Atmospheric Oxidation in a Southeastern US Forest: Sensitivity of Differences Between Modeled and Measured Hydroxyl (OH) to Model Mechanism and Inputs

Forests play a critical role in the atmosphere's oxidation chemistry because of their broad global extent and their prodigious emissions of biogenic volatile organic compounds (BVOCs). The high hydroxyl (OH) reactivity of these BVOCs causes much of the initial chemistry to occur near the forest. Some OH measurements in forests are much greater than calculated with models, leading to close examination of the BVOC oxidation mechanisms and the possibility of significant OH recycling. The 2013 Southern Oxidant and Aerosol Study (SOAS) provides a rigorous test of the BVOC oxidation mechanisms and OH recycling with its extensive measurement suite that was positioned in an Alabama forest for six weeks. OH measurements made with the Ground-based Tropospheric Hydrogen Oxides Sensor (GTHOS) are compared to photochemical box models constrained with other simultaneous measurements in order to test the understanding of this forest photochemistry. In this work, we use a global sensitivity analysis (Random Sampling - High Dimensional Model Representation) to examine the sensitivity of the differences between the modeled and measured OH to the model mechanism and inputs. In this presentation, we will discuss the model reactions and inputs that have the most influence on the modeled OH and its difference with measured OH and will provide recommendations for reducing model and measurement uncertainty.