One recent proposal seeks to infuse the atmosphere with hydrogen peroxide, insisting that it would both oxidize methane (CH4), an extremely potent greenhouse gas while improving air quality.
"Our work showed that the efficiency of the proposed technology was quite low, meaning widespread adoption of the technology would be required to make any meaningful impact on atmospheric CH4," said Mayhew, a postdoctoral researcher with the U's Wilkes Center for Climate Science and Policy. "Then, our results indicate that if this technology is adopted at scale, then we start to see some negative air-quality side effects, particularly for wintertime particulate matter air pollution."
To conduct the study, the Utah scientists modeled what would happen if you deployed the technology patented by a Canadian company, which is proposing to spray aerosolized hydrogen peroxide, or H2O2, into the atmosphere during daylight hours from 600-meter towers. These towers would approach the height of the world's tallest radio towers.
"When that hydrogen peroxide is in the presence of sunlight, it's going to make a really powerful oxidant, the hydroxyl radical OH," said Haskins, an assistant professor of atmospheric sciences. "That's a natural scrubber in the atmosphere, and it's going to help speed up the conversion of methane to CO2."
Methane is a single-bonded molecule combination of carbon and hydrogen, as opposed to the double-bonded compounds that are far more common in the atmosphere. Hydroxyls are more likely to oxidize those double-bonded molecules, such as the isoprene coming off trees or volatile organic compounds, so OH is just not that efficient for breaking down methane, according to Haskins.
"OH doesn't react fast with methane," Haskins said. "It's reacting with so many other things."
The primary ingredient in the natural gas burned in home appliances and power plants, methane, or CH4, packs 76 times more climate-warming punch than carbon dioxide over a 20-year timeframe. Methane persists in the atmosphere for only 12 years, but the gas is blamed for nearly a third of the rise in global temperatures since the Industrial Revolution, according to the International Energy Agency.
Anthropogenic sources, primarily oil, gas and coal operations and landfills, account for 60% of global methane emissions.
Artificially speeding up methane oxidation could slow climate change, but such geoengineering projects could carry adverse environmental impacts, which Haskins's lab seeks to characterize. A recent report from the National Academy of Sciences concluded the unintended consequences of atmospheric methane removal technologies are likely significant but poorly understood. Haskins' study is heeding the report's call to scrutinize these technologies, such as the one that would release vast amounts of hydrogen peroxide.
"We could buy ourselves about 50 years and avoid some of the immediate impacts of climate change if we did this, but no one had actually previously done any side-effects studies to see what was going to happen," Haskins said. "This is very first paper to assess any air quality side effects of such geoengineering solutions."
"There's so many feedbacks that can go on in the climate. Atmospheric chemistry is just one example. You change one thing and you think it's going to do this, but it actually may do the opposite in one place versus the other," Haskins said. "You have to be really careful and do these sorts of assessments. Is this a responsible thing to do? What's the impact going to be?"
By way of example, Haskins raised the troubling history of manmade gasses called chlorofluorocarbons, or CFCs, which ate into the protective layer of ozone that shields Earth from harmful ultraviolet radiation.
"We started using CFCs in industry as propellants and refrigerants, and suddenly we cause the ozone hole," she said. "And we've been dealing with the consequences of that for 40 years. And we still won't have a fully resolved no-ozone-hole year until probably 2060, so we have to be careful of what we're doing."
Mayhew and Haskins used a global chemical-transport model, called GEOS-Chem, to simulate the proposal to release hydrogen peroxide from towers. The goal was to estimate how much methane would be oxidized under three different emission scenarios, from light to extreme.
Their simulation envisioned the use of 50 towers spread around North America. Replicating the company's proposal, the medium-release scenario called for each tower to spray 612 grams, or 1.35 pounds, per second for 10 hours a day for a year.
"This proposed solution just won't remove any meaningful amount of methane from the atmosphere. It's not going to solve global warming. At most, we found 50 towers could reduce 0.01% of annual anthropogenic methane emissions," Haskins said. "You'd need about 352,000 of them to remove 50% of anthropogenic methane. It's an insane number. And if you did 50 high-emission towers, you'd still need about 43,000."
In the meantime, places with poor wintertime air quality could see particulate pollution get much worse.
"There's potential that future research could show that the air quality impacts of placing these towers close to methane point sources is minimal if they're activated at certain times of the year, and far from large population centers," Mayhew said. "If that's the case, then this technology (or similar approaches) could play a very small role in combatting warming, but it's clear from our work that the air-quality side effects should be placed as a central consideration for any proposed real-world implementation of technology like this."
Research Report:Potential Air Quality Side-Effects of Emitting H2O2 to Enhance Methane Oxidation as a Climate Solution,
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