Summer Storms Cause Ozone Depletion In United States
July 27, 2012

Summer Storms Cause Ozone Depletion In United States

Lawrence LeBlond for - Your Universe Online

It was in the 1970s that the British Antarctic Survey discovered a hole in the ozone layer above the Antarctic Circle, and in 1985, when the first measurements were taken, researchers were dumbfounded by the dramatic drop in ozone levels. The concern was so great that it eventually led to the ban of all products containing chlorofluorocarbons (CFCs), which were confirmed to be the root cause of the ozone depletion.

Fast forward nearly 30 years, and a new threat is sitting on our doorstep. Researchers from Harvard, led by Weld professor of atmospheric chemistry James G. Anderson, have discovered a serious and wholly unexpected ozone loss over the United States in summer. The finding, published today in the online at Science´s Science Express website, has startled the experts because ozone depletion was thought to only occur at very cold temperatures over the polar regions where there is little threat to humans.

The ozone layer is important as it blocks a large incoming portion of the sun´s ultraviolet light from reaching the Earth, protecting humans and animals from the damaging radiation that can lead to skin cancer in humans. Previously, it was believed that stratospheric ozone was susceptible to chemical catalysts of manmade origin, such as chlorine and bromine, which are present in the atmosphere because of the formerly widespread use of CFCs. And the chemical reactions that destroy ozone are highly dependent on atmospheric temperature and the presence of water vapor.

What Anderson and his colleagues discovered, is that during strong summer storms in the US, water vapor is thrust by convection high up into the lower stratosphere, higher than researchers thought possible, and alters atmospheric conditions that result in widespread ozone depletion for days after such storms.

This is the first time ozone depletion has been linked to the effects of climate change. And scientists said the risk of ozone damage could increase if global warming leads to more strong storms.

“We were investigating the behavior of convective water vapor as part of our climate research,” Anderson said, “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.”

“It´s the union between ozone loss and climate change that is really at the heart of this,” said Anderson. For years, he and other atmospheric scientists were careful to keep the two concepts separate. “Now, they´re intimately connected,” he said.

Using isotopic signatures, the team demonstrated that water vapor had been carried directly to the stratosphere as a result of convective injection. And in these regions where injected water vapor was found, also found was the catalytic loss of ozone increased by a hundredfold. As a result, rates of ozone loss significantly exceed the natural rates of ozone regeneration. While the data come from experimental evidence gathered over the US, they note that similar conditions may exist elsewhere.

These findings have significant implications for public health because the increased rate of ozone depletion occurs in only a few days within regions with high water-vapor concentration, greatly increasing the incidence of exposure to dangerous UV radiation, which in turn leads to high onset of skin cancer.

Though the findings are sound, Anderson said much more research is needed, including direct measurements in the stratosphere in areas where water vapor was present after storms. Still, the problem deeply concerns Dr. Anderson. “I never would have suspected this.”

While the evidence is conclusive, Anderson noted that it is not yet clear whether the number of such injection events will rise. “Nobody understands why this convection can penetrate as deeply as it does,” said Anderson, who has studied the atmosphere for four decades.

Mario Molina, professor of chemistry and biochemistry at UC, San Diego, and co-recipient of the 1995 Nobel Prize for his research in the 1970s on CFCs and ozone depletion, said the findings add “one more worry to the changes that society´s making to the chemical composition of the atmosphere.”

Molina, who was not involved in the latest research, said the concern was “significant ozone depletion at latitudes where there is a lot of population, in contrast to over the poles.” The NASA-funded study focused on the US because that is where the data was collected.

Ralph Cicerone, an atmospheric scientist and the president of the National Academy of Sciences, who reviewed the study for Science, agreed that more research was needed.

“One of the really solid parts of this paper is that they´ve taken the chemistry that we know from other atmospheric experiments and lab experiments and put that in the picture,” he said. “The thing to do is do field work now -- measure moisture amounts and whether there is any impact around it.”

“The connection with future climate is the most important issue,” Cicerone said. If further research verifies the current findings, then the impacts in “a future climate where the air is getting warmer and moister” will need to be considered.

Recollecting the ozone issue of the 70s and 80s, Molina points out that if there had “been no international agreement to ban CFCs” in the late 1980s, this newly described problem “would have been a lot worse.” He said he hopes that “these types of warnings will make the case even stronger for society to begin to react to the climate-change issue, just like we managed to do with the ozone issue.”

The use of CFCs were phased out beginning in the late 1980s with the signing of the international Montreal Protocol treaty, but experts said it will take decades for the atmosphere to be totally cleansed of the chemical pollutant. Chlorine from the CFCs are the ultimate destroyer of the ozone. Ozone has to undergo a chemical shift in the presence of sunlight that makes it more reactive, and the shift is sensitive to temperature.

By showing that CFC-related ozone destruction can occur in conditions other than the cold ones at the poles, the study suggests that the full recovery of the ozone layer may be further off than previously considered.