MEASUREMENT OF PEROXY RADICALS ABOVE THE FOREST FLOORS: UNRAVELING THE DISCREPANCIES IN MODEL PREDICTIONS

Abstract

Peroxy (HO2 and RO2) radicals play a central role in atmospheric oxidation chemistry, influencing ozone formation and secondary organic aerosols production, through the reaction with nitrogen oxides (NOx). Previous measurements of the concentration of these radicals were overestimated by current models of atmospheric chemistry especially in forested environments where there are high biogenic VOC emissions and low NOx concentrations both within and above the forest canopies. This discrepancy observed between model and observed radical concentration suggests that some processes such as radical cycling, deposition or heterogenous surface reactions may not be fully integrated into existing chemical mechanisms. Measurement of HO2* above the forest floors can provide a better understanding of these discrepancies and improve model representation. During summer 2025, measurements of HO2* (HO2 + a fraction of RO2) below the canopy and near the forest floors were conducted at a rural forest, Northern Michigan. Modeling of the measurements was done using the Framework for 0-D Model (F0AM) with incorporating the Master Chemical Mechanism (MCM) constrained with other measured chemical species. The model output generally underestimated the measured HO2*.