Tropospheric OH and its trends: new insight into atmospheric processes and implications for methane lifetime

The hydroxyl radical (OH) is a critical determinant of global oxidative capacity and trace gas lifetimes, but its variation remains poorly constrained. This study investigates the sensitivity of modelled tropospheric OH concentration changes to new understanding of physical and chemical processes using the FRSGC/UCI chemistry transport model. We show that simulated tropospheric O 3 and NO 2 agree well with satellite observations, but that annual variations in CO do not, likely due to uncertainties in biomass burning emissions in the southern hemisphere and overestimated CO trends over Asia. This discrepancy, along with the background increase in CO, may lead to an underestimation of OH increase or overestimation of OH decrease from 2000 to 2017. Changes in tropospheric OH column show substantial spatial heterogeneity, with increases in OH in high-emission regions and decreases in the tropics. Global mean OH trends are dependent on the assumed trend in emissions: with time-varying emissions there is little change in OH, while under annually invariant emissions, OH increases in response to changes in meteorological conditions. Inclusion of water vapor UV absorption, heterogeneous reactions, and updates to the OH+NO 2 reaction rate have a smaller impact, but decrease OH levels by 3.6 %, 5.8 %, and 7.0 %, respectively, increasing CH 4 lifetimes by 4.2 %, 5.2 %, and 8.4 %. Incorporating oceanic CH 3 CHO emissions reduces global mean OH by up to 1.5 %, increasing the CH 4 lifetime by up to 1.6 %. These results provide a quantitative basis for understanding the drivers of tropospheric OH variability and their implications for global methane chemistry.