Dispersing hydrogen peroxide (H2O2) aerosols into the air has been proposed as a way of producing more hydroxyl radicals (OH), the largest natural atmospheric methane sink. This approach aims to increase the rate of methane oxidation by generating hydroxyl radicals via solar photolysis of hydrogen peroxide or the Fenton reaction. Little is known about the efficacy or safety of the idea as there has yet to be any scientific research on this method and it remains theoretical.
A commercial entity has proposed to implement hydrogen peroxide dispersal, despite a lack of independent scientific evaluation or technical content published by the entity.
There is currently no published evidence hydrogen peroxide dispersal is a feasible atmospheric methane removal approach. Independent scientific research would be needed to address many major unanswered questions before determining feasibility and assessing whether safe deployment scenarios exist.
Its cost and climate impacts will depend on the reaction rates and efficiencies in going from hydrogen peroxide to hydroxyl radicals to methane oxidation. Given the lack of research, little is known about the approach, but there are many potential sources of efficiency loss between hydrogen peroxide dispersion and methane oxidation that need to be characterized. The likely inefficiencies lead to material costs driven by commodity hydrogen peroxide prices that may make the approach cost-implausible.
Some of the efficiency loss may result from hydroxyl radicals reacting with other pollutants, including sulfur dioxide (SO₂), carbon monoxide (CO), ozone (O3), and non-methane volatile organic compounds (NMVOC). The environmental and human health co-benefits should be valued, while possible negative health effects (see section below) should also be weighed.
Since local atmospheric conditions affect efficiency, future estimated costs will always be location- and condition-specific. Any future cost estimates would also need to take into consideration all the greenhouse gas emissions across the lifecycle of this approach, from production of hydrogen peroxide through dispersal. Current hydrogen peroxide production methods have high carbon dioxide emissions, with an estimated 1.19 tons of CO₂e emissions (using GWP100) per ton of hydrogen peroxide, 50% in H₂O, produced.
Even under implausibly optimistic assumptions of zero efficiency loss (perfect photolytic conversion from hydrogen peroxide to hydroxyl radicals), and zero lifecycle carbon emissions from the process itself, this approach does not appear to be cost-effective. One ton of hydrogen peroxide costs as little as ~$375. If a ton of hydrogen peroxide was photolyzed into two hydroxyl radicals per molecule of hydrogen peroxide (as the entity proposing this method suggests they’re targeting), and then ~15% of hydroxyl radicals oxidize a methane, this would result in 0.14 tons of methane oxidized, or approximately a $2,700 cost per removed ton; while this is in the cost-plausible range, it does not include efficiency losses, lifecycle carbon costs, capital costs, or other operating costs.
This estimate also does not take into account the many efficiency losses (see Figure above), which are not well understood. There is also variability in the percentage of hydroxyl radicals that react with methane, likely influenced by atmospheric conditions and altitude, with observations ranging from 10% to 25%. Furthermore, the estimate assumes that hydrogen peroxide is in the gas phase; aerosolized hydrogen peroxide in aqueous form would have a 1:2 rather than 2:1 conversion to hydroxyl radicals, leading to a 4x increase in cost per ton of methane oxidized. The latter consideration, alongside the likely efficiency losses including deposition, make hydrogen peroxide appear cost-implausible.
The potential for atmospheric regeneration of hydroxyl radicals is not well understood; until further scientific studies are conducted, feasibility is evaluated without inclusion of hydroxyl radical regeneration.
Many important questions about hydrogen peroxide dispersal have not been addressed. Scientific transparency and independent review are required for the community to evaluate this method effectively. This effort is currently hindered by the lack of independent scientific research and peer-reviewed literature. Until the underlying basic science is available, and efficacy, safety, and community support are established, any deployment or selling of credits is premature.
Scaling this method of methane removal would require a massive increase in global production of hydrogen peroxide. Assuming the implausibly optimistic 1:0.14 ratio of hydrogen peroxide to methane removal (see explanation in Feasibility section), removing 10 million metric tons of methane (830 Mt CO2e using GWP20), one benchmark for scale, would require the dispersal of 71 million tons per year of hydrogen peroxide. This is more than fourteen times the current global hydrogen peroxide production of 5 million tons per year. Atmospheric process inefficiencies will increase this number, probably dramatically.
If the major unanswered questions are addressed and the approach is determined to be viable, the timing of scaling would likely be largely dependent on hydrogen peroxide manufacturing capacity growth. Today, hydrogen peroxide is produced in large industrial facilities, which depend on fossil fuel inputs. New, more modular, and less polluting production methods are being researched, but haven’t yet been scaled. Ideally any potential future growth of hydrogen peroxide production capacity would be based on cleaner methods. However, there is currently no evidence of feasibility.
Given the very early state of understanding this potential pathway, health and environmental co-benefits and concerns of hydrogen peroxide dispersal are not yet well understood. It would be critical to study them further before considering any future field testing or deployment.
The hydroxyl radicals this method aims to produce may have co-beneficial effects. Besides methane, they may also oxidize VOCs and ozone, which increase the overall negative radiative forcing while also reducing pollutants with negative human health and environmental impacts near ground level.
However, inhaling hydrogen peroxide at certain concentrations is known to irritate the nose, throat, and lungs. OSHA regulations limit average work shift airborne exposure to 1 ppm and warns that hydrogen peroxide may cause mutations. Modeling possible exposure levels from dispersing hydrogen peroxide is needed to understand its potential risks to human health and natural systems.
In the aqueous phase, hydrogen peroxide reacts rapidly with sulfur dioxide to produce sulfuric acid, generating acid rain (Pandis 1989, Seinfeld 2016).
Potential impacts on stratospheric ozone are not understood and could be influenced by the deployment altitude of hydrogen peroxide dispersal.
To assess overall safety of hydrogen peroxide dispersal, scientists need to understand what level of exposure humans and ecosystems would be subject to in any deployment scenario and what positive or negative health or environmental impacts may result from resultant changes in air quality.