Nitrogen oxide and nitrogen dioxide gases in the upper stratosphere climbed to their highest levels in at least two decades in spring 2004, scientists report. The increases led to ozone reductions of up to 60 percent, roughly 40 kilometers [25 miles] above Earth’s high northern latitudes, according to Cora Randall of University of Colorado at Boulder and 10 colleagues in Canada, Norway, Sweden, and the United States. Two natural processes were responsible, they say.
“This decline was completely unexpected,” Randall said. “The findings point out a critical need to better understand the processes occurring in the ozone layer.” Randall, a researcher at the university’s Laboratory for Atmospheric and Space Physics, is lead author of a paper on the subject scheduled for publication 2 March in Geophysical Research Letters. She and her international team studied data from seven different satellites, concluding that both the Sun and stratospheric weather were responsible for the ozone declines.
Winds in the upper part of a massive winter low-pressure system, which confines air over the Arctic region and is known as the polar stratospheric vortex, sped up in February and March 2004 to become the strongest on record, she said. The spinning vortex allowed the nitrogen gases, thought to have formed at least 30 kilometers [20 miles] above the stratosphere as a result of chemical reactions triggered by energetic particles from the Sun, to descend more easily into the stratosphere.
The increases in the two nitrogen gases — collectively known as nitrogen oxides or NOx — are important because they are major players in the stratospheric ozone destruction process, said Randall. The team concluded that some of the extra nitrogen oxides was actually formed after huge quantities of energetic particles from the Sun bombarded Earth’s atmosphere during the massive solar storms of October-November 2003.
“No one predicted the dramatic loss of ozone in the upper stratosphere of the Northern Hemisphere in the spring of 2004,” she said. “That we can still be surprised illustrates the difficulties in separating atmospheric effects due to natural and human-induced causes. “This study demonstrates that scientists searching for signs of ozone recovery need to factor in the atmospheric effects of energetic particles, something they do not now do.”
The 2004 enhancements of nitrogen oxides gases in the upper stratosphere and subsequent ozone losses occurred over the Arctic and the northern areas of North America, Europe, and Asia, said the paper’s authors. Severe ozone losses also can occur during winter and spring in the stratosphere at about 20 kilometers [12 miles] in altitude, driven primarily by very cold temperatures, they said.
Because of seasonal conditions, the researchers are unable to measure the precise contributions of solar storms and stratospheric weather to the nitrogen oxides spike seen in the stratosphere last year. “No observations of upper atmospheric nitrogen gases are available in the polar region in the winter, so the descending nitrogen oxides cannot be traced to its origin,” said Randall.
Stratospheric ozone, a form of oxygen, protects life on Earth from the harmful effects of ultraviolet radiation. The ozone layer has thinned markedly in high latitudes of the Northern and Southern Hemispheres in recent decades, primarily due to reactions involving chlorofluorocarbons and other industrial gases. Scientists credit the 1987 Montreal Protocol, an international agreement that is phasing out the production and use of such ozone-destroying compounds, for helping the protective ozone layer to be restored by the middle of this century.
Randall’s co-authors include researchers from the University of Colorado at Boulder; the National Oceanic and Atmospheric Administration, NASA, the Harvard-Smithsonian Center for Astrophysics; Hampton University and GATS Inc; York University in Canada; Chalmers University of Technology in Sweden; and the Norwegian Institute for Air Research. They analyzed data from satellite instruments, including POAM II, POAM III, SAGE II, SAGE III, HALOE, MIPAS and OSIRIS for the study, which was funded by NASA, the European Union Commission, and the European Space Agency.