such] values is clearly extraordinary" Bojkov says.

The recent northern thinning is the latest chink to appear in the stratospheric ozone armor, which protects Earth's surface from harmful ultraviolet radiation from the sun. British scientists reported the first signs of weakness in 1985, after discovering that the stratosphere above Antarctica loses a large fraction of its ozone eacH year during September and October, a phenomenon dubbed the "ozone hole."

After several expeditions to Antarctica, scientists gathered proof linking the ozone hole to chlorofluoracarbons and other pollutants that carry destructive chlorine and bromine high into the stratosphere. In the frigid Antarctic, these chemicals eat away half the ozone in hte sky during the southern hemisphere's springtime.

In 1987, researchers discovered a less obvious but more pervasive ozone thinning around the globe. The stratosphere over all regions except the tropics has lost a few percent of its ozone since 1979 - a trend also attributed by scientists to chlorine and bromine compounds.

Bojkov thinks that such chemicals caused much of the dramatic ozone thinning over the northern hemisphere this winter, but other factors also contributed, he says. Meteorological data suggest that air movement played a role in setting up the ozone pattern. Masses of tropical air, naturally low in ozone, spread north during winter, lowering ozone concentrations above Europe and other regions, Bojkov says.

U.S. scientists who monitor ozone-sensing instruments on several satellites confirm that northern hemisphere ozone concentrations, which normally increase in winter, did not rise as much as in previous years. Measurements made by the Upper Atmosphere Research Satellite (UARS) also support the idea that destructive chemicals helped thin this winter's ozone. From December through late February, the craft observed extremely high concentrations of ozone-eating chlorine monoxide over much of the Arctic and surroundings regions, says UARS scientist Joe W. Waters of NASA's Jet Propulsion Laboratory in Pasadena, Calif.

From past studies, atmospheric researchers have come to know chlorine monixide as the chief villain that attacks ozone in the Antarctic. Normally, chlorine in the stratosphere is bound in inactive molecules that cannot destroy ozone. But the cold Antarctic stratosphere contains icy particles that provide a surface on which inactive chlorine compounds can convert to harmful chlorine monoxide.

In early February of last year, scientists involved in a major research project expressed dismay at finding extremely high concentrations of chlorine monoxide in the Arctic stratosphere - an indication that the atmosphere of the north was primed to destroy ozone. If temperatures were to remain cold for several weeks, they predicted, sunlight and chlorine monoxide could combine to munch away a significant fraction of the Arctic ozone, perhaps even generating an ozone "hole" in the north. Soon afterwards, however, the atmosphere warmed, saving the Arctic from major ozone loss at that time.

The events of this winter apparently vindicate predictions made last year. Temperatures in the polar stratosphere stayed cold a month longer than last year, and high concentrations of chlorine monoxide persisted longer as well. As if on cue, ozone levels remained well below normal, hitting record lows for the month of February.

"Last year was the warning; this year it happened," says Mark R. Schoeberl of NASA's Goddard Space Flight Center in Greenbelt, Md.

The combination of destructive chemicals and meteorology, however, did not create an Arctic ozone hole, because ozone is normally quite abundant in the higher latitudes of the northern hemisphere during winter. Even with concentrations well below average for this time of year, ozone remained plentiful in the sky during the last few months, providing what was probably adequate protection against the weak winter sunlight, scientists say.

But the current deficit could leave ozone levels lower than normal later in the year, when more ultraviolet radiation streams through the northern hemisphere skies, Schoeberl says.