Methane is destroyed naturally in the atmosphere, limiting its lifetime to about a decade. For the most part this is due to reactions with hydroxyl (OH) radicals, and a few percent of methane is oxidized by reaction with chlorine atoms (Cl). The formation of OH radicals is therefore of great importance to methane oxidation in the atmosphere. OH radicals are mainly produced from the photolysis of ozone (O₃), and O₃ is mainly produced from the photolysis of NO₂. However, these are self-limiting processes, for example because NO_x not only forms O₃, but it also reacts with O₃ to destroy it. This is why atmospheric chemistry is not linear, and it means that the impact of changes to the atmosphere can be counterintuitive [1].

For example, it was found that the release of Cl₂ into the atmosphere by iron-salt aerosols does not always lead to a reduction in methane, and can even increase methane levels in the atmosphere [2,3,4]. The reason for this is that Cl₂ emissions can reduce NO_x and O₃, which leads to lower OH concentrations, resulting in less methane oxidation by OH, while the extra methane oxidation by Cl is sometimes not enough to compensate for this [2,4].

To evaluate when atmospheric methane removal approaches are viable, it is important to gain a deeper understanding of the chemical mechanisms driving these effects. Through modeling, we have shown that for iron-salt aerosol emissions, methane is increased if the iron-salt aerosols are within a certain range of NO_x background concentrations at low emission levels [4]. Methane removal by iron-salt aerosols can be achieved when the NO_x background is very low or high, or if the iron-salt aerosols are at high concentrations [4].

It has been suggested that atmospheric methane removal can be increased by accelerating the OH production from ozone [5], by adding extra ozone to the atmosphere [5], or by adding extra H₂O₂ to the atmosphere [6].

The core question to be addressed is: Which alternative (non-ISA) methane removal mechanisms can be imagined, and how are these impacted by non-linear atmospheric chemistry, especially with regard to NO_x background concentrations? This can be evaluated by determining the methane removal per reactive molecule that is added to the atmosphere.

When successful, this will lead to the identification of methane removal approaches that are worth further study by quantifying viability and key uncertainties. It will also identify the conditions critical to viability for each approach, and this can be used to make an estimation of the scalability, cost-efficiency and climate benefit of each approach.