What methane depletion rate is achievable by ISA?
We estimate increasing the global atmospheric methane depletion rate by around a factor of four. This is based on an emission rate of up to 150,000 tonnes Fe/yr and an empirical experiment carried out in a 3.5 m3 climate chamber. It is a first order estimate, and may in practice be much higher.
How can you claim such a high depletion rate of methane by ISA?
See below how (blue diagram) the addition of HCl increases depletion rate.
The brown diagram (without additional HCl) shows how, within a certain period, only one methane molecule may be depleted to CO2 because the ISA particle is waiting for a °Cl atom to react with methane, and for the resulting HCl to find its way back to complete that ISA particle.
The blue diagram shows how additional HCl can enable one ISA particle to oxidise far more methane molecules within the same time period. In this way °Cl acts like a kind of ‘oxidiser conveyor belt’, returning to ISA particles as HCl. The HCl can come from a variety of sources, such as sea spray coagulation with sulphuric acid aerosols, or the depletion of halo-methanes by oxidation, or supplied as a precursor in the dispersion plume.
When ISA particles nucleate clouds they become dissolved in water droplets. Do they continue removing warming agents at the same rate?
No, at a lower rate. When ISA becomes dissolved in cloud droplets it is still photolysable, but the pH increases from the optimum acidity. However, the process is reversible. As the relative humidity falls and the cloud dissolves back into the atmosphere, ISA concentration in the droplet once again increases, restoring the optimum low pH and thus maximum warming agent depletion rate again.
N.B. the bottom of clouds are a little darker than the top, so ISA is most active at the top of clouds.
In 1.3 your claim that DMS and its derivatives “activate ISA to produce further cloud condensation nuclei (CCN)” was a surprise. Is it not the other way round?
It is true that ISA enhances phytoplankton growth once it settles in the ocean, and phytoplankton excrete dimethyl-sulphide (DMS). However, DMS and similarly excreted halo-methanes activate ISA to produce cloud-whitening CCN aerosol, as shown in the image below. DMS also becomes oxidized within the atmosphere, generating sulfuric acid aerosol that combines with sea salt particles, producing additional CCN independent of ISA.
(Unrelated – in addition, when the acid aerosols and acid gases in the atmosphere fall onto land they activate weathering of silicate minerals, increasing alkalinity transport to the oceans.)