The growth rate of methane increased by 50% in 2020-2022 compared to the 2010s, and one hypothesis attributes this to reduced NO_x and hydroxyl radical (OH) during COVID lockdowns (1). This emphasizes the role of air oxidation in methane removal and highlights the interactions between greenhouse gases and air pollution. However, such climate-carbon-chemistry interactions are poorly established. Global- mean OH simulations can differ by 80% due to uncertainties in chemical mechanisms (2). Furthermore, even though estimates of the global methane budget consider decreases of OH as one factor of the post-2006 methane increases (1, 3), this does not match methane carbon isotope records (4). The lack of reliable quantification on global-scale OH hinders our ability to evaluate atmospheric methane sink and its sensitivity to the level of air pollution, further limiting predictions on the effectiveness of methane removal through increased atmospheric oxidation capability.

The core problem is advancing the ability to predict atmospheric oxidation driven by OH. Specifically, we will focus on the following questions:

1. To what extent can satellite measurements constrain OH? What observing strategies can reduce remaining uncertainties?
2. What is the magnitude of tropospheric OH? How does it vary seasonally and spatially?
3. Will air pollution influence OH magnitude, and further influence methane’s Lifetime?

Key goals include:

1. Quantify tropospheric OH based on satellite remote sensing and ground/aircraft in-situ measurements of multiple tracers.
2. Build a carbon-climate-chemistry focus model to interpret factors determining OH magnitude and variabilities.
3. Predict the lifetime of methane under scenarios of increased/decreased NO_x, carbon monoxide (CO), and ozone (O₃) pollution.

Success can be measured through derivation of OH magnitude, OH temporal-spatial pattern, and OH sensitivity to air pollution, with rigorous uncertainty estimates.

Tropospheric OH is the primary atmospheric oxidant that shapes the lifetimes of methane (greenhouse gas), carbon monoxide (toxic pollutant), and hydrochlorofluorocarbons (sources of ClX, causing damage to stratospheric ozone). Leveraging advanced satellite and in-situ measurements, we will improve the observing capabilities of OH. The improved estimates of OH magnitude, temporal-spatial patterns, and controllers of OH variabilities, will advance predictions of atmospheric methane sink, which is vital to design strategies to increase methane oxidation. The optimized OH is also broadly useful to scientific communities (such as model evaluation and development) and will be publicly available, supporting predictive modeling for scenarios from sustainable path to intensified fossil fuel use.