Wider Damage to Earth’s Ozone Layer Feared

Scientists studying atmospheric ozone depletion have long believed that pollutants ravage the Earth’s protective ozone layer only at certain times of the year, when sunlight and other conditions are just so.

Now, however, satellite measurements made by scientists at the Jet Propulsion Laboratory in Pasadena and at Edinburgh University in Scotland indicate that the ozone-destroying process operates for extended periods.

The data, published in today’s edition of the British journal Nature, suggests that the potentially dangerous ozone loss may be masked in the winter and early spring by the influx of ozone from elsewhere on the planet.

Ozone holes, such as those observed over the South Pole, develop only when shifting winds cut off this replenishing flow, the scientists believe.

If confirmed by later observations, this would indicate that ozone loss over the poles already could be depleting the protective form of oxygen in the upper atmosphere over a much wider area than previously believed.

Joe Waters, the National Aeronautics and Space Administration scientist who heads a team of scientists using the Upper Atmosphere Research Satellite to measure atmospheric chemistry, called the new data “a significant reason for concern.”

However, he said too little is known about atmospheric chemistry to say whether this is the first indication of a potentially catastrophic trend or merely a statistical quirk.

The Nature article says that ozone levels over the Northern Hemisphere in late February and early March were 10% below levels at the same time last year. “This is twice the year-to-year average fluctuation,” Waters said, “and last year was already low.”

Ozone is a lung-searing pollutant at low altitudes, but in the upper atmosphere it absorbs solar ultraviolet rays that cause skin cancer and cataracts. Chlorine-based industrial gases, such as those found in refrigerators and some fire extinguishers, destroy ozone.

Waters and his team discovered that all of the chlorine in the atmosphere--less than 20% of which occurs naturally--is converted to an ozone-eating form such as chlorine monoxide. In the past scientists believed at least part of the chlorine bonded to other elements to form inert combinations.

Evidence of the replenishing ozone flows might explain why high levels of chlorine monoxide are measured over both poles at certain times without producing the expected drop in the ozone level, a paradox that has long perplexed scientists.

Martyn Chipperfield of Cambridge University said the finding about chlorine, coming at the beginning of what scientists worry will be a particularly bad spring and summer for Earth’s atmosphere, is “an invaluable aid in trying to understand and quantify the evolution of stratospheric ozone.”

Until now, it was believed that chlorine assumed its highly reactive form only when exposed to sun for extended periods during longer spring and summer days and to immense clouds of water and nitric acid. Ice crystals in the clouds provide a surface on which certain chemical reactions produce ozone-eating chlorine monoxide.

Wind patterns usually cause such clouds to linger longer over the Antarctic than the Arctic. This, scientists said, is why an ozone hole hovers over the South Pole while the ozone is merely thin over the North Pole.

Last winter, however, the Northern Hemisphere experienced a particularly cold season, letting these “polar stratospheric clouds” form earlier and last longer over the Arctic--allowing the creation of more chlorine monoxide.

Waters confirmed that the atmospheric satellite detected high levels of that gas from early December through early March, or about a month longer than normal. A predictable drop in ozone over the hemisphere has since been measured by the World Meteorological Organization.

Because of this, Chipperfield said, a substantial ozone loss should be expected later in this spring and into summer.