Photocatalytic aerosols as an atmospheric methane removal approach would involve dispersing photocatalysts (likely titanium dioxide or zinc oxide compounds) in the troposphere to oxidize methane. Laboratory research is underway to improve the performance of specific photocatalysts, though there isn’t yet any peer-reviewed literature on photocatalytic aerosols. Research into the potential side effects has not yet begun but will be essential to address the substantial environmental concerns of this approach.

For photocatalytic aerosols to be a feasible approach there would have to be significant increases in catalytic efficiency as well as resolution on potential environmental concerns. Research is underway to improve the catalytic efficiency, while atmospheric modeling of the side effects has yet to begin.

Given the lack of research to date, the feasibility of photocatalytic aerosols for methane removal has not been established. 

Apparent Quantum Yield (AQY), the ratio of incident photons to oxidized methane molecules, is a key metric for determining the costs and climate impacts of photocatalysis. It helps to determine the amount of photocatalytic aerosol required to oxidize a certain amount of methane.

Forthcoming research estimates that photocatalytic aerosols are not yet cost-plausible for atmospheric methane removal. Reaching the cost-effectiveness threshold with 1 micrometer aerosols would require a ~30x increase AQY to at least 1% from its current state-of-the-art of only 0.03% at 2 ppm methane. However, the cost would be lowered if smaller aerosols were used. Smaller aerosols have lower required AQY due to their higher ratio of surface area to volume, though they may have additional negative health effects. Understanding the tradeoffs between these positive and negative impacts requires more research.

Additional work is needed to understand the full atmospheric impacts these aerosols would have during their atmospheric residence times, and the resulting total climate impact. In addition to methane, photocatalysts also oxidize volatile organic compounds (VOCs) and tropospheric ozone, pollutants with negative human health and environmental impacts when at or near ground level. At the same time, their products include other potentially harmful volatile organic compounds. 

These effects, alongside any albedo increases, may be location-specific and dependent on local atmospheric conditions. The mechanism of deployment, such as dispersal from airplanes, may also contribute to process emissions that must be accounted for.

The potential scale for photocatalytic aerosols is primarily determined by the socially acceptable concentrations of these aerosols and the raw material constraints imposed by their production.

Titanium dioxide, the most Earth-abundant photocatalyst, is currently produced at commodity scale as a paint pigment, with a global production volume of 8.4 million metric tons  in 2021. Production of zinc oxide, another promising photocatalyst, is about an order of magnitude smaller. Forthcoming research suggests that the mass of aerosol will be roughly the same order of magnitude as the oxidized methane. Therefore, reaching a benchmark annual scale of 10 million metric tons of methane (830 Mt CO₂e using GWP20) may require a doubling of the existing global production of photocatalysts.

If a feasible approach was found (through increased AQY) and appropriate oversight and transparency were in place, the path to scaling photocatalytic aerosols 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 photocatalyst itself, and any limitations associated with the deployment modality (e.g., appropriately outfitted ships and/or airplanes).

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 are not yet well understood. It is critical to study them before considering any future testing or deployment. There are major environmental concerns around producing VOCs and nanoparticles that would need to be resolved before any deployment.

Photocatalysts oxidize VOCs and ozone, pollutants with human negative health and environmental impacts near ground level. Photocatalysis may also produce VOCs and nanoparticles with detrimental effects. The aerosols themselves are particulate matter that, if inhaled, could irritate human lungs. PM2.5 or PM10 air quality metrics should guide acceptable particle size and deployment locations of these aerosols. It’s also important to assess the post-deposition environmental impacts of these particles. More research is needed to better understand the tradeoffs between the potential positive and negative impacts of photocatalytic aerosols.

Key areas that need to be studied include:

• Possible human and environmental exposures from airborne photocatalyst aerosols from any given set of possible deployments, as well as any potential precipitation risks and impacts
• Full atmospheric impacts of releasing photocatalyst aerosols, and any resulting health and environmental co-benefits and concerns.