Strong Winds in Upper Stratosphere Trigger Increase in Ozone-destroying Gases

In March 2006, the winds allowed near-record amounts of ozone- destroying gases, collectively known as nitrogen oxides or NOx, to descend some 50 kilometers [30 miles] from the mesosphere to the top of Earth's stratosphere.

NOx, is a generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts, especially nitric oxide and nitrogen dioxide. Because NOx destroys ozone, which heats up the stratosphere by absorbing ultraviolet radiation, the naturally occurring gases could trigger atmospheric changes that could have unanticipated climate consequences, according to Cora Randall of the University of Colorado at Boulder, lead author of the study.

In February 2006, winds in the polar upper stratospheric vortex, a massive winter low-pressure system that confines air over the Arctic region, sped up to rival the strongest such winds on record, said Randall. The only time more nitrogen oxides were observed in the upper stratosphere was in the winter of 2003-2004, when huge solar storms bombarded the region with energetic particles, triggering up to a 60 percent reduction in ozone molecules, said Randall.

“We knew strong winds would lead to more NOx in the stratosphere if there were solar storms, but seeing that much NOx come down into the stratosphere when the Sun was essentially quiet was amazing,” Randall said. Her paper on the subject was published 27 September in Geophysical Research Letters, published by the American Geophysical Union. Researchers from the University of Waterloo in Ontario, Canada, and the University of Michigan, as well as the University of Colorado participated in the study.

The upper stratosphere lies several kilometers [miles] higher than the ozone hole of the lower stratosphere, which is caused by man- made gases, including chlorine and bromine, which gobble up ozone molecules. Because there is significantly less ozone in the upper stratosphere, the ozone-destroying nitrogen oxide gases are unlikely to cause immediate health threats, such as increases in skin cancer, Randall said.

The destructive NOx gases, created above the stratosphere when sunlight or energetic particles break apart oxygen and nitrogen molecules, appear to be important players in controlling the temperature of Earth's middle atmosphere, according to Randall. “If human-induced climate change leads to changes in the strength of the polar vortex, which is what scientists predict, we'll likely see changes in the amount of NOx descending into the stratosphere,” she said. “If that happens, more stratospheric NOx might become the rule rather than the exception.”

“The atmosphere is part of a coupled system, and what affects one layer of the atmosphere can influence other layers in surprising ways,” Randall said. “We will only be able to predict and understand the consequences of human activities if we study the entire system as a whole, and not just in parts.”

The 2006 increases of NOx in the upper stratosphere occurred over the Arctic and the northern areas of North America and Europe, according to the paper's authors. The research team used data from Canadian and United States satellites, including the Canadian Atmospheric Chemistry Experiment.

The work was funded by NASA and the Canadian Space Agency.