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.
There is currently no published literature showing that hydrogen peroxide dispersal is a feasible atmospheric methane removal approach. Independent scientific research is needed to address many major unanswered questions before determining feasibility and assessing whether safe deployment scenarios exist.
The cost and climate impacts will depend on the reaction rates and conversion efficiencies 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.
Some of the efficiency losses may result from hydroxyl radicals reacting with other pollutants, including sulfur dioxide (SO₂), carbon monoxide (CO), ozone (O3), and non-methane volatile organic compounds. 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 1.2 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, 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 lifecycle carbon costs, capital costs, or other operating costs.
This estimate above also does not take into account the many efficiency losses (see Figure above), which are not well understood. There is 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 the cost per ton of methane oxidized. The latter consideration, alongside the likely efficiency losses including deposition, make hydrogen peroxide appear cost-implausible.
The highly complex and non-linear atmospheric chemistry that drives the hydroxyl radical budget has many uncertainties, including how it may respond to perturbations and regeneration of hydroxyl radicals under different conditions. While the regeneration of hydroxyl radicals could lower the cost of this approach, further scientific studies must be conducted before including this effect in a feasibility evaluation.
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.
The potential scale for hydrogen peroxide aerosols is primarily determined by the socially acceptable concentrations of H2O2 and the raw material constraints imposed by producing vast quantities of H2O2. 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 annual 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 (outlined above) may increase this number, potentially dramatically.
If a feasible approach was found and appropriate oversight and transparency were in place, the path to scaling hydrogen peroxide could be rapid. It would be limited by the speed at which adverse and unexpected effects could be accurately assessed after a given scale of deployment, material availability for the H2O2 itself, and any limitations associated with the deployment modality (e.g., appropriately outfitted ships and/or airplanes). 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 would be based on cleaner methods.
We estimate that scaling to 10 million metric tons of methane (830 Mt CO2e using GWP20), a benchmark for scale, could occur within a decade after an initial hypothetical first successful methane megaton-scale deployment.
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 beneficial side effects. Besides methane, they may also oxidize VOCs and ozone. This would decrease the overall 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 warn that hydrogen peroxide may cause mutations. Modeling possible exposure levels from dispersing hydrogen peroxide is needed to understand the potential risks to human health and natural systems. 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.
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.
Since there are as yet no publications examining the feasibility, scalability, and safety of the approach, many major open questions remain about hydrogen peroxide dispersal.
Any approach to oxidizing additional methane in the atmosphere that involves altering atmospheric chemistry needs to be well understood prior to any deployment. Independent scientific analysis and social acceptance are prerequisites.
Key questions are currently unanswered for this approach, including: