HOx Radical Chemistry in a Forested Environment During PROPHET-AMOS 2016: Model Comparisons and Radical Budgets

The hydroxyl (OH), hydroperoxy (HO₂) and organic peroxy (RO₂) radicals play important roles in atmospheric chemistry. In addition to contributing to the formation of ozone and secondary organic aerosols, reactions of these radicals control the lifetime of many trace gases that are important to issues of global climate change. Previous measurements of these radicals in remote forest environments have shown serious discrepancies with modeled concentrations, which brings into question our understanding of the atmospheric chemistry of isoprene and other biogenic VOCs under low NO_x conditions.

In the summer of 2016, OH, HO₂ and HO₂* (HO₂ + αRO₂) radicals and total OH reactivity were measured using the Indiana University Laser-Induced Fluorescence - Fluorescence Assay by Gas Expansion (LIF-FAGE) technique as part of the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport- Atmospheric Measurements of Oxidants in Summer (PROPHET-AMOS). This campaign took place in a forested area in northern Michigan characterized by high mixing ratios of isoprene and low mixing ratios of NO_x. Ambient measurements from this campaign were compared to model predictions using both the Master Chemical Mechanism (MCM) and the Regional Atmospheric Chemistry Mechanism (RACM2), including mechanisms of HO_x radical recycling by isoprene. A radical budget analysis will be presented illustrating the contribution of radical recycling to the overall radical budget. While the models are able to reproduce the observed OH radical concentrations, the models over-predict the measured concentration of peroxy radicals. Several hypotheses are tested in order to explain the observed discrepancies.