Copper-based chemicals may be contributing to ozone depletion
Some ozone-destroying chemicals are unaccounted for. Are copper-based fungicides producing them?
Date:
January 13, 2022
Source:
University of California - Berkeley
Summary:
As Earth's ozone layer recovers from past emissions of now-banned
CFCs and halons, other chemicals are emerging as major causes
of stratospheric ozone depletion. Atmospheric scientists have
been searching for the sources of about one-third of the major
threats, methyl bromide and methyl chloride. New research shows
that copper-based compounds in common use generate these compounds
when interacting with soil and seawater, with sunlight boosting
production by a factor of 10.
FULL STORY ========================================================================== Copper released into the environment from fungicides, brake pads,
antifouling paints on boats and other sources may be contributing
significantly to stratospheric ozone depletion, according to a new study
from the University of California, Berkeley.
==========================================================================
In a paper appearing this week in the journal Nature Communications,
UC Berkeley 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.
"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." 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 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 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. Even some replacements for banned refrigerants are
coming under scrutiny.
========================================================================== 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 Berkeley 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. The ozone-depleting power of copper
is another cause for concern, the authors said.
========================================================================== "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 The connection between copper and methyl halides was first revealed through a series of research projects conducted by UC Berkeley undergraduate researchers.
Rhew asked them to test the impact of metal ions, starting by replicating previously published work on iron in soils. When this produced small
amounts of methyl halides, Rhew then asked them to investigate a different metal -- copper -- in the form of copper sulfate, one of the most common
copper compounds used today.
"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." Jiao and Rhew designed
more thorough experiments, obtaining soil samples from an agricultural
research plot called the Oxford Tract located near the UC Berkeley campus
and subjecting them to various treatments, including different amounts
of copper and oxidants. While copper alone in soil and seawater produced
some methyl bromide and methyl chloride, the addition of sunlight and/ or hydrogen peroxide -- which is produced in soil by microbes or sunlight - - generated more than five times the amount of methyl halides and prolonged
the activity of copper from about a week to between two and three weeks.
When Yi sterilized the soil, the amount of methyl halide production rose
even more. On the other hand, after burning off all the organic material,
soil incubated with copper produced no methyl halides. That led him to
focus on chemicals -- catechol and guaiacol -- often used as proxies for
soil organic carbon because they each contain a phenol ring structure,
like those found in organic matter.
Adding increasing amounts of either copper sulfate or hydrogen peroxide to catechol-halide solutions increased emissions of methyl halides, as well, whereas emissions were near zero when any of these substrates was missing.
Subsequently, Yi found that sunlight served a similar function as hydrogen peroxide in boosting methyl halide production. In seawater, exposing
copper- amended solutions to sunlight increased emissions fourfold.
The researchers suspect that one common form of copper ion, Cu(II), is oxidizing organic material to liberate methyl radicals, which readily
combine with chlorine and other halogens in the soil or seawater to
form methyl halides. Both sunlight and hydrogen peroxide subsequently
reoxidize the copper -- from its cuprous (I) to cupric (II) state --
so that it can act again and again to generate more 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." Jiao noted that this also doesn't
take into account the potential oceanic emissions associated with copper
in runoff.
Rhew said that much more research needs to be done to determine which
copper compounds are the most potent producers of methyl halides in soil
and seawater and how much is actually produced.
"There's plenty of halide in soils, and there's plenty of organic matter
in 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.
========================================================================== Story Source: Materials provided by
University_of_California_-_Berkeley. Original written by Robert
Sanders. Note: Content may be edited for style and length.
========================================================================== Journal Reference:
1. Yi Jiao, Wanying Zhang, Jae Yun Robin Kim, Malte Julian Deventer,
Julien
Vollering, Robert C. Rhew. Application of copper(II)-based chemicals
induces CH3Br and CH3Cl emissions from soil and seawater. Nature
Communications, 2022; 13 (1) DOI: 10.1038/s41467-021-27779-3 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2022/01/220113151441.htm
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