Biogenic Emissions Modulate the Tropospheric Hydroxyl Radical (OH) Response to Climate Warming

The hydroxyl radical (OH) sets the oxidative capacity of the atmosphere and determines the lifetime of reactive greenhouse gases such as methane (CH4). The response of OH to climate warming is influenced by uncertain and compensating processes involving weather-sensitive chemistry and emissions. In this study, we extend the idealized aquaplanet configuration of the Community Earth System Model (CESM) Community Atmosphere Model version 6 (CAM6) to include atmospheric chemistry (“AquaChem”). Beyond the aquaplanet’s zonally symmetric sea surface temperatures (SSTs) and lack of seasonality, we further simplify the spatial variability of trace gas and aerosol emissions. We show that the AquaChem configuration generates a robust OH chemical budget, including both production and loss pathways, with relatively short simulations. Thus, the AquaChem model serves as an effective tool for rapidly assessing the sensitivity of OH chemistry, including both production and loss pathways. Rapid convergence allows us to assess the sensitivity of OH chemistry to surface warming. The strongest direct response in OH to increased surface temperatures is an increase in “primary” OH production due to higher water vapor concentrations. We then test the sensitivity of OH sources and sinks to different assumptions regarding the response of emissions to rising temperatures. AquaChem simulations indicate that biogenic emissions are a dominant factor influencing the OH response to climate warming. In tropical regions, climate warming enhances biogenic emissions and increases the OH loss rate, outweighing the increase in OH production resulting from rising water vapor and resulting in a decrease in OH abundance.