Iron Salt Aerosols (ISA) is an approach to atmospheric methane removal that 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.
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, 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 10’s of chlorine radicals per iron.
Chlorine radicals will also remove tropospheric ozone, which is beneficial because ozone is a powerful greenhouse gas and an air pollutant. However, this effect could also be detrimental because ozone is the source of hydroxyl radicals (OH) which remove methane. This suppression of hydroxyl radicals could paradoxically lead to a longer methane lifetime. It is important to note that the efficiency of the process will be limited by factors such as the reaction of chlorine radicals with hydrocarbons other than methane (NMVOC), and by the rate at which the iron salt chloride can regenerate its reactive form. Iron salts themselves have unknown side effects on warming, cloud formation, and deposition products.
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 until they’re resolved we can’t know how much additional methane ISA could oxidize, and therefore what it might cost per ton of methane removed. Costs and performance will also vary according to ISA particle size, deployment modalities, and atmospheric conditions where the reaction is taking place. However, preliminary estimates suggest that it could be cost-plausible given its catalytic nature.
While more research is needed on its safety, efficacy, and full ecosystem impacts, ISA holds promise as a potentially scalable approach to removing > 10 teragrams of methane per year. To achieve globally relevant scale, the proposed ISA mechanism must meet several key conditions: it must prove safe and effective, there must be a sufficient amount of chlorine available in the atmosphere, suitable atmospheric conditions must be found, and potential material limitations (including the iron itself) must not be restrictive. If these conditions are met, ISA appears to be scalable within a decade.
Given the very early state of the science behind ISA, its potential health and environmental co-benefits vs. its potential negative impacts are not yet well understood. It’s therefore critical to study them and establish better understanding before contemplating field testing or 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 pH balance of atmospheric aerosols or even the ocean, and it could potentially produce byproducts.
Airborne iron particles may contribute to particulate pollution, with potential human health impacts for individuals at the deployment site or downstream. The iron will 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.