Unexpected Ozone Loss Observed above United States
A team of Harvard scientists led by Weld professor of atmospheric chemistry James G. Anderson announced today the discovery of serious and wholly unexpected ozone loss over the United States in summer. The finding, published in advance online on July 26 at Science’s Science Express website, is startling because the complex atmospheric chemistry that destroys ozone has previously been thought to occur only at very cold temperatures over polar regions where there is very little threat to humans. (A large hole in the ozone layer persists over Antarctica.) The discovery also links—for the first time—ozone loss (an issue around which world leaders successfully organized to ban chlorofluorocarbons, or CFCs) to climate change (a global problem that has so far proven politically intractable).
The ozone layer blocks a large fraction of the sun’s ultraviolet light from reaching the earth, protecting life forms from potentially damaging radiation that in humans can lead to skin cancer. But stratospheric ozone is susceptible to chemical catalysts of manmade origin, such as chlorine and bromine, which are present in the earth’s atmosphere as a result of the formerly widespread commercial use of CFCs. And the chemical reactions that destroy ozone are highly dependent on both atmospheric temperature and the presence of water vapor.
Anderson’s team has discovered that during intense summer storms over the United States, water vapor is thrust by convection far higher into the lower stratosphere than previously thought possible, altering atmospheric conditions in a way that leads to substantial, widespread ozone loss throughout the ensuing week. The paper links the loss of ozone over populated mid-latitude regions in summer to the frequency and intensity of these big storms, which could increase with climate change resulting from rising levels of carbon dioxide and methane in the atmosphere.
“We were investigating the behavior of convective water vapor as part of our climate research,” Anderson says, “not ozone photochemistry. What proved surprising was the remarkable altitude to which water vapor was being lofted—altitudes exceeding 60,000 feet—and how frequently it was happening.” Anderson and his team realized the significance of the finding because higher water- vapor concentrations in the cold reaches of the lower stratosphere change the threshold temperature at which chlorine is converted to a free radical state: in the presence of water vapor, direct catalytic removal of ozone takes place at warmer temperatures.
http://harvardmagazine.com/2012/07/ozone-destruction
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