Polar clouds cause more ozone loss in Arctic
Researchers conclude that long-lasting polar clouds create greater
ozone depletion over the Artic.
Copyright 2000, Environmental News Network
Tuesday, June 6, 2000
By Robinson Shaw
The ozone layer that protects life on Earth may not be recovering from the damage it has suffered over the Arctic region as quickly as scientists previously thought. The evidence is in the clouds.
More polar stratospheric clouds than anticipated are forming above the North Pole and staying longer than usual, causing additional ozone depletion over the Arctic, according to Azadeh Tabazadeh, a scientist at NASA's Ames Research Center.
The stratosphere is nine to 25 miles above Earth and includes the ozone layer. PSCs generally form 13 miles above the poles, where temperatures can drop to minus 110 degrees Fahrenheit and below.
"Polar stratospheric clouds provide a 'double-whammy' to stratospheric ozone. They provide the surfaces, which convert benign forms of chlorine into reactive, ozone-destroying forms, and they remove nitrogen compounds that act to moderate the destructive impact of chlorine," said Phil DeCola, NASA's atmospheric chemistry program manager.
Most chlorine compounds pumped into Earth's atmosphere in recent decades by human activities initially were tied up as chlorine nitrate or hydrochloric acid, both of which are non-reactive. But the chlorine compounds change into ozone-eating chlorine radicals in late winter and early spring after reacting with sunlight when PSCs are in the picture.
"An ozone hole forms every spring over the Antarctic in the Southern Hemisphere which is colder than the Arctic," said Tabazadeh. "The Arctic has been getting colder and is becoming more like the Antarctic; this could lead to more dramatic ozone loss in the future over the Northern Hemisphere, where many people live."
In some parts of the Arctic, stratosphere ozone concentrations declined as much as 60 percent from November 1999 through March 2000.
Although seasonal ozone loss is more severe in the Antarctic, the ozone loss in the Arctic presents potentially more serious health problems to human beings, said Owen Toon, a professor at the University of Colorado who was one of five project scientists to lead SOLVE.
Ozone-depleted air from the Arctic drifts south toward North America, Europe and Russia each spring, increasing the amounts of ultraviolet light reaching Earth's surface in the highly populated mid-latitudes and potentially causing increases in several types of cancer.
Research during the SAGE 111 Ozone Loss and Validation Experiment, or SOLVE, in Kiruna, Sweden, was conducted during several flights of the ER-2.
The greenhouse effect, which warms Earth near its surface, may also be cooling the stratosphere enough to cause PSCs to form earlier and persist longer. Greenhouse gases are radiating energy and heat away from the upper stratosphere, creating prime conditions for PSC formation.
"The Arctic has become colder and more humid, conditions that promote formation of more polar stratospheric clouds that take part in polar ozone destruction. The main conclusion of our study is that if this trend continues, Arctic clouds will remain longer in the stratosphere in the future," Tabazadeh said.
"With the clouds persisting longer, we are seeing greater ozone losses even though the amount of chlorine in the atmosphere has declined slightly," said Toon.
More than a decade ago, scientists determined that manmade chlorine and bromine compounds cause most ozone depletion. Manufacturers used the chlorofluorocarbons, or CFCs, as refrigerants, aerosol sprays, solvents and foam-blowing agents. Fire fighters used bromine-containing halogens to put out fires. The Montreal Protocol ended the manufacture of CFCs in signatory countries.
The Montreal Protocol has helped to reduce the amount of chlorine in the upper atmosphere.
"Scientists used to believe that as chlorine levels decline in the upper atmosphere, the ozone layer should slowly start to recover. However, greenhouse gas emissions, which provide warming at the Earth's surface, lead to cooling in the upper atmosphere. This cooling promotes formation of the kind of polar stratospheric clouds that contribute to ozone loss," said Tabazadeh. "Several recent studies, including this one, show that ozone recovery is more complex and will take longer than originally thought."