Iron salt aerosols (ISA) involves lofting iron-based particles into the atmosphere (e.g., from ships or towers) to enhance atmospheric chlorine radicals, a natural methane sink. This method mimics a natural phenomenon that is currently being studied to characterize its methane oxidizing impact. In contrast to some of the other methods, this approach is catalytic; the iron uses sunlight to convert abundant chloride (e.g. from sea spray) into reactive chlorine. It may prove feasible, but much more scientific study would be needed to understand the full set of impacts, including risks and side effects, before this can be determined.
Several entities have formed to commercialize ISA. Some propose to sell methane credits for ISA deployment. Given the early stage of the research, this is premature. 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.
While more research is needed to fully assess the approach, ISA is potentially climate beneficial and cost-plausible.
The natural occurrence of iron in mineral dusts is believed to generate chlorine radicals via a natural analogue of the ISA mechanism. This effect is believed to be catalytic, with each iron molecule having the potential to produce multiple chlorine radicals before deposition. This catalytic effect is poorly understood and highly dependent on local conditions, yielding anywhere between zero and tens of chlorine radicals per iron molecule.
The potential release of iron would have impacts beyond only breaking down methane. Iron salts themselves have unknown side effects on warming, cloud formation, deposition products, and other Earth system impacts. The produced chlorine radicals would also break down tropospheric ozone, which has mixed effects depending on atmospheric conditions. Decreased tropospheric ozone is beneficial because ozone is a powerful greenhouse gas and an air pollutant. However, photolysis of ozone is the primary source of hydroxyl radicals (OH) which remove methane. This suppression of hydroxyl radicals could paradoxically lead to a longer methane lifetime in certain atmospheric conditions. It is important to note that the efficiency of the process will be limited by factors such as the percentage of photoactive iron in the emitted iron, the reaction of chlorine radicals with atmospheric species other than methane, and the rate at which the iron salt chloride can regenerate its reactive form.The cost of implementing ISA is not yet known. There are many uncertainties regarding ISA’s efficacy and efficiency in producing chlorine radicals, its effect on net oxidative capacity, and overall impact on warming. Until they’re resolved we can’t know how much additional methane ISA could oxidize and how much it might cost per ton of methane broken down or unit of warming avoided. Costs and performance will also vary according to factors such as ISA particle size, deployment strategy, and atmospheric conditions where the reaction is taking place. However, preliminary estimates suggest that it could be cost-plausible given its catalytic nature.
The potential scale for iron salt aerosols would be primarily determined by atmospheric dynamics, the socially acceptable concentrations of these aerosols, raw material constraints associated with their production, and the amount of chlorine available in the atmosphere.
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 many potential impacts of ISA beyond only methane breakdown, analyses of scale and cost need to look at the full climate and environmental effects, beyond only possible atmospheric methane reduction.
Given the very early state of the science behind ISA, the tradeoffs between its potential health and environmental co-benefits and its potential negative impacts are not yet well understood. It’s therefore critical to thoroughly study them before contemplating deployment.
It is not yet known whether ISA might have harmful side effects on the climate or the environment. The ISA photochemistry shifts chlorine between different forms, which could potentially change the acidity of atmospheric aerosols or even areas of the ocean and it could also potentially produce byproducts.
Airborne iron particles may contribute to particulate pollution which may have potential human health impacts on individuals at the deployment site or downstream. The iron would eventually settle out of the atmosphere, landing on water, land, or ice. This may have potentially beneficial or harmful effects. Iron could fertilize bacterial growth on ice, reducing albedo and increasing melting; in the ocean it could fertilize algal growth, potentially sequestering carbon and/or cause algal blooms that could negatively affect marine habitats. It’s also possible that harmful chlorinated compounds in gaseous, liquid, or solid form could be produced.
ISA is currently being studied in laboratory smog chambers and using reaction mechanism models and quantum chemistry models of fundamental reaction steps to characterize how the approach will respond to a variety of atmospheric conditions. Data from these experiments is being combined with historical datasets to build and improve computational models for ISA’s atmospheric chemistry.
Although ISA is the most studied approach in the field of atmospheric oxidation enhancement (AOE), fundamental questions about its safety and efficacy have not yet been answered.
Modeling has demonstrated that additional chlorine radicals produced from ISA may first oxidize available tropospheric ozone, which is a precursor to hydroxyl radicals, the primary sink of methane. Models indicate this would initially result in a net increase in the lifetime of methane. But above a condition-specific threshold concentration of chlorine radicals, this lifetime trend appears to reverse, thereby enhancing the net methane sink. The threshold and conditions where ISA begins to reduce methane’s lifetime must be better characterized.
ISA research could be accelerated with improvements in modeling and observation technology. Measuring how much methane is oxidized by chlorine in a natural environment is a labor-intensive, logistically complicated, and expensive process. It’s currently done by taking samples on board ships, and sending flasks to a central laboratory for analysis using expensive, specialized lab equipment. We need a more scalable approach to consistently monitor methane, carbon isotopes, nitrogen oxides, ozone, hydroxyl and chlorine radicals, and any other relevant proxy gasses. That would help accelerate research, improve characterization, and narrow uncertainty around positive and negative impacts.
Key questions that still need answering include: