Preliminary work suggests atmospheric methane oxidation enhancement would increase particulate matter pollution, one of the leading causes of global premature mortality [1], regardless of the method employed [2] (Fig. 1). For OH-based methods, ozone air pollution is increased in populated areas [2]. Past work was performed at low spatial resolution (~200-500 km) and focused on annual mean effects [2,3]. The chemistry of air quality and atmospheric oxidation is highly non- linear. The predicted effectiveness of methane oxidation enhancement and its air quality impacts may depend on model resolution. For example, chlorine-based methods can increase methane for certain concentrations of nitrogen oxides (NO_x) [4], which, depending on season and region, can increase or decrease with higher model resolution [5]. A new method to enhance methane oxidation, aerosolized photocatalysts, has recently been shown to be potentially viable [6], but its effects on air quality have yet to be explored.

Figure 1. Simulated change in annual mean PM2.5 (particulate matter <2.5 μm diameter) after implementing global atmospheric methane oxidation enhancement via H₂O₂ release. From ref. [2].

The core problem is we do not yet understand which approaches to enhance atmospheric methane oxidation will 1) be effective at reducing radiative forcing and 2) limit harm to human or ecosystem health via impacts on air quality and stratospheric ozone. Here we will investigate OH enhancement (e.g., via H₂O₂), Cl enhancement (e.g., via iron salt aerosol), and aerosolized photocatalysts.

For each proposed approach, key goals are: 1) quantify the effectiveness in decreasing methane for a range of deployment scenarios, 2) assess air quality impacts at high spatiotemporal resolution, 3) quantify impacts on stratospheric ozone, 4) inform the timing and location of deployments that maximize effectiveness and minimize air quality impacts.

Metrics to assess success will be simulated changes in methane lifetime, air pollutants, tropospheric oxidants, and ozone-depleting substances for each scenario. Key uncertainties that may impact the results will be investigated through sensitivity tests.

If successful, this work will improve understanding of potential approaches to increase atmospheric methane oxidation by:

• Informing which methods are most likely to successfully reduce methane concentrations and least likely to negatively affect air quality, other climate forcers, and/or stratospheric ozone
• Identifying location (regions, altitude) and timing (e.g., seasonal and diurnal) where deployment of a given approach leads to greatest effectiveness and least harm to air quality
• Identifying key species impacted by atmospheric methane enhancement, which will inform experimental and field testing and future monitoring
• Quantifying uncertainties in predictions due to model resolution