According to a new study from the University of California, Berkeley, copper released into the environment via fungicides, brake pads, antifouling coatings on boats, and other sources may be contributing significantly to stratospheric ozone depletion.
Copper in soil and saltwater works as a catalyst for the conversion of organic matter into methyl bromide and methyl chloride, two strong halocarbon chemicals that deplete ozone, according to a report published this week in the journal Nature Communications by UC Berkeley geochemists. The problem is made worse by sunlight, which increases the creation of these methyl halides by a factor of 10.
The findings help to explain, at least in part, the origin of much of the methyl bromide and methyl chloride in the stratosphere. These methyl halides have become the new primary sources of ozone-depleting bromine and chlorine in the stratosphere since the worldwide ban on chlorofluorocarbon (CFC) refrigerants and brominated halons used in fire extinguishers began in 1989. Methyl halides are becoming more important as long-lived CFCs and halons fade from the atmosphere.
“If we don’t know where methyl bromide and methyl chloride are coming from, then how can we make sure that those compounds are reduced along with CFCs?” said the paper’s senior author, Robert Rhew, UC Berkeley professor of geography and of environmental science, policy, and management.
“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.”
The ozone layer protects us from cancer-causing ultraviolet light from the sun, but compounds containing chlorine and bromine, such as CFCs and halons, were discovered in the 1980s to deplete the ozone layer, resulting in thinner stratospheric layers that let in more hazardous radiation.
Please note that organic agriculture is not a major cause for ozone depletion. However, copper-based fungicides appear to have atmospheric side effects that might be considered in terms of overall environmental impact. With the widespread use of copper in the environment, this potentially growing impact should be considered when predicting future halogen load and ozone recovery.
Jiao
“Despite a ban on the production of CFCs and halons, the major sources of halogens, the ozone layer has yet to repair itself. Last year, the hole in the ozone over Antarctica was about as bad as it’s ever been,” Rhew said.
The ozone hole’s endurance is mostly owing to the persistence of banned ozone-depleting substances in the stratosphere, which take decades to evaporate. However, some ozone-depleting substances continue to be released. Even some substitutes for banned refrigerants are being scrutinized.
Methyl chloride and methyl bromide are two of the most important contributors today. Bromine atoms are 50 times more damaging to ozone than chlorine atoms.
Despite the fact that it is no longer used as a soil fumigant in agriculture, methyl bromide is still utilized as a pesticide for quarantine and pre-shipment of agricultural products. And methyl chloride is employed as a chemical feedstock, despite the fact that the majority of its emissions are thought to come from biomass combustion or natural sources.
However, the overall amount of these methyl halides created each year does not match the yearly addition of these compounds to the atmosphere, a phenomenon that has perplexed scientists for more than two decades.
According to Rhew, around a third of the methyl bromide and methyl chloride in the atmosphere comes from unidentified sources. According to the new findings, copper is a significant, if not the primary, 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.
Copper compounds are allowed on organic crops, according to the first author and former UC Berkeley doctoral student Yi Jiao, who is now a postdoctoral fellow at the University of Copenhagen in Denmark. Copper has been used in farming since the 1700s, including as a major antifungal agent in the Bourdeax mixture, which has been used in France since the 1880s to prevent downy mildew on grapes.
Because of this history, copper pollution of soils is a big problem in Europe today. Copper’s ability to deplete the ozone layer is also a cause for concern, according to the authors.
“Please note that organic agriculture is not a major cause for ozone depletion. However, copper-based fungicides appear to have atmospheric side effects that might be considered in terms of overall environmental impact,” Jiao tweeted this week. “With widespread use of copper in the environment, this potentially growing impact should be considered when predicting future halogen load and ozone recovery.”
Copper + soil + sunlight = methyl halides
A series of research studies done by UC Berkeley undergraduate scholars first established the link between copper and methyl halides. Rhew asked them to investigate the effects of metal ions in soils, beginning with a replication of previously published research on iron in soils.
Rhew then requested them to explore a different metal, copper, in the form of copper sulfate, one of the most common copper compounds used today, after this created modest amounts of methyl halides.
“We replicated the iron experiment and then thought, ‘Let’s look at a different transition metal, like copper, and see if it has a similar effect,’” Rhew said. “When we added copper sulfate to soil, it produced a tremendous amount of methyl halides, and this surprised us. And then another undergraduate did the experiment with seawater, and that produced an impressive amount of methyl halides, as well. So, we knew there was a novel process going on, but we only had a few pieces to the puzzle until Yi conducted a suite of creative experiments to put it all together.”
More in-depth tests were designed by Jiao and Rhew, who took soil samples from the Oxford Tract, an agricultural research plot near the UC Berkeley campus, and subjected them to various treatments, including variable quantities of copper and oxidants.
While copper alone produced some methyl bromide and methyl chloride in soil and seawater, the addition of sunlight and/or hydrogen peroxide produced in soil by microbes or sunlight produced more than five times the amount of methyl halides, extending copper’s activity from a week to two to three weeks.
The amount of methyl halide produced increased even more after Yi sterilized the soil. On the other hand, soil incubated with copper produced no methyl halides after all the organic material was burned away. As a result, he concentrated on the compounds catechol and guaiacol, which contain phenol ring structures similar to those present in organic matter and are frequently employed as proxies for soil organic carbon.
When copper sulfate or hydrogen peroxide were added to catechol-halide solutions in increasing volumes, methyl halide emissions rose as well, whereas emissions were near zero when any of these substrates were missing.
Following that, Yi discovered that sunlight had a similar effect on methyl halide formation as hydrogen peroxide. Exposing copper-added solutions to sunshine boosted emissions fourfold in seawater.
The researchers believe that Cu(II), a common type of copper ion, is oxidizing organic matter to free methyl radicals, which readily react with chlorine and other halogens in soil or seawater to generate methyl halides. Both sunshine and hydrogen peroxide reoxidize the copper, converting it from cuprous (I) to cupric (II), allowing it to function repeatedly to make additional methyl halides.
“We did a back-of-the-envelope calculation to see the impact copper sulfate would have and estimated that it could be responsible for 4.1 gigagrams of methyl bromide per year, which would be about 10% of the missing source,” Rhew said.
“That’s pretty substantial, and that’s only looking at copper sulfate. Maybe even more widely used is another copper compound called copper hydroxide. So, this is just the beginning of our understanding of what copper’s impact is on halocarbon chemistry.”
This also doesn’t account for the possible oceanic emissions connected with copper in runoff, according to Jiao.
According to Rhew, further research is needed to understand which copper compounds are the most potent makers of methyl halides in soil and the ocean, as well as how much is created.
“There’s plenty of halide in soils, and there’s plenty of organic matter in the soil, so the magic ingredient is copper, which is regenerated by sunlight,” he said. “This has opened our eyes to a whole new area of inquiry regarding the role of copper in the environment.”
The work was funded in part by National Science Foundation (EAR-1530375). Co-authors with Rhew and Jiao are former UC Berkeley undergraduates Jae Yun Robin Kim and Julien Vollering, former UC Berkeley postdoctoral researcher Julian Deventer, and visiting scholar Wanying Zhang from the University of Science and Technology of China.
