Copper released into the environment from fungicides may be contributing significantly to stratospheric ozone depletion, according to a new study from the University of California (UC).
In a paper published Jan. 14 in the journal Nature Communications, UC geochemists show that copper in soil and seawater acts as a catalyst to turn organic matter into both methyl bromide and methyl chloride, two potent halocarbon compounds that destroy ozone. Sunlight worsens the situation, boosting production of these methyl halides by a factor of 10. The findings answer, at least in part, a long-standing mystery about the origin of much of the methyl bromide and methyl chloride in the stratosphere.
Since the worldwide ban on chlorofluorocarbon (CFC) refrigerants and brominated halons used in fire extinguishers starting in 1989, these methyl halides have become the new dominant sources of ozone-depleting bromine and chlorine in the stratosphere. As the long-lived CFCs and halons slowly disappear from the atmosphere, the role of methyl halides increases.
"By 2050, we should be back to relatively normal ozone, but things like the continued emissions of methyl bromide and methyl chloride are road bumps in the road to recovery. Copper usage in the environment is projected to increase rapidly in the next few years, and this should be considered when predicting future halogen load and ozone recovery," said the paper's senior author, Robert Rhew, UC professor of environmental science and policy.
Earth's ozone layer is critical to protecting us from cancer-causing ultraviolet light from the sun, but chemicals containing chlorine and bromine -- such as CFCs and halons -- were found in the 1980s to destroy the ozone, creating thinner layers in the stratosphere that let in more of the dangerous radiation.
Despite a ban on production of CFCs and halons, the ozone layer has yet to repair itself.
The persistence of the ozone hole is, for the most part, due to the persistence of banned ozone-depleting compounds, which take decades to dissipate in the stratosphere. But some ozone-depleting chemicals are still being emitted.
Among the major contributors today are methyl chloride and methyl bromide. One atom of bromine is 50 times more destructive to ozone than one atom of chlorine.
Though methyl bromide is banned for use as an agricultural soil fumigant, it is still used as a pesticide for quarantine and pre-shipment of agricultural products. And methyl chloride is used as a chemical feedstock, although most of its emissions are believed to be from biomass burning or natural in origin. But the total amount of these methyl halides produced each year still do not add up to the observed yearly addition of these chemicals to the atmosphere, a fact that has puzzled scientists for more than 20 years.
About one-third of the methyl bromide and methyl chloride in the atmosphere comes from unknown sources, Rhew said. The new findings suggest that copper is an important, if not the major, source of the missing methyl bromide and methyl chloride.
"We've banned methyl bromide, but are other changes that we're making in the environment causing large emissions of this compound into the atmosphere? With the increase in the use of copper, it appears that copper-catalyzed production is an increasing source, as well," Rhew said.
First author and former UC doctoral student Yi Jiao, now a postdoctoral fellow at the University of Copenhagen in Denmark, noted that copper compounds are allowed on organic crops, a legacy of its use in farming since the 1700s, including as a major antifungal agent in the Bourdeax mixture used since the 1880s in France to prevent downy mildew on grapes. Copper contamination of soils is a major issue today in Europe because of this history, and the ozone-depleting power of copper is another cause for concern.
"With widespread use of copper in the environment, this potentially growing impact should be considered when predicting future halogen load and ozone recovery. Copper combined with soil and sunlight produce more methyl halides," said Jiao.
