Speaker: Glen Chua (Princeton Univ.)
Topic: Drivers of the hydroxyl radical (OH) and the role of OH in determining the climate and atmospheric composition effects of hydrogen leakage under different future methane trajectories

The hydroxyl radical (OH) is an atmospheric "detergent", removing air pollutants and greenhouse gases like methane from the atmosphere. Thus, understanding how it is changing and responding to its various drivers is important for air quality and climate. In the first part of this talk, I will be presenting results on the historical drivers of OH in the Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric chemistry-climate model, the Atmospheric Model version 4.1 (AM4.1), using which we do"all-but-one" simulations to isolate the impacts of various near-term climate forcers (NTCFs) known to affect OH. We find that modeled tropospheric airmass-weighted mean OH has increased by ~5% globally. This global trend is largely due to the combined effects of NOx emissions and CH4 concentrations. CO emissions and meteorology were also important in driving regional trends.
 
In the next part of this talk, I will be looking at another potential future driver of OH in the form of increased hydrogen (H2) emissions. The H2 economy is growing both in the US and globally, as governments all over the world are hoping that H2 will play an important role in the energy transition. However, when H2 leaks from the supply chain, it has a surface warming effect because, by reaction with OH, it enhances the levels of other greenhouse gases like CH4, ozone (O3) and stratospheric water vapor (H2O). The impacts of H2 on atmospheric composition and thereby climate are similar to that of CH4, in that the latter also reacts with OH and also enhances the same greenhouse gases. Using the GFDL AM4.1 driven by both H2 and CH4 emissions, we show that CH4 mitigation is an effective way to counteract both the atmospheric composition and climate impacts of H2 leakage. Furthermore, after also considering the co-reduction of other NTCF emissions due to the displacement of fossil fuels from the adoption of the H2 economy, our findings further emphasise the importance of CH4 mitigation to counteract near-term warming from both H2 effects and aerosol co-reductions.