Photocatalytic aerosols is a proposed approach to aerosolize photocatalysts such as titanium dioxide or zinc oxide compounds and disperse them in the specific atmospheric conditions, in the troposphere, to oxidize methane. Laboratory research is underway to improve the performance of specific photocatalysts, though there isn’t yet known active research into photocatalytic aerosols. Research into the potential side effects has not yet begun, but will be essential to address the substantial environmental concerns of this method.
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. Researchers have ideas that they believe could make significant progress on catalytic efficiency.
Given the lack of research to date, the potential cost of photocatalyst aerosols per ton of methane removed has not been established. Forthcoming research estimates that this approach is already cost-plausible. However, reaching the cost-effectiveness threshold with 1 micrometer aerosols would require a ~30x increase in catalytic efficiency, as measured by apparent quantum yield (AQY), to at least 1%, while current state-of-the-art AQY is 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 expected 1-20 day 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 still-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.
Any considerations around scaling this approach would first require proving basic feasibility, as noted above.
If proven feasible, photocatalytic aerosol scalability would depend on cost, raw material supply constraints, and deployment location restrictions. 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 10 million metric tons per year of oxidized methane would require at least doubling 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). Thus one could estimate that scaling to 10 million metric tons per year after an initial hypothetical first successful small-scale deployment may take between 5 and 10 years.
Given the very early state of understanding this potential pathway, health and environmental co-benefits and concerns are not yet well understood. It would be 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 these potential positive and negative impacts.
Key areas that need to be studied include: