Ozone Depletion Over Ocean Caused By Sea Surface Iodine
Lawrence LeBlond for redOrbit.com – Your Universe Online
The earth´s ozone layer is critical for protecting life from damaging UV light from the sun. In the late 1970s scientists discovered for the first time that this precious barrier between life and death was depleting, and at a fairly consistent rate. After years of study, the man-made cause of the depletion–mainly seen in the polar regions–was determined to hail from chlorofluorocarbons (CFCs), which were found widespread in aerosol sprays and other products, as well as in nature. With that knowledge, several governments around the world moved to ban all CFC-emitting products to try to reverse ozone loss.
Into the mid-1990s, other governments enacted policies to reduce or remove CFCs from the market, and as early as the year 2000, scientists began noticing a decrease in ozone depletion, indicating the ozone was beginning to repair itself after decades of damage, largely caused by man. However, this is not the end of the ozone story. After a new chapter in the ozone saga came to light, scientists scrambled to make sense of why ozone was continuing to deplete, this time over the oceans.
In their studies, they determined the majority of ozone-depleting iodine oxide observed over the remote ocean comes from a previously unknown marine source. They found that the principal source of iodine oxide comes from emissions of hypoiodous acid (HOI). What was perplexing to the team was the fact that HOI was previously not known as being released from the ocean itself. They also found instances of molecular iodine (I2) also being released.
Methyl iodide (CH3I) was first discovered to be globally present in the world´s oceans in the 1970s. Yet the presence of iodine in the atmosphere at the time was believed to have come from emissions of organic compounds from microscopic phytoplankton.
Earlier research has shown that reactive iodine and bromine in the atmosphere are responsible for destruction of ozone on a wide level–about 50 percent more than predicted by some of the most advanced climate models–in the lower atmosphere over the tropical Atlantic Ocean.
The new research, published in Nature Geoscience, involved the study of gaseous emissions of inorganic iodine, which were found to be prevalent in the atmosphere based on studies of iodide reactivity of ozone in the lab setting.
The reaction of iodide with ozone leads to the formation of both I2 and HOI. Using their lab models, the researchers showed that the reaction of ozone with iodide on the sea surface could account for 75 percent of observed iodine oxide levels found over the tropical Atlantic.
“Our laboratory and modeling studies show that these gases are produced from the reaction of atmospheric ozone with iodide on the sea surface interfacial layer, at a rate which is highly significant for the chemistry of the marine atmosphere,” noted study coauthor Professor Lucy Carpenter, of the Dept. of Chemistry at York.
“Our research reveals an important negative feedback for ozone — a sort of self-destruct mechanism. The more ozone there is, the more gaseous halogens are created which destroy it. The research also has implications for the way that radionucleides of iodine in seawater, released into the ocean mainly from nuclear reprocessing facilities, can be re-emitted into the atmosphere,” Carpenter said in a statement.
“This mechanism of iodine release into the atmosphere appears to be particularly important over tropical oceans, where measurements show that there is more iodide in seawater available to react with ozone,” added coauthor Professor John Plane, from the School of Chemistry at Leeds.
“The rate of the process also appears to be faster in warmer water. The negative feedback for ozone should therefore be particularly important for removing ozone in the outflows of pollution from major cities in the coastal tropics,” he concluded.